Glossary

This is a shortened version of the complete glossary with fewer and more concise definitions. A much more extensive glossary is found online, for free, at http://www.wiley.com/go/biologystg3e. The online glossary defines far more terms has more comprehensive definitions than this printed glossary. Please download a copy.

A-N node
The atrioventricular node (A-N node or atrioventricular bundle) initiates the ventricular contraction.
Abdomen
In vertebrates, the abdomen is the area between the diaphragm and the pelvis. In humans, the abdomen includes the digestive organs and urinary tract.
Abiotic
Abiotic factors are nonliving factors involved in an ecosystem.
Abscission
Abscission in plants is caused by the growth of thin-walled cells that result in the falling of a leaf or fruit from the plant.
Absorption spectrum
The absorption spectrum (= absorbtion spectrum) includes the specific wavelengths of electromagnetic radiation absorbed by a substance.
Acetylcholine
This is one of over 10 neurotransmitters.
Acetylcholinesterace
An enzyme that breaks down the neurotransmitter acetylcholine.
Actin
A protein that, with myosin, forms the contractile filaments of muscle cells.
Action potential
A nervous impulse, or wave of depolarization, is also called an action potential. It rapidly passes down the entire length of the axon.
Activation energy
The minimum amount of energy required under certain conditions before a specific chemical reaction may occur.
Active transport
When a cell expends energy to move molecules from one location to another, the process is known as active transport.
Adenohypothysis
The anterior lobe of the pituitary gland is also called the adenohypophysis; produces hormones.
Adenosine diphosphate
When a phosphate group breaks off of an adenosine triphosphate (ATP) molecule, it forms adenosine diphosphate (ADP).
Adenosine triphosphate
Adenosine triphosphate (ATP) is a high-energy molecule that provides some of the activation energy needed for chemical reactions.
ADP
When one of the phosphate groups breaks off of adenosine triphosphate (ATP), it forms adenosine diphosphate (ADP).
Adrenal cortex
Each adrenal gland has an outer cortex and an inner medulla. The cortex produces over 50 different hormones.
Adrenal gland
Sitting on top of each kidney is an adrenal gland. The adrenal glands produce several hormones, including adrenalin.
Adrenalin
A hormone and a neurotransmitter. It is also called epinephrine. It is produced in the adrenal medulla.
Aerobic
In the presence of oxygen, related to oxygen, or using oxygen.
Aerobic respiration
In the presence of oxygen, the three-carbon pyruvate molecules produced in glycolysis can be further oxidized in the mitochondria.
AIDS
Human immunodeficiency viruses cause HIV infections, which over time can lead to an immune system breakdown called acquired immunodeficiency syndrome (AIDS).
Albumin
A group of water-soluble proteins that occur in egg whites, lymph, tissues, and blood plasma.
Alcohol group
A hydroxyl group is written –OH. When a hydroxyl group is part of an alcohol, it is called an alcohol group. An alcohol group is written –COH.
Algae
Simple, nonflowering, usually aquatic plants. Includes many single-celled and many multicellular forms that lack true leaves, roots, stems, and vascular tissues.
Allele
Diploid cells have genes with one set of chromosomes from the mother and one set from the father. Each gene can have either two identical alleles or two different alleles.
Alternation of generation
Some organisms use both sexual and asexual reproductive strategies at different stages during their life cycle, a phenomenon termed alternation of generation.
Alveoli
The small air sacs throughout the lungs where gas exchange occurs.
Amino acid

All proteins are composed of amino acid subunits that are arranged in a pattern specific to each distinct protein type.

Amino group
All amino acid molecules contain one amino group, which is written this way: -NH2.
Ammonia
A compound composed of nitrogen and hydrogen with the formula NH3.
Ammonium cation
The ammonium cation is NH4+.
Amnion
The innermost membrane of reptile, bird, and mammal eggs.
Amniote
Any animal that has an amnion during development. Such animals include reptiles, birds, and mammals.
Amniotic fluid
The embryo of an amniote is located within the amnion and is bathed in amniotic fluid.
Amoeba
A unicellular organism that can change its shape, usually by extending and contracting its pseudopods.
Amylase
A starch-digesting enzyme in saliva.
Anabolic steroids
Synthetic, man-made steroids.
Anaerobic
In the absence of oxygen.
Anaerobic fermentation
Process by which cells can obtain energy without oxygen (also called anaerobic respiration).
Anaerobic respiration
When respiration occurs without any oxygen, it is called anaerobic fermentation (also called anaerobic respiration).
Anaphase
Stage of cell division that begins when the two complete sets of chromosomes start moving toward opposite ends of the spindle.
Anaphase I
During meiosis I, when the centromeres do not divide. Instead, one homolog from each of the homologous pairs moves toward a separate pole.
Anaphase II
During meiosis II, when the centromeres split and the sister chromatids move toward opposite poles.
Androgens
Male sex hormones, such as testosterone.
Anemic
When the blood's concentration of red blood cells, or the red blood cell's concentration of hemoglobin, drops below normal, a person becomes anemic.
Angiosperms
A large group of plants that includes most trees and shrubs, grasses, and herbaceous plants.
Angstrom
A unit of length equal to 10,000 microns (equal to one ten-billionth of a meter; a micron is equal to one-millionth of a meter; 1 meter = 10 billion angstroms).
Animal kingdom
Includes all living and extinct animals.
Anion
Negatively charged ion.
Annual
A plant that completes an entire life cycle by germinating, growing, flowering, and bearing seeds in one year or less.
Anterior lobe of pituitary
Also called the adenohypophysis. It produces hormones.
Anterior vena cava
Sometimes called superior vena cava. It carries deoxygenated blood from veins in upper part of the body to the heart (to the heart's right atrium).
Anther
Stamens include the pollen grains, which are produced at the specialized portion at the tip of the filament, called the anther.
Antheridium
Male sex organ that produces male gametes in some fungi and some nonflowering plants.
Anthocyanins
Plant pigments that protect leaves and stems from being damaged by strong light by absorbing ultraviolet light.
Anthophyta
The phylum Anthophyta, more commonly known as the flowering plants, or angiosperms, or anthophytes.
Antibiotic
Substances that destroy or help stop growth of certain bacteria.
Antibody
(Ab) Blood protein that neutralizes pathogens (antigens) by inactivating or destroying them.
Antidiuretic hormone
The posterior lobe of the pituitary produces vasopressin; also called antidiuretic hormone (ADH).
Antigens
Substances (such as foreign microbes) that stimulate the body to produce antibodies.
Antigen-antibody reaction
Specific chemical reaction that occurs each time a specific antigen combines with a matching antibody.
Antihistamine
Chemicals that block the receptors that histamines target.
Antioxidant
A substance that inhibits oxidation.
Anus
The external opening of the rectum, through which feces pass out of a body.
Aorta
The largest artery in the human body. It carries oxygenated blood from the heart to all parts of the body except the lungs.
Aposematic coloration
Warning coloration that alerts potential predators.
Arachnoid
One of the three layers of meninges that lies just under the dura mater. It resembles a cobweb.
Archaea
Also called Archaebacteria. Used to be said these were more primitive than bacteria. Now some say they are intermediate between bacteria and eukaryotes.
Archaebacteria
Not long ago it was said that members of the domain Archaea (also called Archaebacteria) were more primitive than bacteria. Now some say they are intermediate between bacteria and eukaryotes.
Archegonium
The archegonium (plural is archegonia) is the female sex organ produced by the gametophyte generation of mosses, liverworts, ferns, horsetails, and many gymnosperms.
Archenteron
The first opening (the blastopore) that connects the internal cavity (the archenteron) with the exterior of the gastrula.
Arteriole
Arteries branch into smaller arterioles, which branch into the smallest blood vessels, the capillaries.
Artery
Arteries are the blood vessels that transmit oxygenated blood away from the heart toward the cells and tissues throughout the body.
Arthropods
Belong to the phylum Arthropoda, most of which are insects (class: Insecta).
Artificial Selection
Often called selective breeding.
Asexual budding
When a new plant sprouts from a cell, leaf, root, or some other part of the parent.
Asexual reproduction
Requires one organism (no partner is necessary).
Association neurons
Association neurons (or interneurons) relay messages from one neuron to another.
Aster
During the early stages of spindle composition, microtubules radiate around each centriole, creating formations collectively known as the asters.
Atmosphere
We call the gases surrounding the earth the air and the atmosphere.
Atmospheric cycles
The carbon cycle and nitrogen cycles are called atmospheric cycles because carbon and nitrogen occur as gases during part of their cycles.
Atmospheric pressure
The pressure exerted by the weight of the atmosphere. On average, at sea level this is about 14.7 pounds per square inch.
ATP
Adenosine triphosphate (ATP) is a high-energy molecule that is present in all living tissue.
Atrioventricular node
The heart has a natural pacemaker called the atrioventricular node (A-N node or atrioventricular bundle), which starts each heartbeat.
Atrium
The thin-walled chambers in the heart are called atria (individually each is an atrium).
Autonomic nervous system
Consists of the nerves that carry nervous impulses from the central nervous system to the heart.
Autotroph
Primary producers that supply organic matter for all the other organisms in the food chain.
Auxins
Plant hormones that are important in controlling cell elongation in stems.
Aves
The technical term for birds.
Axon
Long extension from the cell body of a neuron that can conduct an electrical impulse (action potential).
Axon terminal
The final part of a neuron where the signal transmitted by the neuron is converted to a chemical (a neurotransmitter).
Axonal ending
The final part of a neuron where the signal transmitted by the neuron is converted to a chemical (a neurotransmitter).
B cells
This type of lymphocyte (also called white blood cell, WBC, leukocyte, leucocyte) protects the body from certain bacteria and viruses.
Bacilli
Eubacteria are often described in terms of their shape. Those that are rod-shaped are called bacilli.
Bacteria
Group of single-celled organisms that have cell walls but don't have an organized membrane-bound nucleus and they lack other organelles.
Bacterial virus
Some viruses known as bacteriophages attack only bacteria. A bacteria-attacking virus is called a bacterial virus, or a bacteriophage.
Bacteriophage
The viruses that only attack bacteria are known as bacteriophages. There are viruses that only attack eukaryotic cells. And many viruses are extremely specific with regard to the type of cell they will attack. Some viruses that commonly attack humans include cold viruses, many of which are called rhinoviruses. Americans catch over 60 million common colds each year.
Barometric pressure
Barometric pressure is atmospheric pressure. One atmosphere is the weight of the earth's atmosphere when measured at sea level.
Basal body
Occur at the base of cilia or flagella. They are structurally identical to centrioles. They support and give rise to the rest of the stalk.
Basophils
Type of white blood cell in the immune system of vertebrates. They protect the body from certain bacteria and viruses.
Batesian mimicry
Phenomenon where some animals that are not bad-tasting or poisonous may look just like those that are.
Behavioral ecology
Ecology and behavior are very closely related. The study of one invariably includes the other, and this combination is called behavioral ecology.
Bicarbonate ions
HCO3− is a bicarbonate ion.
Bicuspid valve
Oxygenated blood passes from each lung back to the heart, where it empties into the left atrium, then through the bicuspid valve (also called mitral valve) before entering the left ventricle.
Biennial
Plants that complete their life cycle in two years or less, from the time they germinate until they die.
Bilateral symmetry
When organisms have virtually identical right and left sides, with a different top and bottom.
Bile
A fluid produced by the liver that aids in lipid digestion. It is stored in the gall bladder, and released via the bile duct, into the duodenum.
Binary fission
Simple cell division, when a prokaryotic cell divides, creating two daughter cells, passing to each daughter cell an entire set of genetic material.
Bioaccumulation
This refers to how certain chemicals can become concentrated in living organisms.
Bioconcentration
The accumulation of a chemical in an organism.
Biological classification
Methods used to organize living and fossil organisms, to classify them into related groups and categories.
Biological science
Biological science (biology) is a natural science that involves understanding living organisms.
Biology
The study of life.
Biomass
The total mass of live organisms in a specific area at a specific time.
Biome
The major ecosystems, often called biomes, are regarded as distinct entities with distinct forms of life.
Biosphere
All the Earth's living organisms are sometimes called the biosphere.
Biotic
Created by a living organism, or a part of a living organism (including parts, products, remains, secretions, and wastes).
Birds
Modern birds are animals with feathers and no teeth.
Birth rate
The number of births in a year per 1,000 people (or other species).
Bladder
Urine is produced in the kidneys, then travels through the ureters to the urinary bladder.
Blastocyst
A fertilized egg becomes a solid ball of cells, which becomes a hollow ball of cells called a blastocyst (blastula).
Blastula
A fertilized egg becomes a solid ball of cells, which becomes a hollow ball of cells called a blastocyst (blastula).
Blood
A type of tissue that carries a collection of similar types of cells and associated intercellular substances that surround them.
Blood clot
A coagulated, gelatinous mass of blood cells.
Blood pressure
Blood pressure is defined as the pressure exerted by blood on the wall of any blood vessel.
Blood type
The most widely known is the ABO group. In this group there are four blood types: A, B, AB, and O.
Blue-green bacteria
Cyanobacteria used to be called blue-green algae. Now blue-green bacteria is the preferred term.
Bowman's capsule
In the kidneys, each Bowman's capsule is a membranous structure surrounding each glomerulus.
Brain
This is the most anterior ganglion, or enlarged, organized, integrative mass of nervous tissue.
Budding
When new plants sprout from leaves, roots, or another part of the parent. Or when broken off bacterial cell fragments grow into mature bacterial cells.
Ca2+
Ca2+ is a calcium ion. It is one of the most abundant positively charged inorganic ions (cations) found in human blood plasma.
Calcium carbonate
Calcium carbonate's chemical formula is CaCO3. It is a primary component in limestone, calcite, pearls, shells, dolomite, chalk, and eggshells.
Calcium ion
Ca2+ is a calcium ion. It is an abundant positively charged inorganic ion (cation) found in human blood plasma.
Calcium phosphate
A family of minerals composed of calcium and phosphorus, and other materials, that strengthen bones and teeth.
Calorie
Unit of energy required to raise the temperature of 1 gram or cubic centimeter (cc) of water by 1 degree (1°) Celsius (C). The calories usually referred to in biology and nutrition are kilogram-calories (Kcal); each is 1,000 gram-calories. Therefore, a kilocalorie (Kcal = kcal) equals 1,000 calories. When calories are discussed in Biology: A Self-Teaching Guide, they are kilogram-calories; that is, the amount of heat required to raise the temperature of 1,000 grams (1 kilogram) of water from 14.5 to 15.5 °C.
Calyx
Together, the sepals and the petals of a flower are called the calyx.
Cambium
The plant tissue called cambium is also called vascular cambium, or lateral meristematic tissue. It produces phloem, xylem, and cork.
Camouflage
A means of disguise that conceals organisms. Animals are often colored or shaped in ways that enable them to blend in with their backgrounds.
Capillary
A small, thin blood vessel that connects an arteriole to a venule.
Carbohydrates
A large group of organic compounds. Examples include sugars, starch, and cellulose.
Carbon
Carbon in the atmosphere mostly exists as a component of gaseous carbon dioxide. Carbohydrates are made from water and carbon dioxide (CO2).
Carbon cycle
Passing of carbon from carbon dioxide in the atmosphere into living tissue through photosynthesis and back to the atmosphere through respiration.
Carbon dioxide
CO2.
Carbon fixation
The process that incorporates carbon from carbon dioxide in the atmosphere into other carbon compounds.
Cardiac muscle
The cardiac muscle (heart muscle) is the involuntary muscle that constitutes the primary tissue making up the walls of the heart.
Carnivores
Animals that eat other animals or animal parts.
Carotenes
These yellow pigments are one of two divisions of photosynthetic pigments in the carotenoid group.
Carotenoids
Group of accessory photosynthetic pigments that absorb visible light in wavelengths from 453 to 642 nanometers (which causes carotenoids to be yellow, orange, or red) and transfers the energy to chlorophyll.
Carpel
Carpels represent the innermost whorl of a flower.
Carrier molecules
They help move certain substances through membranes, in and out of cells. They are lipid soluble.
Carrying capacity
Often referred to by the letter K. This is the number of individuals of a species that a specific environment can support.
Cartilage
In addition to bone, another important connective tissue in most skeletal systems is cartilage, which, unlike bone, is both firm and flexible.
Catalyst
An enzyme that accelerates a chemical reaction without being permanently changed by the reaction.
Cation
Positively charged ion.
Cell
Smallest unit of life. Cells have purposes and functions, such as taking up nutrients, removing wastes, growing, and reproducing.
Cell body
The cell body (soma) is the part of a nerve cell (neuron) that contains the nucleus and surrounding cytoplasm, but does not include the axons and dendrites.
Cell division
The ordered mechanism cells use to create new cells by breaking or splitting or dividing into two parts. Each part is a called a daughter cell.
Cell membrane
This is a membrane at a cell's surface that surrounds the cell's cytoplasm. It is also called a plasma membrane.
Cell replication
Also called cell duplication. This is how cells reproduce, while passing their genetic information to succeeding generations of cells.
Cell sap
Also called cytosol. This is the liquid inside the large vacuole in a plant cell that provides mechanical support. Cell sap helps keep a plant cell turgid.
Cell theory
Cells are the fundamental units of life, cells are the smallest entities that can be called living, and all organisms are made up of one or more cells.
Cell walls
Outside a plant cell's plasma membrane is a rigid layer of polysaccharides, called a cell wall that is made of cellulose.
Cellular respiration
A series of chemical reactions that renders energy available to the cells.
Cellulose
The main structural component comprising the cell walls of plants.
Centimeter
(cm) Unit of length equal to one-hundredth of a meter. 100 centimeters = 1 meter. Ten millimeters = one centimeter. 2.54 centimeters equals an inch.
Central nervous system
The brain and the spinal cord comprise the central nervous system.
Centriole
Centrioles are found only in animal cells; generally located near the nucleus, they occur in pairs, and are composed of nine sets of triplets all arranged lengthwise, creating a cylindrical structure. They become active during cell reproduction.
Centromere
This is a specialized DNA sequence that links the pair of sister chromatids (the dyad).
Cephalization
“Headness” is known as cephalization, which is typical of most bilateral, active organisms. The head region contains sensory cells.
Cerebellum
During evolutionary history, the anterior portion of the hindbrain became enlarged and specialized as the cerebellum, which controls balance, equilibrium, and muscular coordination.
Cerebral cortex
Amphibians have more gray matter than fish, indicating that their cerebrum functions less as a simple conduit and more as an integrator of incoming information. With this expansion of internal gray matter, the gray matter moved from the inside part of the brain to the surface, where it is called the cerebral cortex.
Cerebrospinal fluid
Between the pia mater and the arachnoid is cerebrospinal fluid (CSF), which acts as a protective cushion, protecting the brain from mechanical injury.
Cerebrum
This major part of the brain (two-thirds the weight of entire brain) contains two cerebral hemispheres. It is also called the cerebral cortex.
Cervical vertebrae
Most humans possess seven cervical vertebrae (neck bones).
Cervix
This is the lower opening of the uterus.
Chemical
This is a pure basic compound or substance. The production of a chemical involves changing the arrangement or attachments of atoms or molecules.
Chemical homeostasis
The condition of a constant internal environment is referred to as chemical homeostasis.
Chemoreceptors
Sensory cells or sensory organs that aid in the detection of certain chemical signals.
Chloride ions
Cl− is a chloride ion. The primary negatively charged inorganic ions are chloride (Cl−), bicarbonate (HCO3−), phosphate (HPO2−)4, and sulfate (SO2−)4.
Chlorophyll
A green photosynthetic pigment that absorbs light energy used in photosynthesis.
Chloroplast
Organelles inside cells that conduct photosynthesis. They are enclosed within an outer envelope composed of two membranes, an outer and inner membrane.
Cholesterol
An organic molecule. It is also a steroid, and it is also classified as a sterol. Cholesterol is an important component of animal cell membranes.
Chordates
A chordate is a member of the taxonomic group called Chordata.
Chromatids
Once recoiled, the chromosomes become “X” shaped. The X's are composed of two identical chromatids, each held to the other by a single centromere.
Chromoplast
Unlike chloroplasts that contain green pigments, chromoplasts usually synthesize and store orange or yellow pigment.
Chromosomes
These are the structures in the cells that contain the genes.
Cilia
Singular: cilium. Cilia are long, thin organelles that project from the surface of some cells and have the capacity to beat back and forth.
Circulatory system
The circulatory system of vertebrates and many invertebrates delivers nutrients (amino acids, electrolytes), minerals, hormones, blood cells, and dissolved gases (oxygen) and then carries off the metabolites such as carbon dioxide and nitrogenous wastes.
Citric acid cycle
Also known as the Krebs cycle, and tricarboxylic acid (TCA) cycle. A series of chemical reactions in cells of aerobic organisms that generate a useful form of energy from carbohydrates, fats, and proteins.
Class
The taxonomic category that is one step higher than order and one step lower than phylum.
Cleavage
The division of a cell involves the process where one cell becomes two cells.
Cleavage furrow
In animal cells, the division of the cytoplasm (cytokinesis) begins with the formation of an indentation, or cleavage furrow, which forms all the way around the equatorial region of the cell.
Climate
The weather conditions in an area over time (such as for several decades in a row).
Climate change
Long-term changes in weather patterns that last many years (for instance for decades, centuries, or millions of years), as measured statistically.
Climate system
Involves the atmosphere, hydrosphere, cryosphere, biosphere, and lithosphere.
Clitoris
A female sex organ that is present in mammals and some other animals, and contains many nerve endings and erectile tissue.
Clone
Binary fission and budding produce groups of genetically identical cells, known as clones.
Cloning
Production of genetically identical offspring, though they may not look identical.
Closed circulatory system
Higher animals, such as vertebrates, usually have a closed circulatory system through which their blood circulates.
Cocci
Eubacteria are often described in terms of their shape. Those that are spherical are known as cocci.
Coccyx
In humans, the “tail” or tailbone is called the coccyx. This is comprised of the last bones of the vertebral column.
Coelom
A true coelom is a cavity located between the digestive tract and the body wall.
Coelomates
Organisms with a coelom are called coelomates.
Coenzymes
Enzymes are molecules that are made of proteins. Coenzymes are molecules that are not made of proteins, and yet they help enzymes to work properly. Sometimes coenzymes are called cofactors.
Cofactor
A cofactor is a substance that needs to be present for certain enzymes to speed up a chemical reaction by reducing the activation energy needed for the chemical reaction to occur.
Coitus
The same as sexual intercourse, which is also the same as copulation.
Cold-blooded
Organisms that are not homeothermic are sometimes called cold-blooded, or poikilotherms (poikilothermic).
Collagen fibers
Structural proteins that provide connective tissue support.
Colloid
Colloids usually contain particles that are too small to be seen but are large enough not to form a true solution. The particles don't settle out at an appreciable rate.
Colon
The large intestine.
Colonial
Two or more individuals living near one another.
Colonizer
In ecological terms, a colonizer is a species that increases in number or is able to persist by moving to new areas and establishing new colonies, or by increasing the size of an already established colony.
Colony
Several organisms, especially of the same species, living together, usually near each other, sometimes immediately adjacent to each other.
Community
Each of the populations living in a given area constitutes a community. In addition to species diversity, the population size of each species is very significant in understanding the dynamics of that community. Some species are dominant.
Competition
When two individuals or two species are in competition with one another, this is usually a situation where certain resources are limited (for instance food, water, territory).
Competitive exclusion principle
The competitive exclusion principle states that when two or more species compete for the same resource necessary for their livelihood, and they are found or placed together in a situation where the resource is limited, they cannot coexist indefinitely.
Competitor
One individual or one species may compete with another individual or species for such living requirements as food or space.
Concentration
The measure of an amount of a subcomponent in a solution.
Concentration of substances
Concentration can be expressed in terms of the quantity of a chemical, or of a solute that is present in a specific amount of solution.
Condensation reaction
An example of a condensation reaction is a chemical reaction between two molecules that eliminates water.
Conifers
The conifers include all the species of pine, spruce, fir, hemlock, cypress, redwood, larch, juniper, and yew.
Conjugation
When two bacterial cells come together and are joined by a protein bridge through which DNA fragments pass from cell to cell.
Contractile vacuole
Important in expelling excess water and waste from the cell.
Copulation
The same as coitus, which is the same as sexual intercourse.
Cork cambium
In some plants, located outside the cortex, is a cork cambium, which divides mitotically to create the most external tissue, known as the cork.
Coronavirus
The coronaviruses include a number of viruses, some of which are associated with common cold-like ailments. Usually the varieties of coronaviruses that attack humans are not particularly harmful, but on occasion, when a virus jumps from another species to humans, we humans have little or no immunity and the results can be catastrophic. Luckily, the jumping of a “novel” virus from a non-human species to humans is rare, however this occurred in 2003 when the SARS coronavirus (SARS-CoV). SARS is short for “severe acute respiratory syndrome,” which describes the respiratory illness that this coronavirus (CoV) causes in people. This virus is believed to have jumped from a species of bat to people, possibly when someone was handling the bat. Again in 2019 another coronavirus jumped, possibly originating with a bat species, and somehow it jumped to a human, and then spread around the world causing the disease called Covid-19 (Co is short for corona, vi is short for virus, d is short for disease, 19 is for 2019, when it first appeared in humans). This first appeared in Wuhan, Hubei, China, and then it rapidly spread to people all around the world, causing a pandemic (a disease that spreads to many people all around the world) that killed hundreds of thousands of people.
Corpus luteum
A small ductless temporary aggregation of endocrine tissue that forms in a ruptured follicle following the release of an egg.
Covid-19
In 2019 a coronavirus infected humans around the world causing a pandemic. The disease is called Covid-19 (Co is short for corona, vi is short for virus, d is short for disease, 19 is for 2019, when it first appeared in humans). It first appeared in humans in China. This disease killed hundreds of thousands of people.
Cotyledon
The embryonic leaf-like structure (it is not green) that comprises much of the seed with its endosperm.
Covalent bond
A molecule has a covalent bond when atoms are attached together that have a stable balance created by sharing electrons, where in the outer orbital one atom may need another electron to be balanced and stable and the other might have an extra and wants to get rid of it to be balanced and stable.
Cowper's glands
A pair of small glands that transmit secretions into the urethra at the base of the penis, that mix with the sperm.
Cranium
The part of the skull covering the brain.
CRISPR
An acronym for clusters of regularly interspaced short palindromic repeats. CRISPR is a tool used to edit genes.
Cristae
These folds in the inner membranes of mitochondria provide increased surface area for such chemical reactions as aerobic respiration that requires oxygen.
Crossing over
During synapsis the chromatids may exchange segments of genetic material. When this happens, it is called crossing over.
Crossover
During prophase I of meiosis, when two matching homologous chromosomes pair up, and sections of the chromosomes may crossover, during what is called crossover, before segregation.
Crustacea
Crustaceans are a large taxonomic group of arthropods. Included in this group are spiders, ticks, mites, centipedes, millipedes, insects, crabs, and lobsters.
Cryosphere
All the frozen water on earth is called the cryosphere.
Cryptic coloration
Cryptic coloration hides an animal against its background by being the same color or pattern.
Cuticle
A hard, tough, protective, flexible, and often waxy layer covering the epidermis of many plants and invertebrates.
Cyanobacteria
Used to be called blue-green algae. They are prokaryotic, single-celled, blue-green bacteria.
Cyclic photophosphorylation
In the photosynthetic phosphorylation process, the light energy-driven addition of phosphate groups is known as cyclic photophosphorylation.
Cytochrome system
For life to occur, energy is required. For cells to harness the energy available in the atoms and molecules, they use an electron transport chain called the cytochrome system.
Cytochromes
The main components of the electron transport chain are cytochromes (iron-containing protein molecules).
Cytokinesis
Eukaryotic cell division includes nuclear division (karyokinesis) and the division of the rest of the cell, which is called cytokinesis.
Cytoplasm
Except for the nucleus, everything within the plasma membrane of a cell is called cytoplasm.
Cytoplasmic streaming
This is when the cytoplasm moves inside the cell.
Cytosol
Also called cell sap. This is the liquid inside the large vacuole in a plant cell that provides mechanical support.
Darwin, Charles
(1809–1882) English naturalist whose work changed how we think about natural selection, artificial selection, and evolution.
Daughter cells
When a cell reproduces or divides it creates two cells. Each of these are called daughter cells.
Day
One day equals 24 hours.
Deamination
Protein hydrolysis produces amino acids that can be metabolized. One such metabolic pathway is deamination, which involves the removal of the amino groups.
Death rate
The number of deaths in a year per 1,000 people.
Decimeter
One-tenth of a meter = 1 decimeter. Ten decimeters = 1 meter.
Decomposers
Bacteria represent some of the most important groups of decomposers; without them, dead organisms would not decay properly.
Dehydration reaction
Dehydration means the elimination of water. A chemical reaction that eliminates water is a dehydration reaction.
Dendrite
Small extension from the cell body of a neuron that can conduct an electrical impulse received from another cell via the synapse to the body or soma of the cell.
Dendritic cells
Stimulate resting T cells to initiate an immune response.
Denitrifying bacteria
Denitrifying bacteria live under anaerobic conditions and complete the nitrogen cycle by returning nitrogen back to the air.
Density
The density of a substance is its mass per unit volume.
Density dependent
As the population density of one species increases, the mortality within that species may increase. In such a case the mortality is said to be density dependent.
Density independent
As the population density of one species increases, the mortality within that species may increase. In such a case the mortality is said to be density dependent. If the mortality is due to causes other than the species' density, then it is said to be density independent.
Deoxygenated blood
Oxygen-poor blood. This is blood without an adequate amount of oxygen in it.
Deoxyribonucleic acid
DNA for short. Chromosomes are mostly composed of DNA.
Depolarization
At the site where the neuron is stimulated, the membrane becomes a little more positively charged relative to the outside of the cell. This change in membrane potential is called depolarization.
Diabetes
When a pancreas is underactive and does not produce enough insulin, this causes an insulin deficiency, known as diabetes.
Diaphragm
A muscle under the rib cage that significantly improves breathing efficiency.
Diarrhea
When a bowel movement is loose and watery.
Diastolic pressure
Diastolic pressure is the pressure of the blood on the walls of the arteries when the heart is relaxed (between heart beats).
Diffusion
In a system where diffusion occurs, a net movement of particles can be observed over time moving from regions of higher concentration of that substance to regions of lower concentration of that substance.
Digestion
The breaking down of nutrient materials we call food.
Dioecious
Plants and invertebrates with separate individuals that are male and separate individuals that are female are called dioecious.
Diploid
A cell or nucleus that has two complete sets of chromosomes (2N), one set from each parent, is said to be diploid.
Directed movement

Rather than being sessile (attached to the substrate) or drifting about at random, animals that control the direction of their movement have directed movement.

Disaccharide
Any sugar made of two monosaccharides.
DNA
Deoxyribonucleic acid is DNA for short. Chromosomes are mostly composed of this. DNA contains genetic information and is a long, self-replicating molecule.
DNA Virus
Unlike living cells, viruses do not metabolize; that is, they do not generate their own energy. Instead, with genetic information contained in their nucleic acids (in their viral DNA or RNA), they overpower other cells, insert their nucleic acids (DNA or RNA) into their host's cell, and then direct the production of more viruses by utilizing the host's cellular machinery. All other organisms contain both DNA and RNA, however viruses contain only one or the other. Examples of DNA viruses are Epstein-Barr virus (EBV), hepatitis B virus, hepatitis C virus, and human herpes virus.
Domain
The taxonomic category that is one step higher than kingdom.
Dopamine
Dopamine is involved in transmitting signals to other neurons.
Dorsal hollow nerve cord
The phylum Chordata is usually divided into three subphyla, which all have a dorsal hollow nerve cord.
Duodenum
Where the stomach ends and the small intestine begins, the first part of the small intestine is called the duodenum.
Duplication
When speaking of duplication in terms of cells, often the term replication is used. Similarly, cell replication is also called cell duplication.
Dura mater
One of the three layers of meninges, the dura mater lies just under the skull; it is tough and fibrous.
e−
e− is an abbreviation or shortened form for the word electron.
Ebola
Ebola (Ebola hemorrhagic fever) is very rare, but it is also very contagious and deadly. There is no a vaccine that can prevent people from catching this disease. Another disease caused by a virus may have heard of that are caused by virus is yellow fever, which was also brought under control with a vaccine.
E. coli
E. is the abbreviation for the genus Escherichia. Escherichia coli is a species of bacteria that commonly lives in the lower intestine (large intestine) of humans.
ECG
An electrocardiogram (also EKG or ECG) measures a heart's electrical potential during each contraction.
Ecology
A theoretical, quantitative study of organisms, populations, species, communities, and ecosystems.
Ecosystem
The sum total of all the organisms and their environment in a given area.
Ecotone
When the transition from one ecosystem to another is abrupt, such a zone is sometimes called an ecotone.
Ectoderm
In an embryo, the ectoderm is the outer tissue layer.
Ectoparasite
Often parasites are thought of as small animals that either live on or in their host.
Effector cell
A type of neuron (nerve cell) that is also called a motor neuron, or effector neuron.
Effector organ
An effector organ is a muscle or gland that, due to a neural impulse, either contracts or secretes.
Efferent nerve ending
Located at the end of a nerve, there are some sites that carry nervous impulses from the central nervous system to muscles and organs.
Egg
The sex cell or gamete produced by a sexually reproducing female. Also called an ovum (plural = ova).
Ejaculate
The thick liquid released from a penis as a result of an ejaculation. The liquid is called semen. It contains sperm.
Ejaculation
The process of releasing semen through the male's urinary meatus.
EKG
An electrocardiogram (also EKG or ECG) measures a heart's electrical potential during each contraction.
Electrocardiogram
An electrocardiogram (also EKG or ECG) measures a heart's electrical potential during each contraction.
Electrolytes
Substances in solution that enable the solution to conduct electricity when dissolved in water.
Electromagnetic radiation
Travels in waves that we know as gamma rays, infrared light, microwaves, radio waves, ultraviolet light, visible light, and X-rays.
Electromagnetic waves
Electromagnetic radiation travels in waves that we call electromagnetic waves.
Electron acceptor
Any chemical that accepts electrons from another element or compound.
Electron transport chain
The NADH and FADH2 released during the Krebs cycle (citric acid cycle) are transported to the series of electron carriers known as the electron transport chain.
Embryo
When an egg is fertilized it goes through many cell divisions and organization of tissues. While going through many developmental stages, and while still unborn or not yet hatched, it is known as an embryo.
Emigration
Movement from an area.
Endocrine glands
Hormone-producing tissues are often called endocrine glands.
Endocrine system
A series of tissues, glands, and cells found throughout the body that secrete chemicals called hormones that, when active, exert specific effects on specific cells and tissues.
Endocytosis
A form of active transport that moves particles via invagination of a membrane into a cell via pinocytosis or phagocytosis is called endocytosis.
Endometrium
The uterine lining.
Endoplasmic reticulum (ER)
Network of tubules inside a eukaryotic cell that is attached to the nuclear membrane. The rough endoplasmic reticulum has many ribosomes attached.
Endoskeleton
The skeletal structure of animals with cartilage or bones inside the body.
Endothelium
Capillary walls are made of endothelial cells, which are thin, flattened cells that line inside surfaces of various organs, the abdomen, and the inside of lymphatic vessels, and blood vessels.
Energy
Energy cannot be created or destroyed, it can be transferred to other objects, and it can be converted in form. Cells need to convert energy into a form that they can use. All living organisms need energy to continue living.
Enzyme
A protein that speeds up a chemical reaction by reducing the activation energy needed for the chemical reaction to occur.
Epidermis
In animals the epidermis is the thin layer of skin (epithelium) on the outside surface of the animal, which lies over the dermis.
Epididymis
Sperm is stored in the epididymis, which is attached to the testes.
Epinephrine
A hormone and a neurotransmitter. It is also called adrenalin. It is produced in the adrenal medulla.
Equilibrium
A stable system in a state of balance.
Erectile tissue
Biological tissue that can become engorged with blood. This usually causes it to become turgid (stiff).
Escherichia coli
A species of bacteria that commonly lives in the lower intestine (large intestine) of humans. Some strains can make you sick.
Esophagus
After food is swallowed, it passes through the esophagus to the stomach.
Essential amino acids
The only amino acids our bodies cannot make, that we must consume in our diets, are called essential amino acids.
Estrogen

Sometimes spelled oestrogen, this is a primary female sex hormone that helps with sexual development during puberty.

Estrous cycle
Estrous, also spelled estrus and oestrus, occurs when an ovarian follicle matures (in some animals several follicles mature), estrogen is released, and when females are said to be in heat during the estrous phase of their cycle.
Estuary
The region where fresh water from streams and rivers drains into salt water.
Eubacteria
Distinct from Archaebacteria. Eubacteria (also called true bacteria and cyanobacteria) are simple cells with rigid cell walls, no nucleus, and many species have flagella.
Eukaryote
Organism whose cell or cells contain a nucleus enclosed within membranes. Eukaryotic cells are enclosed within a plasma membrane. They contain mitochondria, Golgi bodies, endoplasmic reticulum, and most other organelles.
Evolution
Changes in heritable traits in organisms through successive generations.
Excretion
The process of ridding the body of excess water, salts, and nitrogenous wastes such as ammonia, urea, and uric acid.
Excretory system
Helps maintain chemical homeostasis by removing unnecessary materials from an organism's bodily fluids.
Exocrine glands
Secrete their products into ducts.
Exocytosis
A form of active transport that moves particles out of a cell via pinocytosis or phagocytosis.
Exoskeleton
The tough external cuticle covering arthropodan bodies functions as an external skeleton.
External genitalia
The female external genitalia are the vulva or pudendum. The male external genitalia are the penis and scrotum.
Extinction
Termination of a species. Extinct is when no living representatives of a species exist.
Extirpation
If a species has been extirpated from a specific area, it used to live there, but no longer does.
Extremophile
Organism that does well in very hot, very cold, etc.
Facilitated diffusion
One of the two main types of transport that involves active, as opposed to passive, transport. Facilitated diffusion involves carrier molecules that move substances either in or out of a cell.
Fallopian tube
Also known as the oviduct, this tube passes from the ovary to the uterus.
Family
The taxonomic category that is one step higher than genus and one step lower than order is family.
Fats
One for the four basic macromolecules found in living things. The other three are proteins, carbohydrates, and nucleic acids. Our bodies can break down fats into glycerol and fatty acids. The fatty acids can be used to make glucose. Our bodies can also store fats.
Fat droplets
Also called a lipid droplets or a lipid bodies, or oil bodies, or adiposomes. These are cellular organelles that are rich in lipids.
Fatigue
When the muscle can no longer sustain a contraction, the muscle fatigues.
Fat-soluble vitamins
Vitamins are classified into two groups: fat-soluble vitamins and water-soluble vitamins. The fat-soluble vitamins are A, D, E, and K.
Fatty acid
A carboxylic acid. It is defined by its molecular structure. Each fatty acid molecule is composed of a long hydrocarbon chain (chain of carbon atoms with hydrogen atoms bonded to the carbon atoms), and at one end of this long chain of carbon atoms is a -COOH group (carboxyl group).
Feces
Indigestible wastes, called the feces or excrement, pass out the anus.
Female
The sex that produces nonmobile haploid (1N) gametes. In animals, these female haploid cells are called ova or egg cells and are produced by ovaries. In plants, the female sex cells are called ovules.
Femur
The largest bones in the body. These are the thigh bones that connect the upper leg with the pelvis.
Fermentation
Many cells, particularly those in most plants and in some microorganisms, can obtain energy without oxygen by the anaerobic process of fermentation (also called anaerobic respiration).
Fern
Vascular plants that reproduce with spores. They don't have flowers or seeds. They have true roots, stems, and complex leaves.
Fertile
Often used when speaking about an individual who is able to produce seed (sperm or eggs) or conceive and produce young.
Fertilization
The steps that lead to a zygote, via the fusion of the male and female gametes, are a process, or an act, that we call fertilization.
Fetus
After two months of development, a human embryo is often called a fetus.
Fission
The biological definition of fission is when something living (can be a cell, or an organism) divides into two or more parts and then regenerates the parts that were missing, so the new individual looks like the original individual.
Fixing nitrogen
Nitrification (also called nitrogen fixing or nitrogen fixation) is the biological oxidation of nitrogen.
Flagella
Long, thin organelles that project from the surface of some cells. They have the capacity to beat back and forth in a corkscrew fashion.
Flatworms
The simplest animals to possess a bilaterally symmetrical body plan. Examples include planaria, flukes, and tapeworms.
Flower
The typical flower consists of four whorls of modified leaves called sepals, petals, stamens, and carpels, all of which are attached to the receptacle.
Flowering plants
Angiosperms are the plants that produce flowers and fruit, and they are often called flowering plants.
Follicle
Gametogenesis in females is known as oogenesis, the process that produces eggs in the small cavities or sacs in the ovaries called follicles.
Follicle-stimulating hormone
The anterior lobe of the pituitary gland in the brain (also called adenohypophysis) produces follicle-stimulating hormone (FSH).
Food
A substance an organism ingests that is used by the organism to provide sustenance, support, energy, and nutrition.
Food vacuole
Vacuoles are large, membrane-bound, fluid-filled spaces found in many cells, particularly plant cells.
Forebrain
The brain has three principal divisions, the forebrain, midbrain, and hindbrain. The most anterior of these three divisions is called the forebrain. It is composed of the olfactory lobes, cerebrum, thalamus, hypothalamus, and pituitary.
Fragmentation
A form of asexual reproduction in which part of an organism separates from the whole, and a new individual regenerates from that part.
Free radicals
Short-lived, highly reactive, uncharged molecules that are extremely reactive because of their tendency to gain or lose electrons, and, therefore, they can react with other molecules.
Fructose (C6H12O6)
A simple sugar, which means it is one of the simplest forms of a carbohydrate.
Fruiting bodies
Reproductive structures produced by some fungi and by some bacteria.
Functional group
In organic chemistry, functional groups are specifically organized groups of atoms with specific bonds that are attached to molecules (or within molecules).
Galactose (C6H12O6)
A 6-carbon sugar.
Gall bladder
Also spelled gallbladder, this is a small hollow organ that stores bile before releasing it into the duodenum.
Gametogenesis
The cell division process called meiosis that creates the gametes (eggs and sperm).
Gametophyte generation
One of two alternating phases in certain plants (such as bryophytes) and algae. The gametophyte stage of a plant or algae is the part that develops sex organs that produce gametes.
Ganglion
An enlarged, organized, integrative mass of nervous tissue.
Gas vacuole
Also called gas vesicle. Located in some bacteria. The gas vacuole is surrounded by a membrane that is permeable to gas. The inflation of the vesicle provides buoyancy.
Gaseous nitrogen (N2)
An important element in living organisms. 78% of the atmosphere is nitrogen.
Gastric juice
Also called gastric juice and stomach acid, this is important in the digestion of proteins.
Gastrointestinal tract
Often called the digestive system.
Gastrovascular cavity
Another name for the internal digestive cavity.
Gastrula
A fertilized egg becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst (blastula). The next stage is called the gastrula, which is a simple double-layered embryo, from which the organism develops.
Gel state
A soft, solid jelly-like material where the colloidal particles interact and form a spongy network. The colloid in a gel state forms a semisolid.
Gelatinous layer
A layer that is thick, slimy, viscous, and may also have the consistency of jelly.
Gametogenesis
The entire developmental process during which gametes (egg cells and sperm) divide during meiosis and mature and develop.
Genes
Discrete units of information that inform the cell what to do and how and when to do it.
Genetic engineering
Manipulation of an organism's genetic material to deliberately modify the characteristics of that organism.
Genetically modified organisms (GMOs)
Organisms whose genetic material has been modified using genetic engineering techniques.
Genetic recombination
During meiosis, especially during prophase I, when homologous chromosomes line up in pairs, fragments of DNA may crossover. This is called genetic recombination. It is also called crossing over.
Genitalia
Also called genitals. The sexual organs.
Genotype
The genes that an organism has.
Genus
The taxonomic category that is one step higher than species and one step lower than family.
Geotropism
Also called gravitropism. Sensing and growing toward the direction where gravity is pulling from (down).
Germ cell
Gametes (1N) are also called germ cells. Examples are eggs and sperm. Each germ cell contains half the number of chromosomes of a somatic cell (2N). Each egg cell is 1N and each sperm cell is 1N.
Gills
Respiratory organs that enable many aquatic organisms to extract dissolved oxygen from the water and excrete dissolved carbon dioxide into the water.
Gill slits
Openings to the gills.
Glial cells
Cells that support the neurons. They are also called neuroglia. They are the central nervous systems cells that are not neuronal.
Global cooling
The earth's climate continually changes. Sometime in the distant future it is expected that there will be another ice age (a long-term decrease in average temperatures).
Global warming
Global warming is usually defined as a long-term increase in the average temperature of the earth's climate system.
Glomerulus
In the kidneys, oxygenated blood travels through small spherical balls of capillaries called the glomeruli. Each one is a glomerulus.
Glucose
In addition to glucose, two other examples of simple sugars are fructose and galactose. All three each have 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. They share the same chemical formula (C6H12O6).
Glycogen
A multibranched polysaccharide that is stored as a long-term energy reserve in some animals. Glycogen can be converted to glucose later.
Glycolysis
The first series of chemical reactions in cellular respiration, in which glucose is broken down in controlled incremental steps. Literally, glycolysis means the splitting of glucose.
GMO
The acronym that refers to genetically modified organisms. This is the term used to describe organisms whose genetic material was modified using techniques commonly called generic engineering.
Golgi apparatus
Also called Golgi body. This cellular organelle looks like a stack of thin, bowl-shaped membranes. It contains chemicals synthesized in the ribosomes. Here these hormones and digestive enzymes are concentrated and stored until released by secretory vesicles produced by the outer portion of the Golgi apparatus.
Golgi body
Sometimes called the Golgi apparatus. These cellular organelles look like a stack of thin, bowl-shaped membranes. They contain chemicals synthesized in the ribosomes.
Gonads
Organs that produce gametes (sperm and eggs).
Gonorrhea
A sexually transmitted bacterial-borne disease.
Gradient
A change in concentration (or temperature, pressure, or elevation) from one point to another.
Grain
A small, single, hard structure, such as a seed.
Gram (gm)
Unit of mass equal to one-thousandth of a kilogram. There are 454 grams per pound. One milliliter of water = 1 gram.
Grana
Stacks of thylakoids. Thylakoids are stacks of flattened sacs in chloroplasts, bordered by pigmented membranes where the photosynthetic light reactions of photosynthesis occur.
Gravitropism
Also known as geotropism. Involves the turning of a plant or fungus toward or away from gravity.
Gray matter

An important part of the central nervous system that mostly consists of cell bodies and synapses.

Greenhouse effect
The process whereby the atmosphere traps much of the sunlight's energy (as warmth) in the lower atmosphere.
Greenhouse gas
Some gases in the atmosphere can cause the earth and its atmosphere to increase in temperature. These gases are called greenhouse gases.
Growth hormone
The anterior pituitary produces many hormones, including growth hormone (somatrophic hormone, STH), which is important in regulating growth.
Growth ring
Inside tree bark, the old xylem continually forms additional rings of tissue that gradually add to the diameter of the stem or trunk. These are the growth rings.
Gymnosperm
Plants with seeds that aren't protected with an ovary or fruit. Examples of gymnosperms are conifers, cycads, and the ginkgo.
H+
A hydrogen ion. Hydrogen is a proton with an electron. A hydrogen ion is a proton without that electron.
Habitat
An organism's habitat is the place where it lives.
Hair
A term used for many similar-looking long, thin structures.
Haploid
A cell or nucleus that has one set of chromosomes (1N) is called haploid. Gametes have one set of chromosomes.
Haversian canals
Compact bones are penetrated by blood vessels and nerves through small narrow openings, some of which are known as Haversian canals.
Head
Upper part of a body, in humans it is separated from the rest of the body by a neck.
Heart
The force maintaining the flow of blood through the arteries, veins, and capillaries is usually provided by the pumping action of a hollow muscular organ, called a heart.
Heart muscle
Heart muscle (cardiac muscle) is the involuntary muscle that constitutes the primary tissue making up the walls of the heart.
Heart valve
One-way valves that help regulate the movement of blood in a hollow muscular organ called a heart.
Heartbeat
Occurs when the heart contracts. The heartbeat is sometimes seen, or felt, or heard. When felt, it is sometimes called a pulse.
Hemocyanin
The blood of many arthropods and mollusks contains an oxygen-carrying pigment called hemocyanin.
Hemoglobin
Human red blood cells (and the red blood cells of other vertebrates) contains the oxygen-carrying pigment called hemoglobin.
Hemolymph
Insect blood, also called haemolymph, is composed of water, plasma, minerals, nutrients, waste products, and blood cells, also called haemocytes.
Hemolysis
The bursting (breaking apart) of red blood cells, and thus their contents are released.
Hepatitis
A disease that causes liver inflammation. Different types of viral hepatitis are caused by different viruses.
Herb
Plants that don't have woody parts.
Herbaceous
Plants without woody parts.
Herbivore
Animals that eat plants or algae.
Hermaphroditic
A hermaphrodite has complete or partial female and male reproductive organs and produces both male and female games.
Herpes virus
There are different forms of herpes virus. One can cause chicken pox (and when reactivated, it can cause shingles). Other forms cause cold sores in the mouth and on our lips, as well as sores in the genital area, and yet another, Epstein-Barr virus, causes mononucleosis. Another type of virus that no longer infects people, was smallpox, which killed hundreds of millions of people until it was eradicated from the wild in 1980.
Heterotroph
Organism that requires other plants and animals for its nutritional requirements.
Hindbrain
Connects the nerve tracts from the spinal cord to the rest of the brain.
Hindgut
The posterior part (caudal) of the intestines.
Histamines
Chemicals that dilate, or relax, the muscles in the walls of blood vessels.
HIV
Human immunodeficiency virus, also called AIDS.
Homeostasis
A constant internal environment.
Homeothermic
Both mammals and birds are homeothermic, or warm-blooded.
Homolog
During the metaphase stage of cell division, the chromosomes are lined up along the equator. The centromeres have divided in two. Each is attached to one of the two corresponding chromosomes from the pair. The individual chromosomes are called homologs, and together, both chromosomes in each pair are called homologous chromosomes.
Homologous chromosomes
A pair of chromosomes, one from each parent. Each is comparable in shape, size, centromere location, and gene position. The genes of each homolog (that's what we call each one) may contain a different allele (a different form of a gene that can result and a different observable characteristic, or said differently, may result in a different phenotype).
Homologous pairs
During the metaphase stage of cell division, the chromosomes are lined up along the equator. The centromeres have divided in two. Each is attached to one of the two corresponding chromosomes from the pair. The individual chromosomes are called homologs, and together, both chromosomes in each pair are called homologous chromosomes.
Hormones
The tissues, glands, and cells of the endocrine system secrete chemicals called hormones which, when active, exert specific effects on specific cells and tissues.
Host
The organism that the parasite parasitizes is the host.
Hour
Equals 60 minutes, equals 1/24th of a day.
HPV
Human papillomavirus infection, often called HPV. Papillomaviruses cause plantar warts, genital warts, and certain wart-like rashes.
Humidity
The amount of water vapor (in a gaseous state) in the air.
Hydrogen
Chemical element with the symbol H. Has one proton and an atomic number of 1.
Hydrogen ion
H+ is a hydrogen ion. Hydrogen is a proton with an electron. Without its electron, the hydrogen becomes a hydrogen ion. Therefore, a hydrogen ion is a proton without an electron.
Hydrogenated fats
Food labels listing the ingredients often term an oil or fat as hydrogenated when hydrogen atoms have been added to the fatty acids to thicken the consistency of the product.
Hydrogenated oils
Food labels listing the ingredients often term an oil or fat as hydrogenated when hydrogen atoms have been added to the fatty acids to thicken the consistency of the product.
Hydrogenation
The process of adding more hydrogen atoms to a molecule.
Hydrolysis
Hydro means water. Lysis means to unbind. Hydrolysis is the unbinding (or breakdown) of chemicals while reacting with water.
Hydrolytic enzymes
Proteins that speed up the breakdown of molecules using hydrolysis (see hydrolysis).
Hydrosphere
Waters surrounding the earth are called the hydrosphere.
Hymen
Girls are born with a thin membrane across the vagina that forms a border around the vaginal opening. This membrane, the hymen, has an opening through which fluids may pass.
Hyperosmotic
A hyperosmotic solution has more solutes. When a cell is in a medium with a higher concentration of osmotically active dissolved or colloidal particles (solutes), giving it a higher osmotic pressure, water moves across the cell membrane into the medium that is hyperosmotic (also called hypertonic) relative to the solution inside the cell.
Hypertonic
When a cell is in a medium with a higher concentration of osmotically active dissolved or colloidal particles (solutes), the fluid has higher osmotic pressure, so the water moves out, across the cell membrane into the medium that is hyperosmotic (also called hypertonic) relative to the solution inside the cell. The fluid moves from the hypertonic solution to the hypotonic solution, from high osmotic pressure, to lower osmotic pressure.
Hyphae
When fungal spores germinate and grow, they absorb food through long, thread-like hyphae.
Hypoosmotic
When a cell is in a medium with a lower concentration of osmotically active dissolved or colloidal particles, and the solution thus has a lower osmotic pressure than that inside of the cell, the cell is said to be in a medium that is hypoosmotic (or hypotonic) relative to it, so water moves into the cell.
Hypophysis
The pituitary (also called hypophysis) is a small endocrine gland (pea-sized) that is centrally located in the brain that produces hormones.
Hypothalamus
In the forebrain, located directly above the pituitary. The hypothalamus gland produces hormones.
Hypothyroidism
This is another word for an underactive thyroid gland.
Hypotonic
When a cell is in a medium with a lower concentration of osmotically active dissolved or colloidal particles, and the solution thus has a lower osmotic pressure than that inside of the cell, the cell is said to be in a medium that is hypotonic (or hypoosmotic) relative to it, so water moves into the cell.
Immigration
Movement to a new area is often called immigration.
Immune system
An organism's defense system against disease. It builds resistance to fight off infections and diseases.
Impermeable
An impermeable membrane does not allow fluids (or gases) to pass through.
In heat
When an ovarian follicle matures (in some animals several follicles mature), estrogen is released, and at this time females are said to be in heat during the estrous phase of their cycle. This coincides with the time that they have a bloody vaginal discharge.
Inch (in.) 1 inch
= 1/12th of a foot. 12 inches to 1 foot. 1 inch = 2.54 centimeters.
Inferior vena cava
Sometimes called posterior vena cava (located posterior to abdominal cavity). This large blood vessel (vein) carries deoxygenated blood from smaller veins in the lower half of the body, back to the heart (to the heart's right atrium).
Influenza
Influenza, which is also called the flu, is caused by different strains of the influenza virus. The influenza virus is the fastest-mutating virus known, capable of rapidly changing the outer protein coat through succeeding generations of the flu. People can therefore catch the flu more than once a year, since they have no antibodies to the new strain. In 1918 a strain of this virus, called Spanish flu, spread around the world and killed 500 million people. It is hoped that a similar version of this deadly and highly contagious virus never starts to make people sick again, although there's no way of knowing.
Influenza virus
An infectious virus that causes a disease most people contract many times during their life (each season the strain is slightly different).
Infrared light
A type of electromagnetic radiation (radiant energy) that travels in waves, that we feel as heat.
Infundibulum
The anterior and posterior lobes of the pituitary are attached to the hypothalamus via a stalk called the infundibulum.
Inguinal canal
A short passage through each side of the abdominal wall in male humans through which the spermatic cords (tubes) pass sperm from the seminal vesicles, through the vas deferens, to the urethra.
Inhibitors
Compounds that are important in inhibiting or blocking cell division activity, thereby maintaining the dormancy of buds, seeds, and shoots.
Inner labia
Inner lips of the vulva.
Inorganic ions
Also known as electrolytes. They are ions that are necessary for cellular activities, and they are essential for electrical activities that make muscles contract and nerves work.
Inorganic molecules
All molecules without any carbon.
Insecticides
Pesticides and insecticides are chemicals that hurt or kill insects and other pests, and can affect other life systems.
Insertion of muscle
The muscle end attached to the farther bone (distal), such as in the hand or foot, is known as the insertion.
Insulin
The islets of Langerhans (islet cells) in the pancreas make the hormone insulin, which reduces the concentration of glucose in the blood.
Insulin shock
It is rare, but sometimes a pancreas becomes overactive and it produces too much insulin. Too much insulin in one's bloodstream can produce insulin shock, when blood sugar levels fall too low.
Intercourse
The same as sexual intercourse, which is the same as coitus, and copulation.
Interkinesis
During the brief period after meiosis I ends, before meiosis II begins, the chromosomes are still double-stranded. This period is called interkinesis.
Internal digestive cavity
Another name for the internal digestive cavity is gastrovascular cavity.
Internal fertilization
Occurs in a range of different animal groups. This usually involves a male inseminating a female with sperm.
Internally fertilized
Reptiles are internally fertilized. The male inseminates the female with sperm.
Interneurons
Also called association neurons, these relay messages from one neuron to another.
Interphase
The longest and most physiologically dynamic part of the cell's life history. It is not considered part of cell division. During this stage of the cell's life, the cell is growing, metabolizing, and maintaining itself.
Interspecific
Ecologists speak of interspecific dynamics. This means they are considering interactions among individuals that are members of the different species.
Interspecific competition
Competition between different species.
Interstitial fluid
Arteries are the blood vessels that transmit oxygenated blood away from the heart toward the cells and tissues throughout the body.
Intraspecific
Ecologists speak of interspecific dynamics and intraspecific dynamics. Intraspecific dynamics are the interactions among individuals of the same species.
Intraspecific competition
Competition that may occur between individuals within a single species is intraspecific competition.
Iodine
The chemical element with the symbol I, and its atomic number (the number of protons in the nucleus of each iodine atom) is 53.
Ion
An atom or molecule with a nonzero charge. This means it has a net electric charge that can be either positive or negative, due to the addition or the loss of one or more electrons. The reason the ion has either a net positive or net negative charge is because its number of protons and electrons is not even. By convention, one electron has one negative charge, and one proton has one positive charge. Positive ions are called cations, due to the loss of one or more electrons. Negative ions are called anions due to the gain of one or more electrons.
Islet cells
The pancreas contains islet cells, or islets of Langerhans, which produce the hormone insulin, which reduces the concentration of glucose in the bloodstream.
Islets of Langerhans
The pancreas contains islet cells, or islets of Langerhans, which produce the hormone insulin, which reduces the concentration of glucose in the bloodstream.
Isomer
Compounds that are composed of the same kinds and numbers of atoms but whose atoms are arranged differently, and therefore have different properties. Some of the simple sugars have the same number of carbon, oxygen, and hydrogen atoms, but their atoms have different spatial arrangements, resulting in properties specific to each compound. These simple sugars are isomers.
Isosmotic
When the osmotic pressure is the same on both sides of a semipermeable membrane, each solution is said to be isosmotic or isotonic. This means that each medium has the same concentration of osmotically active particles, and each has the same osmotic pressure, so neither tends to lose or gain appreciable quantities of water via osmosis.
Isotonic
When the osmotic pressure is the same on both sides of a semipermeable membrane, each solution is said to be isosmotic or isotonic.
K
Symbolizes carrying capacity, which is the number of individuals of a particular species that a particular environment can support indefinitely.
K+

A potassium ion. Potassium ions are among the most abundant positively charged inorganic ions (cations) found in human blood plasma.

Kcal
A kilocalorie (kilogram-calorie). It equals 1,000 calories. A calorie is a unit of energy required to raise the temperature of 1 gram or cubic centimeter (cc) of water by 1°C.
Kidney

Vertebrates have one pair of kidneys, which are excretory organs. Kidneys filter the blood.

Kilocalorie

A kilogram-calorie (Kcal) equals 1,000 calories. When calories are discussed in Biology: A Self-Teaching Guide, they are kilogram-calories; that is, the amount of heat required to raise the temperature of 1,000 grams (1 kilogram) of water by 1°C.

Kilogram
(kg) Unit of mass equivalent to 2.2 pounds (lbs).
Kilometer
(km) Unit of length equal to 1,000 meters = 0.62 miles.
Kingdom
The biological definition of kingdom is the taxonomic term (used for classification of organisms) that contains phyla. Depending on the classification system being used, there are either five or six recognized kingdoms.
Krebs cycle
Also known as the citric acid cycle, and tricarboxylic acid (TCA) cycle. It is a series of chemical reactions used in the cells of aerobic organisms to generate energy from carbohydrates, fats, and proteins.
K-selected species
These have very few young. Their young tend to need considerable parental care, and they breed later in life than many r-selected species. K-selected species have long gestation periods and have long lifespans.
Labia
External folds of skin outside the vagina.
Labia majora
Also called the outer labia, or outer lips of the vulva.
Labia minora
Also called the inner labia, or inner lips of the vulva.
Lachrimal glands
They produce tears.
Lactic acid
Contributes to muscle fatigue.
Lactose
Lactose (C12H22O12) is a dissacharide sugar found in milk.
Lamella
A thin layer or a membrane.
Large intestine
The part of the gastrointestinal tract between the small intestine and the anus is called the large intestine.
Larva
Immature forms are larvae (larvae are plural, larva is singular); especially true of animals whose juveniles must metamorphose into adults.
Larynx
The voice box in humans, located in the throat area. It is an air passage, and is where our vocal cords are located.
Latent period
The latent period of a muscle is the time from when the initial stimulus (action potential) is administered until the muscle begins to contract.
Leaf
The plural of leaf is leaves. Leaves are specialized structures that are attached to the stems (slender stalks that usually contain vascular tissue) of plants.
Leeuwenhoek, Antonie Philips-Van (1632–1723)
A pioneer in microbiology. A Dutch scientist, businessman, lens maker, microscope maker, and microbiologist.
Left atrium
Oxygenated blood flows from the lungs back to the heart, entering the storage chamber on the left side of the top of the heart, called the left atrium. From here the blood flows back to the muscular left ventricle, which then contracts and pumps the oxygenated blood to the rest of the body.
Left ventricle
Oxygenated blood flows from the lungs back to the heart, entering the storage chamber on the left side of the top of the heart, called the left atrium. From here the blood flows back to the muscular left ventricle, which then contracts and pumps the oxygenated blood to the rest of the body.
Leucocyte
White blood cell (WBC, also called leukocyte or leucocycte) is another name for lymphocyte.
Leucoplast
Type of plastid. Leucoplasts are colorless (nonpigmented) starch-storing organelles in plant cells.
Life
Entities that can grow and maintain themselves, and can construct and organize themselves and can reproduce are called living. They possess life.
Life expectancy
A synonym for longevity. Longevities are usually the average number of years that an organism or species is expected to live.
Ligament
Flexible, fibrous connective tissues that attaches bones to one another, and allow some joints to be bendable.
Light reactions of photosynthesis
Convert light energy into chemical energy.
Lipids
Composed of a fatty acid that is connected to a glycerol molecule. Lipids are nonpolar solvents.
Lipid soluble
If a compound or substance dissolves in lipids, fats, oils, or grease, it is lipid soluble.
Lithosphere
The lithosphere refers to the rocks, minerals, and soils surrounding the earth.
Littoral zone
The intervening area between where the water reaches during high and low tide.
Liver
Large organ in the upper right abdomen of vertebrates that manufactures bile, blood-clotting factors, some proteins, cholesterol, and triglycerides.
Logistic curve

Curve that illustrates the exponential increase in number of a species until it reaches its carrying capacity in that environment.

Longevity
The average number of years that you are expected to live depends on how old you already are, and what sex you are, and what ethnic group you may be part of.
Long-day plants

Bloom when the day length exceeds a specific critical value, which is usually during the summer.

Loop of Henle
Each nephron consists of several structures, including a loop of Henle.
Lumbar vertebrae
The vertebrate below the ribcage and above the pelvis.
Lungs
Two elastic sacs in our chest, protected by the ribcage. Air travels from the throat to the lungs, and deoxygenated air from the lungs back to the throat.
Luteinizing hormone (LH)
LH is produced in the anterior pituitary gland in the brain. In females, LH initiates ovulation and corpus luteum development. In males, LH initiates testosterone production.
Lymph
Similar to blood plasma. A clear to yellowish liquid that contains lymphocytes.
Lymph nodes
Also called lymph glands. They are composed of lymphatic tissue (lymphoid tissue).
Lymphatic system
Lymph passes through the lymph nodes and lymph vessels. Together these comprise the lymphatic system.
Lymphocyte
A type of white blood cell.
Lymphoid tissue
Lymphoid or lymphatic tissue is composed of lymphocytes and accessory cells (macrophages and reticular cells).
Lysis
Means splitting. Glycolysis means the splitting of glucose.
Lysosomes
These spherical membrane-enclosed storage vesicles are organelles in animal and most plant cells that contain hydrolytic enzymes that break down old parts of the cell.
Macromolecule
Large organic molecules that are made of many smaller organic molecules.
Magnesium ion
Mg2+ is a magnesium ion. It is one of the most abundant positively charged inorganic ions (cations) found in human blood plasma.
Male
In animals, this is the sex that produces mobile (motile) haploid (1N) gametes, often called sperm or spermatozoa. In plants, the males have haploid cells that are produced by stamens.
Malpighian tubules
Insect excretory organs.
Mammal
After birth, mammals nourish their young with the milk that is secreted from the mother's mammary glands.
Mammary glands
Due to the correct hormone secretions at the right time, mammary glands will produce milk, which is then conveyed through ducts to a nipple (teat).
Mandible
The lower jawbone, Lower teeth are located on this.
Marine environment
Aquatic environment that is either saltwater or brackish. Examples include oceans, seas, estuaries, and bays.
Marrow
The spongy part of the bone contains many spaces filled with soft, blood-forming tissue called marrow (composed of fat).
Maximal stimulus
When speaking about muscle contraction, by increasing the nervous stimulus, more muscle fibers contract until all contract, and that is the maximal stimulus.
Medulla oblongata
Connects the brain to the spinal cord. It deals with blood pressure, heart rate, breathing, sneezing, and even vomiting.
Medusae
Jellyfish are free-swimming adult forms known as medusa, which usually look like rounded domes with tentacles hanging below.
Meiosis
The type of cell division that reduces the number of chromosomes in a cell by one-half is called meiosis. Meiosis is the cell division process that creates haploid cells (the gametes: eggs and sperm). Meiosis involves one round of DNA replication (duplication), followed by two rounds of cell division.
Meiosis I
The development of gametes, or gametogenesis, occurs as a series of cell divisions known as meiosis. The first main part of meiosis is called meiosis I, when the chromosomal DNA is replicated (duplicated), producing two diploid daughter cells.
Meiosis II
The development of gametes, or gametogenesis, occurs as a series of cell divisions known as meiosis. The first main part of meiosis is called meiosis I, which produces two diploid daughter cells. Meiosis II produces four haploid daughter cells.
Melanophores
Pigment-containing cells in some lower vertebrates such as fish, frogs, and lizards. MSH increases skin pigmentation by stimulating the dispersion of melanin granules in mammal melanocytes; mammals, however, lack melanophores.
Membrane potential
Like most other living cells, neurons have an asymmetric distribution of ions across their plasma membranes. The interior of a resting neuron (one that is not transmitting an impulse) contains more negatively charged ions than the outside of the cell, where there are more positively charged ions. This uneven distribution of electrical energy, which might be described as an electrical potential difference across the membrane, is usually referred to as membrane potential and is critical to the neuron's ability to transmit an impulse along its entire length.
Menarche
The occurrence of the first menstrual cycle.
Meninges
Inside the skull, the brain is protected by a system of membranes, the meninges.
Menopause
This is the end of a woman's natural childbearing years. It usually begins at about 45 years of age.
Menses
Also known as menstruation, or the menstrual period or menstrual cycle. This period involves a vaginal discharge of blood and cells that result from the breakdown of the uterine lining.
Menstrual cycle
The female monthly cycle is known as the menstrual cycle. Blood and cells that had lined the uterus are expelled in the menstrual flow, which is a result of the breakdown of the uterine lining.
Menstrual period
When a girl reaches puberty, she has her first menstrual period, and her periods recur every 28 days or so during the rest of her childbearing years.
Menstruation
Also known as a woman's period. After puberty and before menopause, a woman menstruates approximately once every 28 days. The female monthly cycle is known as the menstrual cycle. As a result of the breakdown of the uterine lining, together the blood and cells that had lined the uterus, as well as cervical mucus and vaginal secretions, are discharged in the menstrual flow.
Meristematic cells
Higher plants have meristematic cells that are capable of active cell division and differentiation into specialized tissues that develop into phloem and xylem cells.
Mesoderm
Early on in an embryo, the mesoderm refers to a middle layer of cells.
Mesodermal muscles
Muscles form from mesodermal tissue.
Mesosome
Inside the cell wall of some bacteria is a plasma membrane that coils and loops, creating a unique structure known as a mesosome, which may be important in cell division.
Messenger RNA
The molecules of messenger RNA (= mRNA) transport genetic information from DNA (the genes) to the ribosomes, where protein synthesis occurs.
Metabolism
Involves the chemical processes inside living organisms that sustain life.
Metabolite
Metabolites are produced, often as end products, due to metabolic processes.
Metamorphosis
A succession of genetically controlled changes, thereby incrementally becoming more adult-like.
Metaphase
Lasts only as long as all the chromosomes remain lined up along the equator. The centromeres have divided in two. Each is attached to one of the two corresponding chromosomes from the pair. The chromosomes align along these microtubules at metaphase, and then the chromosomes move apart during the remaining stages of cell division. The individual chromosomes are called homologs, and together, both chromosomes in each pair are called homologous chromosomes.
Metaphase I
During meiosis I, after prophase I, when the homologous chromosomes have paired up and moved toward the equatorial plane of the spindle, the centromeres line up along the middle, and the centromeres attach to the spindle fibers, each connected to a synaptic pair of chromosomes.
Metaphase II
During meiosis II, in metaphase II, the chromosomes line up along the equatorial plane.
Meter
(m) One meter is equal to 100 centimeters. One meter = 1.09 yards = 3.28 feet.
Methanogen
Bacteria that produce methane. Archaebacteria that reduce carbon dioxide into methane in the presence of hydrogen are methanogens.
Methyl
This is a CH3 group that is usually part of a larger molecule. Methyl groups can be found on their own.
Methyl group
The methyl group is a -CH3 group that is usually part of a larger molecule.
Mg2+
A magnesium ion. One of the most abundant positively charged inorganic ions (cations) in human blood plasma.
Microclimate
A climate in a relatively small area that is different than the climate in the general area at large.
Microfibril
Term often used when referring to a structure of a protein fiber, such as hair or sperm tail. It is a very fine fiber-like strand or fibril that usually consists of cellulose and glycoproteins.
Microfilament

Rod-like structure present in the cytoplasm of many eukaryotic cells that is only 4–7 nanometers in diameter.

Micrometer

micron (μm) Unit of measurement equal to one-millionth of a meter. One millimeter = 1,000 microns (micrometers). One micron = 0.001 millimeter = 1 × 10−6 meter. There are about 25,000 microns in an inch. An angstrom is a unit of length equal to 10,000 microns.

Micron
micrometer (μm). Unit of length equal to one-millionth of a meter (see micrometer).
Micropyle
On the surface of a plant's ovule is a small opening called the micropyle. The pollen tube penetrates through the mycropyle.
Microscope
Instrument that allows us to see objects that are so small we might not be able to see them with the naked eye.
Midbrain
The brain has three principal divisions, the forebrain, midbrain, and hindbrain. The most important parts of the midbrain are the specialized areas known as the optic lobes.
Mile
Unit of linear measurement = 5,280 feet or 1,760 yards, or 1,609 meters.
Milk
The whitish liquid produced by mature female mammals after giving birth. It contains fats, protein, lactose, vitamins, and antibodies.
Milliliter
1 milliliter of water weighs a gram.
Millimeter
(mm) 10 millimeters = 1 centimeter. 1 millimeter = 1 tenth of a centimeter. 1 millimeter = 1 thousandth of a meter. 1 inch = 25.4 millimeters. 1 millimeter = 0.039 inch.
Millisecond
1/1000th of a second.
Mimicry
Different species sometimes resemble one another for reasons of defense. One is said to mimic the other. Sometimes one is called the model and the other, which may have evolved to resemble the model, is the mimic.
Mineral
Solid mass of a naturally occurring chemical compound found in a pure form.
Minute
1/60th of an hour.
Mitochondria
Large organelles in the cytoplasm of cells that supply the cell with chemical energy in the form of the high-energy molecule adenosine triphosphate (ATP).
Mitosis
The type of cell division that results in two daughter cells with copies of the same chromosomes that were in the nucleus of the parent cell.
Model
When speaking of biological mimicry, where one species (the mimic tastes fine) has evolved to look like the model (the poisonous one).
Mole
Unit of measurement that always contains the same number of elementary units. That number, by definition, is 6.02 × 1,023, which is called Avogadro's Number.
Mollusca
The phylum Mollusca includes snails, clams, octopuses, and chitons.
Monera
This kingdom includes bacteria and blue-green bacteria (sometimes called cyanobacteria or blue-green algae) that are composed of true cells. Monerans are all prokaryotic; that is, their cells lack most organelles, they do not have a membrane-bound nucleus, and most occur as single-celled organisms.
Monocotyledonae

A major lineage of angiosperms. Monocots (Monocotyledonae) have embryos with one cotyledon.

Monocytes
A type of white blood cell. They protect the body from certain bacteria and viruses.
Monoecious
Some plants and animals have both male and female structures and don't have separate sexes. Hermaphroditic plants (with both male and female parts) are called monoecious.
Monomer
A single molecule that can bond to many more of the same (identical) kind of molecule to form a polymer.
Monosaccharide
A simple sugar. Monosaccharides are the simplest forms of carbohydrates.
Morphogenesis
When cells undergo a series of complex movements establishing the shape and pattern of the early embryo, this transformation process is known as morphogenesis.
Motor neuron
The three main types of neurons are sensory neurons, motor neurons, and interneurons. Motor neurons transmit messages from the brain or spinal cord to an effector organ, such as a muscle or gland.
mRNA
Messenger RNA molecules transport genetic information from DNA (the genes) to the ribosomes, where protein synthesis occurs.
Mucocomplex
Prokaryotic cell walls are composed of a polymer of glucose derivatives attached to amino acids. This substance is termed a mucocomplex.
Mucopolysaccharides
Commonly called glycosaminoglycans. In living organisms, these are long chains of sugar molecules that usually occur in mucus, often around joints.
Mucosal lining
The mucous (mucus) membrane lining the inside of the mouth, nose, palate, colon (large intestine), rectum, and vagina.
Mucus membrane
The mucous (mucus) membrane lining the inside of the mouth, nose, palate, colon (large intestine), rectum, and vagina.
Mullerian mimicry
When two or more species of unpalatable animals look or act similarly, which serves them well because predators easily learn to avoid all of them because they taste bad and look similar.
Multicellular organisms
Organisms that are comprised of more than one cell.
Multinuclear
Some cells have many nuclei. These multinuclear cells are sometimes called coenocytic.
Muscle
In unison, the protein filaments in the bundles of muscle cells contract and relax. Together, these bundles comprise muscles.
Muscle cell
Muscle cells contain chains of muscle fibrils (myofibrils), which are composed of proteins that hold the fibrils together.
Muscle contraction
This is the case when muscle cells contract (get shorter).
Muscle fiber
Also called muscle fibers, myocytes, and myofibers.
Mutation
Inheritable changes in an organism's genetic material. Under normal circumstances, these occur infrequently.
Mutualism
An arrangement between species in which each benefits from the other and neither is harmed by the relationship.
Mycelium
Network of fungal thread-like filaments (hyphae) that creates the body of a fungus, called a mycelium. Mycelia grow, spreading throughout their food source.
Mushroom
Many fungi produce fleshy, spore-bearing, fruiting bodies. These are typically above ground, though some fungi produce them underground.
Mycoplasma
A genus of bacteria whose members are the smallest bacterial species known.
Mycorrhizae
In many plant species, a symbiotic relationship exists between the plant's root hairs and the filamentous elements of specific fungi, which form the mycelium. The product of such a close association is known as mycorrhizae.
Myelin sheathing
Some Schwann cells' plasma membranes envelop certain axons and nerves. Such membranes are called myelin sheathing.
Myofiber
Muscles are composed of muscle cells, which look like long, thin fibers (often 3 cm long). They are also called muscle fibers, myocytes, and myofibers.
Myofibrils
Muscle cells contain chains of muscle fibrils (myofibrils), which are composed of proteins that hold the fibrils together.
Myoglobin
A related oxygen-binding protein called myoglobin is found in the muscle tissue of vertebrates.
Myosin
A protein that, with actin, forms the contractile filaments of muscle cells.
N2
Gaseous nitrogen (N2) is an important element in living organisms. 78% of the atmosphere is nitrogen; atmospheric nitrogen is in the form of N2.
Na+
Na+ is a sodium ion. The most abundant positively charged inorganic ions (cations) found in human blood plasma are sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), and magnesium ions (Mg2+).
NaCl
Ordinary table salt, sodium chloride.
NAD
A nicotinamide adenine dinucleotide. It is also seen as NAD+. It is a cofactor in all living cells. That means it is important in many cellular metabolic pathways. It exists in two forms, as NAD and as NADH. NAD is the oxidized form, NADH is the reduced form.
NADPH2
Nicotinamide adenine dinucleotide hydrogen phosphate (NADPH2) serves as an electron carrier.
Nanometer (nm)
A unit of length in the metric system equal to one-billionth of a meter = 0.000 000 001 m. One nanometer also equals one-millionth of a millimeter.
Natural Selection
Natural selection favors organisms that tend to pass on more offspring, which in turn pass on more offspring, and so contribute to succeeding generations. This is the main mechanism whereby heritable traits are passed down through generations without human involvement.
Nectar
Many flowers secrete a sugary fluid called nectar.
Negative geotropic response
A shoot growing away from the direction in which gravity pulls.
Nekton
A diverse group of tiny aquatic organisms that are larger than plankton and can swim against the current.
Nematodes
Elongate roundworms that taper to a point at both ends.
Nephron
Each human kidney has about a million nephrons. The nephrons consist of capillaries (the glomerulus) and a Bowman's capsule. As blood flows through the capillaries, it is filtered.
Nerve
The thin fibers in living organisms that conduct (send and receive) messages that provide information and help an animal control its reactions.
Nerve cells
Nerve cells are also called neurons. There are two types of nerve cells: receptor-conductor cells and effector cells (effector neurons).
Nerve cords
The major lines of nerve fibers in an animal's body are called nerve cords.
Nervous impulse
A nervous impulse is rapidly transmitted by a nerve cell, which is used to communicate with another cell.
Nervous system
The part of all animals that transmits signals throughout the body and is integral to detecting internal and external information and coordinating responses.
Net productivity
The gross productivity minus the respiration rate (energy used for respiration) leaves the net productivity.
Neural impulse
Rapidly transmitted by a nerve cell, used to communicate with another cell.
Neural stimulus
When speaking of the nervous system, a stimulus that affects a neuron is the neural stimulus.
Neuroglia
All the glial cells are called the neuroglia. These cells account for at least half of the nervous system's volume.
Neurohypophysis
The posterior lobe of the pituitary. It produces oxytocin and vasopressin hormones.
Neuron
Another term for neuron is nerve cell. Neurons are among the most excitable cells in the body. Neurons have specialized parts, including the soma, dendrites, and axons.
Neuropeptide
Usually considered a subcategory of neurotransmitters. These protein-like molecules are comprised of amino acids.
Neurotransmitter
A chemical that travels across the short gap (synapse) from one neuron to the next neuron.
Neutrophils
A type of white blood cell. Neutrophils are cells in the immune system of vertebrates.
Niacin
A water-soluble member of the vitamin B complex. Important for digestion, the skin, the nervous system, and it helps maintain blood cholesterol levels within the recommended range.
Niche
Defining an organism in terms of its role in the ecosystem describes its niche. An organism's niche is what it does.
Nitrate
Nitrifying soil bacteria oxidize ammonia, ammonium, or other nitrogen compounds in the soil, converting them into nitrates (NO3−) or nitrites (NO2−). This process releases energy by adding oxygen.
Nitric oxide
This is one of over 10 neurotransmitters.
Nitrification
The nitrogen cycle is a series of different biological processes that take nitrogen and move it from one molecule to another. Nitrification (also called nitrogen fixing) is the biological oxidation of nitrogen compounds such as free ammonia (NH3) and the ammonium cation (NH4+).
Nitrifying bacteria
Some bacteria in the soil are known as nitrifying bacteria. They oxidize (remove electrons, usually by removing hydrogen, which carries the electrons with it) ammonia to nitrate ions (NO2−).
Nitrite
Nitrifying soil bacteria oxidize ammonia, ammonium, or other nitrogen compounds in the soil, converting them into nitrates (NO3−) or nitrites (NO2−).
Nitrogen
Nitrogen's elemental symbol is N. This element is important for life to exist. Nitrogen is an important part of proteins, DNA, and RNA.
Nitrogen cycle
The nitrogen cycle involves nitrogen going from the gaseous form in the air, into being part of other molecules that play an important role in living organisms, and eventually the nitrogen is returned to the air.
Nitrogen fixing
Some bacteria and blue-green bacteria (cyanobacteria) can convert, or fix, gaseous (molecular) nitrogen (nitrogen fixing) into an aqueous form, ammonia (NH3) by reducing it.
Nitrogenous waste
Also called nitrogen waste. These are metabolic wastes such as ammonia, urea, and uric acid.
Noncyclic photophosphorylation
The process in which the high-energy chlorophyll molecules initially donate electrons and then accept them when in a low-energy state. It is called noncyclic because the same electrons are not continually passed around the system; rather an outside source is required.
Noradrenalin
A hormone and a neurotransmitter. Also called norepinephrine. Its effects are similar to those of adrenalin (epinephrine), and it helps to mobilize the body during times of stress.
Norepinephrine
Noradrenalin is a hormone and a neurotransmitter. Also called norepinephrine, produced in the adrenal medulla, it affects the brain and the body.
Notochord
The rod-like notochord is a cartilaginous skeletal support in the embryo of all chordates in the adults of some chordates.
Nuclear membrane
The nucleus is bounded by a membrane that is called the nuclear membrane.
Nucleic acids
One for the four basic macromolecules found in living things. The other three are carbohydrates, fats, and proteins. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids. They are comprised of many nucleotides linked together in a long chain.
Nucleolar organizer
Sometimes a nucleus contains several nucleoli, which are formed by a specific region on one chromosome called the nucleolar organizer.
Nucleolus
A dark area within the nucleus of most cells that contains a high concentration of ribonucleic acid and protein.
Nucleoplasm
The nucleoplasm consists of the contents within the nuclear membrane, surrounding the nucleus.
Nucleotide
An organic molecule that serves as a monomer unit when forming a nucleic acid polymer such as DNA or RNA. Nucleotides are composed of three molecules: a nitrogenous base (nucleobase), a five-carbon sugar (ribose or deoxyribose), and one or more phosphate groups.
Nucleus
An organelle, enclosed by the nuclear membrane, contained in eukaryotic cells. Inside is the majority of the cell's genetic material (DNA, chromosomes).
Ocelli
Simple eyes.
Olfactory bulbs
Projections connected by the olfactory nerves, from each cerebral hemisphere.
Olfactory lobes
Projections connected by the olfactory nerves, from each cerebral hemisphere.
Omnivore
Omnivores eat plants and animals, and are sometimes considered to be generalists as opposed to specialists.
Oogamete
Organisms that are oogamous have two types of gametes, one of which, the female, is typically the large, nonmotile egg cell, the oogamete.
Oogenesis
Oogenesis comprises the series of steps that produce an egg from a primary oocyte, which is also called an ovum.
Open circulatory system
An open circulatory system is characterized by large open sinuses through which the blood flows.
Optical isomer
Molecules that are functionally equivalent, mirror images of each other.
Optic lobes
The most important parts of the midbrain are the specialized areas known as the optic lobes. These are the visual centers connected to the eyes by the optic nerves.
Optic nerves
Optic nerves carry nervous impulses from behind the eye to the optic lobes in the brain, where this information is converted into what we see.
Order
The taxonomic category that is one step higher than family and one step lower than class.
Organelle
Subcellular structure in a cell that performs functions for the cell.
Organic compounds
A compound containing carbon is an organic compound. The most common organic compounds used in living organisms are carbohydrates, lipids, proteins, and nucleic acids.
Origin of muscle
The end of a muscle attached to the bone nearest to the axis of the body (proximal) is known as the origin.
Osmoregulation
The method organisms use to regulate their internal osmotic concentrations. These are the methods organisms use to maintain the homeostasis of their water content. This is how they maintain fluid balance and the proper concentrations of electrolytes (salts in solution). This keeps their bodily fluids from becoming too concentrated with these electrolytes, and from becoming too dilute (not having enough electrolytes).
Osmosis
An example of passive transport, which involves movement of water through a selectively permeable, also called a semipermeable membrane. The water usually moves from a solution of less concentration of a particle, ion, or molecule in water, to a solution of higher concentration of that particle, ion, or molecule.
Osmotic pressure
The pressure exerted by dissolved particles in a solution or a colloid that moves water across a semipermeable membrane (a biological or synthetic membrane that allows certain ions or molecules to pass through it by diffusion, facilitated diffusion, passive transport, or active transport).
Osseous tissue
Animal bone, or osseous tissue, is a hard, white or yellowish tissue that provides structural support and makes up the skeletal system.
Ounce
(oz) Unit of weight equal to 1/16th of a pound. 1 ounce = 29.6 grams per ounce of water. 1 ounce = 29.6 milliliters.
Outer labia
Outer lips of the vulva.
Ovarian cycle
This cycle occurs in ovaries. It is characterized by hormone release, follicle development, and the maturing and release of the egg (or eggs).
Ovary
Female reproductive cells (female gametes are called ova) are produced in the ovaries.
Oviduct
The oviduct (fallopian tube) passes from the ovary to the uterus. It helps move egg cells from the ovary to the uterus.
Ovulate
Ovulation occurs when an ovary discharges an egg.
Ovule
The ovule is inside the female reproductive structures of a seed plant. Inside the ovary, the ovule contains the female germ cells.
Ovum
The sex cell or gamete produced by a sexually reproducing female is called an egg or ovum (plural = ova).
Oxidation
The removal of an electron (releasing energy).
Oxidative phosphorylation
A metabolic pathway that takes place in cellular organelles called mitochondria. Uses enzymes to oxidize nutrients and release energy.
Oxidizing agent
Substance (element or compound) that causes the oxidation of another substance by removing an electron from something else. It is one part of an oxidation-reduction (redox) reaction.
Oxygen
A colorless, odorless gas that is important in respiration (except in the case of anaerobic respiration). By volume, at sea level, approximately 20% of the atmosphere is composed of oxygen.
Oxygen debt
Anaerobic respiration causes oxygen debt, which means that after strenuous muscular activity, we breathe deeply to acquire the needed oxygen to convert the potentially dangerous lactic acid to glycogen
Oxygenated blood
Oxygen-rich blood.
Oxytocin
The posterior lobe of the pituitary produces oxytocin, which stimulates uterine contraction and constriction of the ducts in the mammary glands.
Pacemaker
The heart has a natural pacemaker, which spontaneously begins each heartbeat.
Pancreas
The pancreas aids digestion by producing pancreatic digestive enzymes, and the peptide hormones glucagon and insulin that regulate sugar and fatty acid levels in the bloodstream.
Pancreatic juice
Comprised of several enzymes, proenzymes, and electrolytes that help digest proteins, fats, and carbohydrates.
Panspermia
Theory that life exists around the universe and is carried through space on space dust.
Papillomavirus
There are about 50 different known types of papillomaviruses. They cause plantar warts, genital wards, and certain wart-like rashes.
Parasites
Take resources from other living organisms, and in the process, they cause the other organisms harm.
Parasitism
A relationship between two species where one species benefits and the other is harmed.
Parasympathetic nervous system
The autonomic nervous system is divided into two parts, the sympathetic and parasympathetic nervous systems. These function in opposition to one another; the first inhibits organs, while the latter usually excites organs.
Parthenogenesis
In parthenogenesis, an egg can develop into an adult without being fertilized by a sperm cell.
Passenger molecule
A passenger molecule can't fit through a biological membrane on its own, so it needs help from a carrier molecule.
Passive transport
When the movement of molecules occurs on its own, without any organized energy input from the cell, the movement is termed passive transport. Diffusion and osmosis are examples of passive transport.
Pathogens
Agents that can cause disease.
Pelvis
The pelvis is formed by the fusion of the right and left ilia (two ilium bones), ischia (two ischium bones), and pubis (two pubic bones).
Penis
This male sexual organ transfers sperm to the female during copulation.
Pentose
A pentose is a 5-carbon monosaccharide.
Peptide bond
Two amino acids are held together by a specific type of covalent bond (resulting from a shared pair of electrons) called a peptide bond.
Period
After puberty, approximately every 28 days, until a woman reaches menopause, she has her period. This means she menstruates. Her period may involve days of a vaginal discharge of blood and cells that result from the breakdown of the uterine lining.
Peripheral nervous system
All the parts of the nervous system, excluding the brain and spinal cord.
Peristalsis
In the throat when swallowing, or in the intestine, when a wave of smooth muscle contractions.
Permafrost
Soils that remain frozen all year round.
Permeable
A permeable membrane that allows liquids (or gases) to pass through.
Peroxisome
Membrane-enclosed organelle that contains enzymes that oxidize fatty acids and amino acids.
Petals
The typical flower consists of four whorls of modified leaves called sepals, petals, stamens, and carpels. The petals are usually colorful and showy.
pH
The abbreviation for potential hydrogen. It is a measure of hydrogen ion (H+) concentration. Acidity and alkalinity of a solution are measured on a pH scale of 0 to 14. A solution that is neither acidic or alkaline is neutral, and has a pH of 7. Acidity is measured by numbers below 7. Alkalinity is measured by numbers above 7.
Phagocytosis
On a larger scale than pinocytosis, phagocytosis is when liquids or macromolecules, or larger particles are engulfed by a cell membrane and moved to the other side of the membrane.
Phenotype
Observable characteristics caused by the interaction of genes and the environment.
Phloem

A complex tissue in higher plants that conducts water and nutrients (including sugars).

Phospholipid
An important component of cell membranes. Phospholipids are comprised of lipids that contain phosphorus.
Phosphorus cycle
A sedimentary cycle. Phosphorus erodes from rocks in the form of phosphate (PO43−). It is an important inorganic chemical in ecology that is taken up by plants, excreted by herbivores, and absorbed again by plants.
Phosphorylation
During photosynthesis, the process that uses light energy to convert ADP into ATP is called photophosphorylation.
Photoperiodism
The response by an organism to the duration and timing of light and dark is known as photoperiodism.
Photosynthesis
Process that green plants and some other organisms use to harness the energy in sunlight, which converts carbon dioxide and water into carbohydrates (sugar) and oxygen.
Photosynthetic phosphorylation
In the photosynthetic phosphorylation process, the light energy-driven addition of phosphate groups, is known as cyclic photophosphorylation.
Photosynthetic pigments
Light energy can excite photosynthetic pigment molecules. Examples of photosynthetic pigments are chlorophylls, carotenoids, carotenes, xanthophylls, and anthocyanins.
Phototropic response
The growth or movement of a cell or an organism toward light.
Phototropism
This is when plants or other organisms move toward light (positive phototropism) or away from light (negative phototropism).
Phycobilins
Red algae and blue-green bacteria contain red or blue photosynthetic pigments called phycobilins.
Phylum
The taxonomic category that is one step higher than class and one step lower than kingdom.
Phytochrome
A blue-green pigment in plants that helps them responds to the presence or absence of light by measuring the time lapse between the onset of darkness until the next exposure to light.
Pia mater
One of the three layers of meninges. It covers and is tightly molded to the brain.
Pilus
This is the protein bridge that joins two bacterial cells when they come together during conjugation.
Pinocytosis
When liquids or macromolecules are engulfed by the cell membrane and are then moved to the other side of the membrane, the process is called pinocytosis.
Pisces
The Chordate class or superclass Pisces includes all the fish.
Pistil
The female portion of a flower is the pistil. It is a seed-bearing structure of the flower. It consists of the stigma, style, and ovary.
Pith
A soft spongy tissue in the center of some plant stems.
Pituitary gland
Also called hypophysis. A small endocrine gland (pea-sized), centrally located in the brain.
Planaria
Some flatworms are called planaria.
Plankton
Small organisms that live in salt or fresh water, that float or drift in great numbers and can't swim against the current.
Plantae
In most classification systems, the plant kingdom Plantae includes several groups of mostly multicellular, mostly photosynthetic eukaryotes.
Plasma
The fluid portion of both blood and lymph.
Plasma membrane
This is a membrane at a cell's surface that surrounds the cell's cytoplasm. It is also called a cell membrane.
Plasmid
A genetic element that is a segment of DNA that is not attached to the bacterial cell's circular DNA, that cannot become integrated into its host's DNA, is a plasmid.
Plasmodium
Some slime molds exist as a plasmodium, which is a multinuclear (coenocytic) mass of cytoplasm lacking cell walls.
Plasmolysis
When the extracellular fluids surrounding plant cells become hyperosmotic relative to the fluids inside the cells, the cell walls keep their shape while the cells lose water. The cell membrane shrinks, pulling away from the cell wall. Such a cell is plasmolyzed, and this process is called plasmolysis.
Plastids
Rather large cytoplasmic organelles that can be seen with a light microscope. Chloroplasts are plastids containing chlorophyll.
Platelets
Also called thrombocytes. These are small, colorless, plate-shaped (hence the name) cell fragments in blood that are involved in blood clotting (they help stop bleeding); they don't have a nucleus.
Platyhelminthes
The simplest animals to possess a bilaterally symmetrical body plan.
Poikilotherms
Organisms that are not homeothermic are sometimes called cold-blooded, or poikilotherms (poikilothermic).
Polarization
After a neuron goes through a wave of depolarization, the cell repolarizes and returns to its original negative ionic balance (of polarization). This neuron has more potassium ions (K+) outside the membrane than sodium ions (Na+) inside the membrane. Polarization is also a technical term describing the orientation of certain light waves.
Pollen grains
Pollen is produced by male seed plants, or by the male structure on a seed plant. A single pollen grain is a tiny male gametophyte, sometimes called the microgametophyte of a seed plant.
Pollen tube
When a pollen grain lands on a flower's stigma, a hollow pollen tube grows out of the pollen grain and through the style. It brings male gametes to the female gametes (ovules, eggs).
Pollination
When pollen from one plant moves to another, usually from one flower to another, and enables fertilization to occur.
Polymer
A large molecule (macromolecule) composed of many smaller molecules, all of which, or nearly all, are identical.
Polymerization
The bonding together of small molecules to form long chains.
Polyp
Has a body form that is basically an upside-down medusa.
Polypeptide
A molecule consisting of a short sequence of amino acid monomers linked together with peptide bonds.
Polypeptide chain
Amino acids when linked together form polypeptide chains, also called peptide chains. Together, one or more polypeptide chain creates a protein.
Polyploid
When a cell or nucleus contains more than two sets of homologous chromosomes.
Polyribosomes
Also called polysomes. When ribosomes occur in clumps or clusters, they are collectively called polyribosomes or polysomes.
Polysaccharide
Each polysaccharide is composed of many smaller sugar molecules that are chemically bonded together.
Population
All the members of a certain species living in a specific area at a specific time are defined as a population.
Porifera
This phylum includes all the sponges.
Positive geotropic response

When the root grows toward gravity, this is called positive geotropism.

Posterior lobe of pituitary

Also called the neurohypophysis. It produces oxytocin and vasopressin hormones.

Posterior vena cava
Sometimes called inferior vena cava. This large blood vessel (vein) carries deoxygenated blood from the lower and middle body to the heart (to the heart's right atrium).
Potassium ions
K+ are positively charged inorganic ions (cations) found in human blood plasma.
Pound
(lb) Unit of weight equal to 16 ounces = 0.45 kilograms.
Precipitation
The physical moisture (water either in liquid or solid form) that falls from the atmosphere to the earth or onto a body of water.
Predation
Results in the death of the prey.
Predator
An organism that feeds on another living organism (or sometimes on other animal parts).
Pressure
Force that presses, squeezes, crushes, or moves.
Prey
The organisms that predators eat.
Primary consumer
Ecologists define primary consumers as animals that eat plants. Animals that eat plants are called herbivores.
Primary oocyte
During oogenesis in meiosis I, a primary oocyte becomes a secondary oocyte and a polar body.
Primary spermatocyte
During the first stage of spermatogenesis, after the first meiotic division (meiosis I), two primary spermatocytes are formed. Both are diploid cells (2N).
Primordial atmosphere
This is what some call the atmosphere surrounding the earth that existed early in the life of our planet.
Prions
Prions are usually much smaller than viruses, sometimes they are 100 times smaller, though some are as large as bacteria. Prions are known to cause some diseases and are implicated as the cause of others. Included among the diseases that prions are associated with are Mad Cow Disease, Creutzfeld-Jacob Disease, scrapie, and several similar degenerative brain diseases. Prions may be radically different from any other known self-replicating entities. There is no evidence that prions contain any nucleic acids, DNA, and/or RNA. Instead, they appear to be little more than dots of protein. Even if they were found to contain nucleic acids, prions are so small that there is little chance they contain a nucleic acid any longer than 50 nucleotides. This is not large enough to encode a protein containing more than about 12 amino acids. Yet, this is sufficient to cause proteins to fold the wrong way in animals that suffer from such a prion transmission.
Producers
Primary producers are autotrophs that supply organic matter for all the other organisms in the food chain.
Progesterone
One of the sex hormones. This steroidal hormone is produced by the corpus luteum.
Prokaryote
Single-celled organism without a distinct nucleus. Prokaryotic DNA differs from eukaryotic DNA in that each is associated with different proteins, and instead of being paired, it is circular.
Prolactin
Prolactin (luteotropic hormone, also called luteotropin) enables milk production.
Prophase
During the first stage of mitosis (cell division), called prophase, the DNA, which is the primary constituent of the chromosomes, recoils.
Prophase I
The first phase of meiosis I is prophase I, when individual chromosomes coil up and condense and when homologous chromosomes move next to each other.
Prophase II
During meiosis II, in prophase II, each chromosome is double-stranded. The chromosomes condense and move toward the equatorial plane, where their centromeres will attach to the spindle fibers.
Prostate gland
The prostate helps control urination in males, and it makes and contributes secretions that become part of the mixture known as semen.
Protein
Proteins are composed of subunits, amino acids, arranged in a pattern specific to each distinct protein type.
Proteinaceous
Contains protein or consists of protein.
Prothrombin
Thromboplastins are proteins that are released by damaged tissues. Once released, thromboplastins convert the plasma protein prothrombin into thrombin, which helps form a blood clot.
Protista
A diverse kingdom containing thousands of species of single-celled organisms.
Protobiont
This is another word for a protocell.
Protozoa
This kingdom consists of several widely divergent, unicellular phyla.
Proximal convoluted tubule
The convoluted part of a nephron between the Bowman's capsule and loop of Henle.
Puberty
The transitional period when an adolescent's body is maturing into a sexually reproductive adult.
Pubic bone
The bone in the area in adult human males and females that is covered with pubic hair.
Pubis
This is where the pubic bones join at the pubic symphysis, at the front of the pelvis in humans.
Pulmonary artery
When the right ventricle contracts, the blood is forced out through the pulmonary artery to the lungs.
Pulmonary circulatory system
The portion of the circulatory system that travels from the heart to the lungs and back.
Pulmonary vein
Veins that carry blood from the lungs to the heart.
Pulse rate
The number of heart contractions per minute.
Pyruvic acid
The same as pyruvate. Important intermediate molecule in cellular metabolic pathways.
Rabies
The rabies virus is usually transmitted to humans through the bite of an infected mammal. The disease it causes is rabies.
Radial symmetry
Radial symmetry means if you cut vertically through the axis, it is possible to produce two halves that are mirror images of one another.
Radiata
Cnidarians and ctenophores are sometimes called the radiate phyla.
Receptor
Any site on or in a cell that receives and binds specific substances.
Receptor site
Location on a cell surface that can react to certain chemicals.
Recombination
When talking about cells and cell division, if the term recombination is used, it usually means genetic recombination. During meiosis, especially during prophase I, when homologous chromosomes line up in pairs, fragments of DNA may crossover. This is called genetic recombination. It is also called crossing over.
Recruitment
When new species move into an area, ecologically this is called species recruitment.
Rectum
The last 4 to 5 in. (about 12 cm long) of the large intestine is the rectum.
Red blood cells
Red blood cells (erythrocytes) are involved in the transportation of oxygen throughout the body. They are distinct in color and shape.
Redox reaction
A reduction-oxidation (redox) reaction involves the transfer of electrons between elements or compounds.
Reducing agent
Substance (element or compound) that in the chemical reaction becomes oxidized (loses an electron, or donates an electron) to another element or compound in a redox reaction.
Reduction
The addition of an electron (storing energy). A chemical reaction where hydrogen is added or oxygen is removed from a compound.
Reflex arc
A pathway of neurons that controls a reflex, but the pathway of neurons never goes to the brain. It begins with a receptor-conductor cell and ends at the effector cell (effector neuron).
Regeneration
A biological process that involves repair to damage or disturbance.
Relaxation period
The time during which the muscle relaxes, before it can contract again.
Releasing factors
Proteins that control the release of hormones.
Renal artery
Sometimes called kidney arteries, they branch off of the descending aorta. One renal artery goes to the left kidney, the other goes to the right kidney.
Renal vein
There is a left renal vein, and a right renal vein. One drains the left kidney, the other drains the right kidney.
Replication
Also cell duplication. When a cell reproduces, each daughter cell must receive more than just a portion of the vital information; it must receive a complete copy of all the essential genetic material.
Reptile
Modern reptiles include turtles, crocodiles, alligators, lizards, snakes, and the tuatara.
Resolution
Seeing something more clearly, or with enhanced resolution.
Respiration
Usually involves the intake of oxygen and the release of carbon dioxide. The products of cellular respiration are carbon dioxide (CO2) + water (H2O).
Resting potential
A neuron's resting potential is what a resting nerve has when it is not transmitting an impulse; it contains more negatively charged ions than found outside of the cell, where there are more positively charged ions.
Retrovirus
Retroviruses are a group of viruses named for their backward (retro) sequence of genetic replication as compared to other viruses. HIV/AIDS (human immunodeficiency virus, acquired immunodeficiency syndrome) is caused by a virus that has been brought under control with medications. Another well-known disease caused by a viral infection is rabies.
Rh-factor
The Rh-factor is the protein found on the surface of red blood cells that are Rh+ (Rh positive).
Rhinovirus
The majority of viruses that cause common colds are members of the genus Rhinovirus.
Riboflavin
A water-soluble vitamin. Vitamin B2 is also called riboflavin.
Ribonucleic acid
Ribonucleic acid is RNA for short. A long molecule composed of four molecular bases (adenine, cytosine, guanine, and uracil). Ribosomes translate RNA into proteins.
Ribosomal RNA = rRNA
Located in ribosomes, necessary for protein manufacturing in the cell.
Ribosomes
Tiny particles in a cell's cytoplasm that consist of RNA and proteins. Ribosomes are where proteins are manufactured when they bind transfer RNA (tRNA) and messenger RNA (mRNA).
Right atrium
Deoxygenated blood flows from the body to the heart, first entering the right atrium, which acts as a storage chamber.
Right ventricle
The thick-walled chambers in the heart are called ventricles, and the thin-walled chambers in the heart are called atria.
RNA
Ribonucleic acid is abbreviated RNA. It is a long molecule that is composed of four molecular bases (adenine, cytosine, guanine, and uracil). Ribosomes translate RNA into proteins.
RNA polymerase
This is the main enzyme used when DNA is copied, or transcribed, into an RNA molecule.
RNA Virus
A virus contains either RNA or DNA, not both. Those that contain RNA are called RNA viruses. Examples of RNA viruses are those that cause measles, rubella (German measles), and mumps – all childhood diseases.
Root
Underground plant organs involved in holding a plant in place, and involved in water and nutrient absorption, as well as aeration and nutrient storage.
Root hairs
Short, thin structures attached to roots. Much of the absorption of water and minerals in plants takes place through the root hairs.
Root pressure
In higher plants, root pressure is responsible for some of the movement of water across the root tissues.
Rough endoplasmic reticulum
Has many ribosomes attached.
rRNA = Ribosomal RNA = Ribosomal ribonucleic acid
rRNA is required for cells to manufacture proteins. rRNA is located in the ribosomes.
r-selected species
r-selected species produce many small offspring.
S-A node
This specialized area located in the wall of the right atrium is called the sinoatrial node (S-A node or S-A bundle).
Sacral vertebrae
Located between the lumbar and the caudal vertebrae. Usually fused to one another, connecting firmly with the pelvic girdle.
Sacrum
In the lower back, the large triangular bone formed by fused sacral vertebrae (S1–S5). This is part of the pelvis.
Saliva
Food is chewed in the mouth where it is mixed with saliva, which is a watery liquid, produced by glands. Saliva aids in chewing, swallowing, and digesting food.
Salivary glands
These are located in the mouth and throat region, and through narrow tubes they convey saliva to the mouth.
Salt
Salts are formed when an acid and a base combine chemically, creating a neutral ionic compound.
Salt water
Salt water is a solution. In this solution, the solute is salt, and the solvent is water.
Sarcomere
Each unit within a single myofibril, is called a sarcomere.
Saturated fatty acids
Fatty acids without any double bonds between the carbon atoms are called saturated fatty acids.
Schwann cells
Some axons are encircled by one type of glial cell, the Schwann cell, which provides nutrition to the neuron.
Science
The body of knowledge that enables us to understand the physical and natural aspects of the world.
Scientific method
This method allows scientists to test hypotheses and theories and to develop concepts and ideas.
Scientist
Someone who utilizes the scientific method in the process of learning new information while answering questions.
Scrotum
In some animals, the testes are located within the scrotal sac, or scrotum.
Second
A measure of time. It is 1/60th of a minute.
Second polar body
The first polar body either disintegrates or divides in the second part of meiosis (meiosis II), creating two second polar bodies that disintegrate.
Secondary cell wall
Is composed of cellulose microfibrils and lignin. It is located between the primary cell wall and the plasma membrane of many plant cells.
Secondary consumers
Carnivores are also called secondary consumers.
Secondary oocyte
The primary oocyte divides into a smaller first polar body and a larger secondary oocyte, which receives a greater share of the cytoplasm during this meiotic division.
Secondary productivity
The rate of formation of organic matter in the heterotrophs.
Secondary spermatocyte
During the second stage of spermatogenesis, after the second meiotic division (meiosis II), four secondary spermatocytes are formed.
Seed plants
Seed plants (spermatophytes) produce seeds. They do not produce spores.
Semen
The ejaculate fluid that is released from a penis during an ejaculation.
Semipermeable membrane
A biological or synthetic membrane that allows certain ions or molecules to pass through by diffusion, facilitated diffusion, passive transport, or active transport.
Seminal vesicles
Pair of glands that transmit secretions into the duct (the vas deferens) that transmits sperm from a testicle to the urethra.
Seminiferous tubules
Where sperm cells are produced and where they mature.
Sensory nerve ending
Located at the end of a nerve where the nerve cell can detect specific environmental stimuli.
Sensory neuron
Nerve cells that convert stimuli into action potentials, and transmit the sensory information to the brain or spinal cord.
Sepals
The parts of the flower that enclose the petals. Usually not as showy as the petals.
Septum
Something in the middle that divides two sides.
Serotonin
One of over 10 neurotransmitters.
Serum
When the red and white blood cells and proteins involved in clotting are removed from blood plasma, the remaining liquid is called serum.
Sessile
Animals that are attached to the substrate, are called sessile.
Sex cells
Most multicellular organisms have two different types of sex cells known as gametes, such as eggs and sperm.
Sex hormones
Steroid hormones that are produced by the adrenal cortex, as well as in the ovaries and testes.
Sexual intercourse
Is the same as coitus, and same as copulation. This is when the male's penis enters the female's vagina, and sperm cells pass from the male to the female.
Sexual reproduction
Compared with asexual reproduction that requires one organism (no partner is necessary), sexual reproduction requires both a male and a female to produce offspring.
Sieve elements
The conducting cells of the phloem are the sieve elements.
Sieve plates
The conducting cells of the phloem are the sieve elements, which are joined together in vertical columns, creating the sieve tubes, each of which is joined at the end, at the sieve plates.
Sieve tubes
The conducting cells of the phloem are the sieve elements, which are joined together in vertical columns, creating the sieve tubes.
Silica
Silicon dioxide (SiO2) is an oxide of silicon.
Simple sugar
The simplest form of a carbohydrate. Another name is monosaccharide.
Single-celled organism
Same as unicellular organism, consists of only one cell.
Sinoatrial node
The heart has a natural pacemaker, which spontaneously begins each heartbeat, called the sinoatrial node (S-A node or S-A bundle).
Skeletal muscles
They respond to conscious control. Commonly called the voluntary muscles.
Skeletal system
System of connected bones, cartilage, and joints that protects organs, supports the body, stores minerals, and produces blood cells.
Skin
A soft tissue that is a multi-layered barrier and a cushion that covers vertebrates.
Skull
A bony covering protecting the head.
Small intestine
The part of the gastrointestinal tract that is between the stomach and the large intestine.
Smooth endoplasmic reticulum
Endoplasmic reticulum that is not covered with ribosomes.
Smooth muscles
These line internal organs such as the intestines and bladder.
Society
A long-lasting arrangement of animals in which individuals are constantly changing, but the continuity of the social structure remains intact.
Sodium ion
Na+ is an abundant positively charged inorganic ion (cation) found in human blood plasma.
Sodium-potassium pump
In a neuron, immediately following the inrush of sodium ions and the outflow of potassium ions, the sodium ions (Na+) are pumped back out of the nerve cells and potassium ions (K+) are pumped back in (against their concentration gradient, so energy is required to fuel the active transport). The pump that restores the original ionic balance is the sodium-potassium pump.
Sol state
Sol state is when very fine particles, often called colloidal particles, which are usually macromolecules, are randomly dispersed, usually throughout a liquid.
Soluble
A soluble substance is one that can dissolve, especially in water. Sometimes you have to heat the water up to make it dissolve.
Solute
The component that is not water (or dissolved some other liquid) that is dissolved in a solution (in the solvent). The solvent is the liquid in which the solute is dissolved.
Solution
A solution is a homogeneous (uniform) mixture of two or more components in which the particles of the different substances in the liquid are so small that they cannot be distinguished.
Solvent
The liquid in which a solute is dissolved.
Soma
The soma is the cell body of the nerve cell (neuron).
Somatic cell
Any cell in a living organism other than a gamete or germ cell, and other than a gametocyte or undifferentiated stem cell. Somatic cells are diploid (2N).
Somatic nervous system
Part of the peripheral nervous system.
Species
The taxonomic category that is one step higher than subspecies and one step lower than genus.
Species diversity
Species diversity can be defined as the number and kinds of species present in any specific community.
Sperm cell
The haploid male reproductive cell is called a sperm cell (sperm). It is a male gamete.
Sperm duct
A duct through which sperm moves.
Sperm tail
Sometimes called a flagellum. This thin bundle of filaments connects to the middle portion of a sperm, which connects to the head of the sperm cell.
Spermatids
Immature or undifferentiated male sex cells.
Spermatogenesis
The cells lining the seminiferous tubules are always dividing meiotically, producing haploid sperm cells; this process is called spermatogenesis.
Spermatozoa
Fully differentiated mature sperm cells are also called spermatozoa.
Sphincter muscles
Ring-like bands of muscle fibers with the capacity to close and open an orifice by constricting and relaxing.
Spinal cord
Nervous tissue enclosed within the spine (the vertebral column) that connects with the brain.
Spindle
A series of microtubules that groups together during nuclear division (part of cell division). The spindle aligns and moves chromosomes.
Spindle fibers
The microtubules that altogether constitute the mitotic apparatus called the spindle.
Spleen
Largest organ in the lymphatic system. Under the rib cage, above the stomach.
Spontaneous generation
The idea that organisms automatically appeared from nonliving matter.
Spore
Spores are usually single haploid cells. When conditions are favorable, spores start dividing mitotically and become a new adult organism.
Sporophyll
Sporophylls are greatly modified spore-bearing leaves. Or stated slightly differently, sporophylls are leaves that bear sporangia.
Sporophyte
Sporophyte generation is the asexual, usually diploid phase that grows from gametophyte generation part or stage of a plant. This is the dominant form in ferns that bears the fronds (leaves), and bears the spores.
Stamens
The stamens are the male part of the flower. Stamens include the pollen, as well as the part that produces the pollen, called the anther.
Staminate cones
A staminate cone (or male pollen cone) is a spiral series of fertile male leaves (microsporophylls).
Starch
Starches are polysaccharides that are composed of long chains of glucose units that are joined by glycosidic bonds (type of covalent bond that joins sugar molecules).
Steroids
A large group of organic compounds that includes many hormones and vitamins. Steroids are characterized by containing four rings of carbon atoms.
Stigma
The part of the flower that receives pollen, is the stigma. The stigma is the receptive portion of the style extending above the ovary.
Stoma
A plant stoma is a hole, typically on a leaf, formed by guard cells that surround the hole (the stoma).
Stomach
After food is chewed in the mouth and swallowed, it passes through the esophagus and goes to the stomach.
Striated muscle
Skeletal muscle cells (fibers) are crossed by many thin dark lines, which is why they are called striated muscles.
Stroma
A botanical term (referring to plants) for the colorless fluid in chloroplasts surrounding the grana. Grana are in the stroma.
Style
The part of the flower that bears the stigma. The style is usually a slender and elongate extension of the ovary.
Subspecies
The taxonomic category that is a step lower than species. Subspecies can usually interbreed with other nearby subspecies.
Subvirus

The smallest infectious agents known to researchers are termed subviral infectious agents, or subviruses. Scientists have identified three major types, the satellite viruses, viroids, and prions.

Viroids are minute rings of RNA that infect certain plants. Satellite viruses are tiny pieces of RNA that make full-size viruses work for them. These tiny nucleic acids multiply inside viruses that are inside cells.

Succession
A sequence of communities where over time one replaces another.
Succulent
Plants with fleshy parts that are thicker and juicier than most plants.
Sucrose
A disaccharide. This is our common table sugar.
Sulfate ions
(SO2−)4 is a sulfate ion. It is a negatively charged inorganic ion.
Sulfur
This element's symbol is S. It is important for life to exist.
Summation
If one continually stimulates the muscle, eliciting stronger and stronger contractions until the maximum contraction is reached, the increased contraction achieved during this period is called summation.
Superior vena cava
Sometimes called anterior vena cava. This large vein carries blood from the head, arms, and upper body, back to the heart.
Survivorship curve
The survivorship curve graphically depicts a species in terms of how many of that species or population survive at each age over its lifetime.
Suture
A junction between bones that is immovable, as in the sutures between fused bones in the skull.
Symbionts
A symbiont is either of two organisms living in a symbiotic relationship with one another.
Symbiosis
When two different organisms (usually different species) interact in an intimate way, from which both species derive a mutual benefit.
Symbiotic
A symbiotic relationship is a jointly beneficial relationship between two different organisms (usually different species).
Sympathetic nervous system
The autonomic nervous system is divided into two parts, the sympathetic and parasympathetic nervous systems. The sympathetic nervous system inhibits organs, while the parasympathetic excites organs.
Synapse
Short gap or junction between two neurons (nerve cells) where nervous impulses are transmitted.
Synapsis
At the start of meiosis, during prophase I, synapsis is the fusion of chromosome pairs, when two matching homologous chromosomes pair up, and sections of the chromosomes may crossover, during what is called crossover, before segregation.
Synaptic cleft
The gap or space between one neuron's cell membrane at the very edge of the axon terminal and the neighboring cell membrane of the next neuron's dendrite.
Synaptic pair
The two matching homologous chromosomes that pair up during prophase I of meiosis, when crossover may occur, before segregation, are called a synaptic pair.
Synaptic vesicles
Synaptic vesicles in the axons of neurons store and release neurotransmitters.
Systole
The blood pressure measured when the heart is contracting.
Systolic pressure
Systolic pressure is exerted while the heart is beating (or pumping).
T cells
Lymphocytes that originate in the bone marrow and while developing, they pass through the thymus gland, and therefore are called T cells, which is short for thymus-dependent lymphocytes.
Telomeres
At ends of the chromosomes are regions of repetitive DNA called telomeres that protect the chromosome from fusing with neighboring chromosomes. Telomeres also protect the chromosomes from deteriorating.
Telophase
This is the last stage of mitosis, when the cytoplasm separates in two parts of the cell, and the cell's plasma membrane pinches in from both sides, creating two distinct cells.
Telophase I
During meiosis I, in telophase I, the parent cell splits into two, and the double-stranded chromosomes in the new haploid nuclei fade from view.
Telophase II
During meiosis II, in telophase II, the chromosomes unwind, the nuclear membranes re-form, and the cells divide.
Temperate
Where temperatures are considered moderate. The climate is not considered excessive.
Temperature
A measure of energy that is often described as warmth or coldness.
Tendon
Flexible, fibrous connective tissue that connects muscles to bones.
Testes
Human male gametes, or sperm, are produced in the testes (singular testis).
Testicle
Human male gametes, or sperm, are produced in the testicles (sometimes called balls).
Testosterone
Testosterone is a primary male sex hormone. It is also found in females and males.
Tetanus
If one stimulates a muscle more and more, when it finally has one sustained, continual contraction, this is called tetanus.
Tetrad
During synapsis, the two homologous chromosomes that pair up, possess two chromatids, and together a tetrad is formed. Each tetrad consists of four chromatids.
Tetrapod
A four-limbed vertebrate.
Tetrose
A 4-carbon monosaccharide.
Thalamus
Small structure in the brain that relays sensory and motor signals, and regulates sleep, consciousness, and alertness.
Thallus
Thallus stage of some plants has no stem, leaves, true roots, or vascular tissues.
Thermoacidophiles
Archaebacteria that flourish in hot acidic environments (pH is between 2 and 3) where the temperature is 70–80 °C.
Thoracic vertebrae
The vertebrae between the cervical vertebrae and the lumbar vertebrae. Between these bones are the thoracic nerves that serve the chest.
Thorax
The part of the body between the neck and abdomen. Insects have a thorax that has three pairs of legs attached.
Threshold
The intensity of stimulus required to activate a nervous impulse is called threshold.
Thrombocytes
Platelets (also called thrombocytes) are small, colorless, plate-shaped, cell fragments involved in blood clotting (they help stop bleeding). They don't have a nucleus.
Thromboplastins
Proteins that are released by damaged tissues and ruptured platelets into the blood that help blood coagulate.
Thylakoids
Membrane-bound compartments inside chloroplasts and inside cyanobacteria.
Thymus gland
Located in the upper chest and lower neck, composed of tightly packed lymphocytes (white blood cells).
Thymus-dependent lymphocytes
Also called T cells.
Thyroid gland
Located just below our larynx, around the front and sides of the trachea. It produces the hormone thyroxin.
Ticks
Ectoparasites that belong to the phylum Arthropoda, class Arachnida, and order Acari.
Tissue
A collection of similar types of cells and the associated intercellular substances that surround them.
Toxic
Poisonous or unsafe.
Trachea
In humans, the trachea is often called the windpipe.
Tracheal respiratory system
In insects, this is a network of hollow tubes that delivers air to the cells throughout the insect's interior.
Tracheids
Xylem's conducting cells, the tracheids, are long, narrow cells tapering at the ends where they come in contact and overlap with each other.
Tranquilizer
Tranquilizers can interfere with neurotransmitters by stimulating or retarding neurotransmitters.
Transcription of a molecule
Transcription is a term used instead of “copied,” when talking about copying a strand of DNA when making an RNA molecule.
Transduction
When a bacteria-attacking virus, known as a bacterial virus, or bacteriophage, carries bacterial DNA from one bacterial cell to another.
Transfer RNA
Each tRNA carries an amino acid to a ribosome, where proteins are synthesized.
Translation
Assembling a protein from an mRNA template. Proteins are produced in ribosomes where mRNA and tRNA are bound together.
Transpiration
In plants, transpiration is the exiting or exhalation of water vapor through the stomata.
Transpiration theory
This is used to explain how water rises in trees to great heights.
Tricuspid valve
From the right atrium the blood is forced through the tricuspid valve into the right ventricle.
Triglycerides
Triglycerides comprise three fatty acid molecules bonded together with a molecule of glycerol.
Triploid
A cell or nucleus that has three homologous sets of chromosomes.
tRNA
Transfer RNA (tRNA) reads the information in the nucleic acids and translates it into proteins.
Trophic levels
Positions organisms occupy in food chains are called trophic levels. Plants occupy level 1, herbivores are level 2, and carnivores are level 3.
Tropism
The turning of part or all of an organism toward a particular stimulus.
True bacteria
Also called eubacteria and cyanobacteria. They are simple cells with rigid cell walls, no nucleus, and many species have flagella.
Turgor
Rigidity of cells due to the absorption of water.
Ultraviolet light
UV light is electromagnetic radiation with shorter wavelengths than visible light.
Unicellular organisms
Same as single-celled organism; unicellular organisms consist of only one cell.
Universal donor
A universal donor has blood type O.
Unsaturated fatty acids
Fatty acids containing some double bonds. They are found mainly in plants.
Urban ecosystems
The newest, fastest-growing distinct ecological entities that humans create where we live in large numbers.
Urea
Animals produce urea as a result of metabolizing nitrogenous compounds. Its chemical formula for urea is CH4N2O.
Ureter
Urine is produced in the kidneys. From each kidney is a ureter. Urine travels through the ureters to the urinary bladder.
Urethra
The duct that empties urine from the bladder. The urethra transmits urine out of the body, from the bladder, to and out of the urinary meatus.
Uric acid
Animals produce nitrogenous waste as a result of metabolizing nitrogenous compounds. Such nitrogenous wastes include ammonia, urea, and uric acid.
Uric acid crystals
When an organism has high levels of uric acid in its blood, urate crystals can form. Urate crystals are often called uric acid crystals.
Urinary bladder
A muscular sac situated behind the pubic bone in the pelvic area.
Urinary meatus
The small opening from which urine exits the urethra in males and females.
Urine
A by-product of metabolism that many animals produce in the kidneys.
Uterine cycle
Involves monthly changes in the endometrium's lining.
Uterus
An easily stretched, muscular organ where the embryo develops.
Vacuoles
Large, membrane-bound, fluid-filled spaces found in many cells, particularly plant cells. There are different kinds with different functions.
Vagina
Also called the birth canal. This hollow, stretchable, muscular tube (a passage) leads from the uterus to the vulva. It provides lubrication and sensation.
Valve
The one-way pumping action of a heart or hearts is often accompanied by a series of one-way valves that help regulate, direct, and control the movement of fluids or gases.
Van Leeuwenhoek, Antonie Philips
(1632–1723) Dutch scientist/businessman and microbiologist who improved the microscope with better lenses.
Vas deferens
Ducts in the human male reproductive system that transmit sperm from the testes (testicles) to the urethra.
Vascular
Long-distance internal transport systems that are responsible for moving fluids.
Vascular cambium

A layer of cells in a tree's trunk that produces tissues that allow plants to grow in height, width, and weight.

Vascular rays
The vascular cambium has cells oriented at a right angle to the stem's axis, known as vascular rays, that carry food to the cambium and xylem.
Vascular tissue
Long-distance internal transport systems that move fluids and their dissolved contents throughout the bodies of higher plants.
Vasopressin
The posterior lobe of the pituitary produces vasopressin (antidiuretic hormone, ADH), which stimulates the kidneys to absorb more water.
Vein
After blood passes through the capillaries, it passes into small veins, or venules, which merge into larger veins.
Vena cava
Large vein that carries deoxygenated blood from smaller veins in the body to the heart.
Ventricle
A hollow compartment. The term is used when speaking about parts of the heart, and parts of the central nervous system.
Vertebrae
Bones that form the backbone that is the supporting axis holding up the body. These bones also protect the spinal cord.
Vertebral column
The backbone or spine.
Vertebrates
Animals with a vertebral column (fish, amphibians, reptiles, birds, and mammals).
Vesicle
Small fluid-filled sacs or bladders inside an organism, often inside of cells. Another name for a vesicle is vacuole.
Villi
Nutritional contents in the small intestine are absorbed into the bloodstream through these tiny, finger-shaped structures lining the small intestine.
Viral capsid
The DNA or RNA of a virus is enclosed in a protective protein coat or sheath (protein capsid or viral capsid). Some types of viruses have a viral capsid that in addition to being composed of protein, it also contains fatty acids. Such viruses can be destroyed with soap or detergent. For a virus to reproduce, it must come in contact with and then attach to a cell, penetrate the cell's exterior, travel to the cell's genetic material, and then change what the cell normally does, into doing what the virus requires to reproduce more viruses.
Virinos
Were thought to be subviral agents. Now scientists believe prions cause what they used to think virinos caused.
Virogenes
Genes believed to have originated as the genetic material in a virus that has become part of the genome of a host's cells.
Viruses
Small, nonliving, noncellular, infectious agents that infect all life forms.
Virusoids
Tiny infectious agents composed of short, circular, single strands of RNA.
Visceral muscle
More commonly called smooth muscles. They line internal organs (intestines and bladder).
Visible light
The portion of the electromagnetic radiation spectrum that human eyes can see.
Visible spectrum
Portion of the electromagnetic spectrum that is visible to the human eye with wavelengths that are 380–740 nanometers long.
Vitamins
Organic molecules that are vital in small quantities to all living organisms.
Viviparous
Instead of laying eggs, such organisms bear live young.
Vulva
External female genitalia. Composed of the protective, sensitive inner and outer labia.
Warm-blooded
Mammals and birds are homeothermic, or warm-blooded. They can maintain a stable internal body temperature.
Water balance
Also called homeostasis. Involves taking in and excretion of correct amounts of water.
Water cycle
Refers to the circulation processes that move water (H2O) from the earth's atmosphere to rivers, lakes, and oceans, and then back to the atmosphere.
Water-soluble vitamins
Include B vitamins and vitamin C.
Water vascular system
Unique to the echinoderms. This is a series of fluid-filled vessels that use hydraulic pressure to operate their hollow tube feet.
Wavelength
Length of up-and-down movement that a wave travels before repeating itself.
Waxy cuticle
A hard, protective, waxy layer covering the epidermis of many plants and invertebrates.
Weather
Atmospheric conditions in specific area at a specific time.
Weed
Weeds are wild plants that people don't want.
Wetland
Ecosystem that contains water permanently or seasonally.
White blood cell
Also called WBC, leukocyte, leucocyte, and lymphocyte.
White matter
In brain and spinal cord, tissue composed of myelinated axons is called white matter.
Windpipe
Trachea is often called the windpipe.
Womb
Used interchangeably with uterus.
Wood
The hard, lignified part of a plant (stems, branches, and trunk).
Xanthophylls
One of two types of yellow photosynthetic pigments in the carotenoid group.
Xylem
Most plants have two major pathways (xylem and phloem) for internal transport (vascular tissue). The xylem carries water and dissolved ions upward from the roots to the other parts of the plant.
Zygote
A zygote is a diploid cell that forms when two haploid gametes (a sperm and an egg) fuse, when a sperm fertilizes an egg.