Biologists see life as existing at five “levels of organization,” of which the cellular level is the first (see Chapter 1 for more on the levels of organization). A basic principle of biology says that all organisms are made up of cells and that anything that has even one cell is an organism. Understanding the basics of cell biology is necessary for understanding any aspect of biology, including human anatomy and physiology.
The first thing to know is that cell biology is astoundingly complex. You can gain a detailed understanding of this complexity only from years of hard study. This chapter aims only to give you some idea of how complex cell biology is, so as to provide context for the various physiological miracles we describe in later chapters.
The Functions of Cells
Almost all the structures of anatomy are built of cells, and almost all the functions of physiology are carried out within cells. A comprehensive list of cell functions would be impossible, but we can group cell functions into a few main categories, which we do in the following sections.
Building themselves
Cells arise from other cells and nowhere else. Once in an organism’s lifetime, at the beginning, two cells fuse to form a new cell. Ever after in that organism’s lifetime, two cells arise from the division of one cell, and all the cells ultimately derive from the first one. This process is how an organism builds itself from one single generic cell, called the zygote, to a complex organism comprising trillions of highly differentiated, highly specialized, and highly efficient cells all working together in a coordinated way. Here’s a look at how a cell goes from one to many.
Fusing: The zygote
The organism’s first cell is the zygote, made by the fusion of sex cells: an ovum (egg cell) from the female parent and a sperm cell from the male parent. (See Chapter 14 for more information about sex cells, and see Chapter 15 for more about the zygote.) The zygote has two complete copies of DNA: one from its male parent and one from its female parent. The two copies combine in the zygote’s nucleus. The zygote is said to be diploid — having a complete double-set of DNA.
Dividing: Mitosis
In the form of cell division called mitosis, one cell divides into two daughter cells, each of them complete but smaller than the original cell. Details of this process are discussed later in this chapter.
The process of mitosis occurs only in diploid cells — cells that have two copies of the DNA. That is, all cells except the mature sex cells, which are haploid — they have only one copy of the organism’s DNA.
Differentiating
After mitosis is complete, each daughter cell goes on to its own separate life. One or both may start or continue down a path of differentiation, the name for processes that give cells their particular structures and functions. A cell destined to become a nerve cell starts down one path of differentiation; a cell destined to become a muscle cell starts down another path.
A variation on this mechanism involves a special kind of cell called a stem cell. A stem cell divides by mitosis, and one daughter cell remains a stem cell and goes on dividing again and again, while the other daughter cell goes on to differentiate into a specific type of cell in a particular tissue. Only some tissues have their own special stem cells, such as the skin and the blood.
The details of cellular differentiation are beyond the scope of this book. Its complexity is beyond imagining. This is the only explanation you really need: It’s under genetic control.
Building tissues
All tissues are made of cells, and those cells build and maintain it. Cells in a tissue are to one degree or another differentiated or specialized for their anatomical or physiological function in the tissue.
In addition to the cells, many tissues also contain structural proteins (which are made by the cells). Differentiated cells produce different proteins: Some produce only a few different proteins, and some produce many different proteins in response to signals they receive from other cells. The process of protein construction is basically the same in every cell and for every protein.
For the purposes of anatomy and physiology, remember that all cells have certain very important features in common, but they differentiate into a vast array of shapes and sizes, containing vastly diverse structures, and having different functions and life cycles.
Transforming energy
Most cells make ATP to fuel their own metabolism. They use glucose in the process of cellular respiration to do so. Flip to Chapter 2 for a description of how cells make ATP.
Some cells, like those lining the small intestine, absorb the glucose only to send it out the other side — allowing other cells access to this valuable resource. Sometimes, the glucose made available by the digestive system is more than the body can use at that moment. Thus, some cells, like those in the liver, function to corral the extra glucose molecules and store them. Later, when glucose levels are low, these cells release some of their stores, making it available to other cells.
Making and transporting products
A great many types of cells make special chemicals that are incorporated into tissues and participate in metabolic reactions. Cellular products include thousands of specific proteins and polypeptides, signaling chemicals like neurotransmitters and hormones, small molecules and ions, lipids of many kinds, and structural molecules of many kinds.
Some specialized cells do essentially nothing else but make and export one product for use by other cells; others make products and perform other functions.
Some cells specialize in transporting the products of other cells around the body, or in transporting metabolic waste products out of the body. Some of these transporting cells have other functions as well. Others do nothing but that one job through their entire life cycle. Red blood cells are an extreme example of the one-job model. They lose their nuclei during differentiation and thereafter do nothing but transport gas molecules from one place to another. They don’t divide, don’t produce ATP, and don’t maintain themselves. When their gas-transporting structures wear out, RBCs have nothing to do. They’re removed from circulation and broken down in the liver.
Communicating
Some cells transmit signals of various kinds while remaining in one place in the body. Some nervous cells’ sole purpose is to generate and conduct electrical signals. They typically live for years, often until the death of the organism itself. Other cells produce various kinds of signaling molecules, like hormones and neurotransmitters, or receive and react to those signaling molecules.
Seeing the Inside of Eukaryotic Cells
Although they’re astoundingly varied, cells are also remarkably alike. (This is a theme of cell biology.) It’s not just that all the cells of one organism or even one species are a lot alike. All cells, at least all eukaryotic cells, are alike. Plants, animals, and fungi are eukaryotes (organisms made up of eukaryotic cells), and all their cells, in all their enormous complexity and variation, are fundamentally alike. Yes, your skin cells, your kidney cells, and your bone cells are fundamentally similar to the leaf cells and root cells of a carrot; the cells of a mold, mushroom, or yeast; and the single cell of microorganisms called protists that live in water and soil.
Here’s a simplistic description of a eukaryotic cell: It’s a membrane-bound sac containing smaller but distinctive structures, called organelles (“little organs”), suspended in a gel-like matrix called the cytoplasm. As their name suggests, organelles are functional subunits of a cell, as organs are functional subunits of an organism. One of the largest and most prominent organelles, the nucleus, controls a cell’s functioning, similar to the way the nervous system controls an organism’s functioning. The term eukaryote is derived from the Greek term karyos, meaning “nut” or “kernel,” which early biologists used to refer to the nucleus. Figure 3-1 shows the general structure of a eukaryotic cell. Refer to this figure as you read about the various cellular structures in the following sections. Table 3-1 gives you an overview of the structures found within a eukaryotic cell.
Illustration by Kathryn Born, MA
FIGURE 3-1: A cutaway view of a basic animal cell and its organelles.
TABLE 3-1 Organelles of Animal Cells (Including Humans)
Organelle
Function
Nucleus
Controls the cell; houses the genetic material
Mitochondrion
Cell “powerhouse”; site of cellular respiration
Endoplasmic reticulum
Plays an important role in protein synthesis; participates in transporting cell products; involved in metabolizing (breaking down) fats as well as drugs
Ribosome
Binds amino acids together under the direction of mRNA to make protein
Golgi apparatus
Modifies proteins into functional form; “packages” cellular products in sacs called vesicles in which products can cross the cell membrane to exit the cell
Vacuoles
Membrane-bound spaces in the cytoplasm that aid in endo- and exocytosis
Lysosomes
Contain enzymes that break down harmful cell products and waste materials and actively transport them out of the cell
The organisms called bacteria (singular, bacterium) are made up of prokaryotic cells. Prokaryotic cells are very different from and much simpler than eukaryotic cells in their basic structure and organization. This difference between eukaryotic and prokaryotic organisms is the great divide in biology. At the cellular level, differences among animals, plants, fungi, and protists are almost negligible compared with the differences between these groups on the one hand and prokaryotes (bacteria) on the other.
Containing the cell: Cell membrane
A cell is bound by a membrane, the cell membrane, also called the plasma membrane or the plasmalemma. The cell membrane of all eukaryotes is made of phospholipid molecules. The molecules are made by cells, a process that requires energy. The molecules assemble spontaneously (without input of energy) into the membrane, obeying the forces of polarity. Turn to Chapter 16 for a discussion of polarity and how the membrane takes its distinctive form, often called the phospholipid bilayer.
Don’t confuse cell membranes with cell walls. Every cell has a membrane, and a cell membrane is a fundamental characteristic of a cell. Some cells also have cell walls outside and separate from the cell membrane. No animal cells have cell walls, but some plant cells and fungal cells have them. They’re dissimilar to cell membranes in structure and function.
Permeating the membrane: The fluid-mosaic model
The phospholipid bilayer is embedded with structures of many different kinds. Though the bilayer itself is essentially similar in all cells, the embedded structures are as various and specialized as the cells themselves. Some identify the cell to other cells (very important in immune system functioning); some control the movement of certain substances in or out of the cell across the membrane. Figure 3-2 is a diagrammatic representation of the phospholipid bilayer and embedded structures. This model of the cell membrane is called the fluid-mosaic model. “Fluid” describes the ability of molecules in the bilayer to move; “mosaic” pertains to the embedded structures.
Illustration by Kathryn Born, MA
FIGURE 3-2: The fluid-mosaic model of the cell membrane.
The chemical properties of the phospholipid bilayer and the embedded structures contribute to a very important feature of the membrane: It’s able to control which substances pass through it and which do not. This means the membrane is semipermeable.
Crossing the membrane passively
Some substances, mainly small molecules and ions, cross the membrane by a passive transport mechanism, meaning they more or less flow unimpeded across the bilayer, driven by the forces of “ordinary” chemistry, such as concentration gradients, random molecular movement, and polarity. Here are some ways that substances cross a membrane passively:
Diffusion: A substance moves spontaneously down a concentration gradient (from an area where it’s highly concentrated to an area where it’s less concentrated). If you drop a teaspoon of salt into a jar of water, the dissolved sodium and chloride ions will, in time, diffuse (spread themselves evenly through the water). You can measure the time in seconds if you stir the solution or in days if you keep the solution perfectly still at room temperature. (To find out why, see Chapter 16.) Cellular and extracellular fluids are constantly being “stirred” and are at temperatures between 95 and 100 degrees Fahrenheit. Molecules to which the cell membrane is permeable (such as oxygen and carbon dioxide) may diffuse into or out of the cell, constantly attempting to reach equilibrium.
The cell membrane is generally not permeable to ions and larger molecules like glucose. They must enter (or leave) the cell through a transport protein via facilitated diffusion. This still doesn’t require energy because the molecules are moving down their concentration gradient — they just need a door to open for them.
Osmosis: The diffusion of water molecules across a selectively permeable membrane gets a special name: osmosis. As with diffusion, a concentration gradient drives the mechanism. The pressure at which the movement of water across a membrane stops (that is, when the concentration of the solutions on either side of the membrane is equal) is termed the osmotic pressure of the system.
Filtration: This form of passive transport occurs during capillary exchange. (Capillaries are the smallest blood vessels — they bridge arterioles and venules; see Chapter 9). Capillaries are only one cell layer thick, and the capillary wall acts as a filter, controlling the entrance and exit of small molecules. Small molecules dissolved in tissue fluid, such as carbon dioxide and water, are pushed through the capillary wall, sliding between the cells and into the blood, while substances dissolved in the blood, such as glucose and oxygen, do the same in the opposite direction. The pulsating force of blood flow provides a steady force to drive this movement.
The blood pressure in the capillaries is highest at the arterial end and lowest at the venous end. At the arterial end, blood pressure pushes substances through the capillary wall and into the tissue fluid. At the venous end, lower blood pressure (thus higher net osmotic pressure) pulls water from the extracellular fluid (and anything dissolved in it) into the capillary.
Does passive transport contradict the idea that the cell controls what comes in and out through the membrane? No. The substances that move by passive mechanisms are “ordinary” small molecules and ions that are always present in abundance within and between every cell and kept within a physiologically healthy concentration range by the forces of homeostasis, the first line of defense against physiological abnormality. If at any time the physiological levels get too high or too low, the cell has pumps that can counteract the passive transport.
Crossing the membrane actively
Active transport allows a cell to control which big, active, biological molecules move in and out of the cytoplasm. Active transport is a fundamental characteristic of living cells (whereas you can set up a system for diffusion, as we note earlier, in a jar of water).
Like many matters in cell biology, active transport mechanisms are numerous and widely varied. When there is a molecule outside the cell that it needs, a simple active transport mechanism is used. Cell membranes have embedded proteins for the active transport of a single, specific molecule. They must be activated, or opened, for the molecule to be pumped in or out. This is generally done by a binding site on the same protein, but it can also be triggered by another protein with the membrane.
With very large molecules, another energy-requiring transport method is used. For example, a large protein made within the cell could require far too much space to exit — effectively rupturing the cell. Instead, the protein is packaged in a vesicle whose outer membrane is the same phospholipid bilayer as the cell membrane. During this transport process, called exocytosis, the lipids realign, letting the protein out of the cell without ever breaching the seal. This same process can occur in reverse, called endocytosis, to bring large molecules in.
Controlling the cell: Nucleus
As we mention previously, the defining characteristic of a eukaryotic cell is the presence of a nucleus (plural, nuclei) that directs the cell’s activity. The largest organelle, the nucleus is oval or round and is plainly visible under a microscope. Refer to Figure 3-1, earlier in the chapter, to see the relationship of the nucleus to the cell; Figure 3-7, later in the chapter, shows a closer view of the nucleus’s structure.
All cells have one nucleus, at least at the beginning of their life cycle. As a cell develops, it may lose its nucleus, as do red blood cells and the keratinocytes of the integument; or the cell may merge with other cells, with the merged cell retaining the nuclei of all the cells, such as the fibers of skeletal muscle. This type of cell is called a syncytium.
The nucleus contains one complete (diploid) copy of the organism’s genome — the DNA that embodies the organism’s unique genetic material. Every nucleus of every cell in an organism has its own complete and exact copy of the entire genome. It’s bound by a semipermeable membrane called the nuclear envelope.
The cells produced from this identical DNA are unimaginably varied in structure, in function, and in the substances they produce (proteins, hormones, and so on). The differentiation of the cell (the structure it takes on) and everything about its products are directed by the nucleus, which controls gene expression, the selective activation of individual genes.
Cytoplasm
Within the cell membrane, between and around the organelles, is a fluid matrix called cytoplasm or cytosol and an internal scaffolding made up of microfilaments and microtubules that support the cell, give processes the space they need, and protect the organelles. The organelles are suspended in the cytoplasm.
The cytoplasm is gelatinous in texture because of dissolved proteins. These are the enzymes that break glucose down into pyruvate molecules in the first steps of cellular respiration (see Chapter 2). Other dissolved substances are fatty acids and amino acids. Waste products of respiration and protein construction are first ejected into the cytoplasm and then enclosed by vacuoles and expelled from the cell.
Organelles — including the nucleus, the mitochondria, the endoplasmic reticulum, and the Golgi body — contain a fluid with a particular composition, similar to the cytosol and to one another but each suited to the particular organelle’s needs.
Internal membranes
The plasma membrane isn’t the only membrane in a cell. Phospholipid bilayer membranes (without the “mosaic” of embedded structures) are present all through the cell, encapsulating each organelle and floating around, waiting to be useful. The network of membranes is sometimes called the endomembrane system. When an organelle makes a substance that must be expelled from the cell, a piece of bilayer moves in and encapsulates (surrounds) the material for exocytosis.
Powering the cell: Mitochondria
A mitochondrion (plural, mitochondria) is an organelle that transforms energy into a form that can be used to fuel the cell’s metabolism and functions. It’s often called the cell’s “powerhouse.” We discuss the role of the mitochondrion in cellular respiration in Chapter 2.
The number of mitochondria in a cell depends on the cell’s function. Cells whose function requires only a little energy, like nervous cells, have relatively few mitochondria; muscle cells may contain several thousand individual mitochondria because of their function in using energy to do “work.” A mitochondrion can divide, like a cell, to produce more mitochondria, and it can grow, move, and combine with other mitochondria, all to support the cell’s need for energy.
Mitochondria are very small, usually rod-shaped organelles (see Figure 3-3). A mitochondrion has an outer membrane that covers and contains it. The fluid inside the mitochondrion, called the mitchondrial matrix, is filled with water and enzymes that catalyze the oxidation of glucose to ATP. A highly convoluted (folded) inner membrane sits within the matrix, increasing the surface area for the chemical reactions.
Unique among organelles, the mitochondrion contains a small amount of DNA in a separate chromosome. This DNA behaves separately and independently from the chromosomes in the nucleus. It duplicates and divides to give birth to new mitochondria within the cell, a separate event from mitosis.
The mitochondrion’s two membranes aren’t the same as the bilayer membrane of the nucleus.
The protein factory
The process of protein construction is a truly elegant system, as you see in the “Synthesizing protein” section later in the chapter. Here we look at the structures of the protein-construction system: the organelles and other intracellular structures and their relationships with one another.
The process of protein construction begins in the nucleus. In response to many different kinds of signals, certain genes become active, setting off the production of a specific protein molecule (gene expression). Think of the nucleus as a factory’s administrative department.
The endoplasmic reticulum (or ER; literally, “within-cell network”) is a chain of membrane-bound canals and cavities that run in a convoluted path, connecting the cell membrane with the nuclear envelope. The ER brings all the components required for protein synthesis together. The ribosomes, another organelle involved in protein synthesis, adhere to the outer surface of some parts of the membrane, sticking out into the cytoplasm. These areas are called rough ER in contrast to smooth ER, where no ribosomes adhere. Think of the ER as the factory’s logistics function.
The ribosome is the site of protein synthesis, where the binding reactions that build a chain of amino acids are performed. Ribosomes may float in the cytoplasm or attach to the ER. Ribosomes are tiny, even by the standard of organelles, but they’re highly energetic, and a typical cell contains thousands of them. Think of the ribosomes as the production machinery.
The Golgi body forms a part of the cellular endomembrane system. It functions in the storage, modification, and secretion of proteins and lipids. Think of it as the shipping department. The boxes used to ship the product are the vesicles.
Lysosomes
Old, worn-out cell parts need to be removed from the cells; if they aren’t, they can become sources of toxins or severe energy drains. Lysosomes are organelles that do the dirty work of autodigestion. Lysosomal enzymes destroy another part of the cell, say an old mitochondrion, through a digestive process. Molecules that can be recovered from the mitochondrion are recycled in that cell or in another cell. Waste products are excreted from the cell in a membrane-bound vacuole.
Building Blocks That Build You
Though the processes of life may appear to be miraculous, biology always follows the laws of chemistry and physics. Biochemical processes are much more varied and more complex than other types of chemistry, and they happen among molecules that exist only in living cells. Molecules many thousands of times larger than water or carbon dioxide are constructed in cells and react together in seemingly miraculous ways. This section talks about these large molecules, called macromolecules, and their complex interactions.
Joining together: The structure of macromolecules
The four categories of these macromolecules, often called the biomolecules of life, include polysaccharides (carbohydrates), lipids, proteins, and nucleic acids. All are made mainly of carbon, with varying proportions of oxygen, hydrogen, nitrogen, and phosphorus. Many incorporate other elements, such as magnesium, sulfur, or copper.
Macromolecules, as the term suggests, are huge. Like a lot of huge things, they’re made up of smaller things, as a class is made up of students. A student is a subunit of the class, almost identical in many important ways to the other students but unique in other important ways. Macromolecules are made up of molecular subunits called, generically, monomers (“one piece”). Each type of macromolecule has its own kind of monomer. Macromolecules are, therefore, polymers (“many pieces”).
In popular usage, polymer suggests plastics. Well, plastics are polymers, but not all polymers are plastics. The term refers to molecules that are made up of repeating subunits — its monomers. The chemical behavior of polymers is different from that of their constituent monomers.
The chemistry of macromolecules is like an infinite Lego set. Any block (monomer) can connect to another block if the shape of their connectors match. With enough blocks, some special connectors, and the energy to do so, you can eventually create a complex structure with bells, whistles, and wheels that turn. Then, you can do that 1,000 times more, and then connect all the complex structures together into one very large, very complex, highly functioning structure. (What’s that? You don’t have the materials or the energy to do that, and anyway, you wouldn’t know how to make a structure fit for the Lego museum? That’s okay. Your cells build much more complex things all day every day. All you need to do is keep supplying them with fuel.)
Polysaccharides
The simple carbohydrate molecule glucose is the main energy molecule in physiology. A common polysaccharide (polymer of carbohydrate monomers) is glycogen, which is made by linking numerous glucose molecule together to function as fuel storage.
Polysaccharides are also used for cell structures. Carbohydrate chains can be found attached to proteins embedded in the cell membrane for both recognition by and attachment to other cells.
Lipids
Lipids are polymers of glycerol and fatty acids (three chains of them) and are insoluble in water. The most common lipids are fats, which are an incredibly efficient energy source (which is unfortunate for us but explains the body’s propensity for storing it!). Cholesterol is also an important lipid that is used to manufacture steroid hormones (see Chapter 8) and can be found in cell membranes providing stabilization.
The phospholipids I discuss earlier also belong in this category. They replace one of the fatty acid chains with a phosphate group — giving them a hydrophilic head that interacts with water (see Figure 3-2).
Proteins
Proteins are polymers of amino acids. The amino acid monomers are arranged in a linear chain, called a polypeptide, and may be folded and refolded into a globular form. Structural proteins comprise about 75 percent of your body’s material. The integument, the muscles, the joints, and the other kinds of connective tissue are made mostly of structural proteins like collagen, keratin, actin, and myosin. In addition, the enzymes that catalyze all the complex chemical reactions of life are also proteins.
Amino acids
Twenty different amino acids exist in nature. Amino acids themselves are, by the standards of nonliving chemistry, huge and complex. A typical protein comprises hundreds of amino acid monomers that must be attached in exactly the right order for the protein to function properly.
The binding proclivities among all the amino acids result in the structural precision that makes proteins functional for the exacting processes of biology. That is, the order of amino acids determines how the protein twists and folds. Every protein depends on its unique, three-dimensional structure to perform its function. A misfolded protein won’t work and in some cases can result in disease.
Enzymes
Enzymes are protein molecules that catalyze the chemical reactions of life. Enzymes can only speed up a reaction that is otherwise chemically possible. How effective are enzymes in speeding up reactions? Well, a reaction that may take a century or more to happen spontaneously happens in a fraction of a second with the right enzyme. And better yet, they are not “used up” in the process. Enzymes are involved in every physiological process, and each enzyme is extremely specific to one or a very few individual reactions. Your body has tens of thousands of different enzymes.
Any time an enzyme is discovered, it is named, usually with some physiology shorthand for its function, frequently with the suffix –ase. An enzyme named pyruvate dehydrogenase removes hydrogen atoms from pyruvate molecules in the processes of cellular respiration (see Chapter 2).
The malfunctioning of a single enzyme, which can be caused by a single nucleotide being out of order, is responsible for some very nasty, sometimes fatal diseases. Phenylketonuria (PKU) is an inherited metabolic disease caused by faulty phenylalanine hydroxylase. The inability to properly metabolize the amino acid phenylalanine, found in many foods, results in mental retardation, organ damage, unusual posture, and even death, unless the afflicted person can limit the ingestion of foods containing phenylalanine, which is a very difficult thing to do.
Nucleic acids
The nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers made of monomers called nucleotides and arranged in a chain, one after another … and another, and another. DNA molecules are thousands of nucleotides long (see Figure 3-4). The functioning of genes is inseparable from the chemical structure of the nucleic acid monomers.
FIGURE 3-4: DNA is made up of thousands of nucleotides.
A nucleotide is made up of a sugar molecule and a phosphate group attached to a nitrogenous base. The sugar molecule is either deoxyribose (in DNA) or ribose (in RNA). The nitrogenous base is one of four:
Cytosine (C), guanine (G), thymine (T), or adenine (A) in DNA
Cytosine (C), guanine (G), uracil (U), or adenine (A) in RNA
The bases connect with each other in specific pairs. (Refer to the section “Gene structure” later in the chapter for a discussion of the biological significance of this complementary pairing.) The complementary pairs are
C with G
A with T in DNA
A with U in RNA
The structural similarities and differences between DNA and RNA allow them to work together to produce proteins within cells. The DNA molecule remains stable in the nucleus during normal cell functioning, protected from damage by the nuclear envelope. An RNA molecule is built on demand to transmit a gene’s coded instructions for building proteins, and then it disintegrates. Some of its nucleotide subunits remain intact and are recycled into new RNA molecules.
Genes and Genetic Material
Your anatomical structures are specified in detail, and all your physiological processes are controlled by your very own unique set of genes. Unless you’re an identical twin, this particular set of genes, called your genome, is yours alone, created at the moment your mother’s ovum and your father’s sperm fused. The genome itself (all your genes) is incorporated in the DNA in the nucleus of each and every one of your cells.
The genome consists of many thousands of genes. Early research into the number of individual genes in the human genome has produced varying estimates, from about 23,000 to around 75,000, with estimates toward the lower end of the range predominating. Within any given cell, only a very few of these genes is ever expressed — that is, activated and used in protein formation.
Traiting you right
Your genes are responsible for your traits. If your genes specify that you’ll grow to 6 feet tall (a trait), they cause bone and tissue to grow until your body reaches that height, and they maintain it thereafter (assuming a favorable environment), until the cells the genes work through age and die. If you have the genes for brown eyes (a trait), your genes direct the production of pigments that color the eyes. And if your genes include those for abundant production of low-density cholesterol, you have a tendency toward atherosclerosis, another trait. In an environment where the available nutritional resources include abundant red meat, this trait could give you trouble. Otherwise, this trait is harmless.
Calling a trait “harmless” may be technically incorrect. Assuming that any trait has some survival-enhancing purpose, at least under some environmental conditions, is probably more correct, even if you don’t know what the purpose is.
Gene structure
The physical structure of the DNA molecule, called the DNA double helix, is key to the functioning of genes (refer to Figure 3-4). The DNA double helix can be compared with a ladder or a zipper. In function, it’s more like a system of code.
Code “you”
The nucleotides are the symbols in the genetic code. In modern English, a written word is a code made of certain subunits (letters) set down in a specific order. For example, this, than, then, and ten are different words with different meanings and functions in expressing a thought. A gene is made of certain nucleotides in a certain order. A gene may be a few or many nucleotides in length (imagine a single word 20 pages long), but a given gene is always exactly the same nucleotides in exactly the same order along one strand of DNA. The order of the nucleotides is absolutely crucial: “ACTTAGGCT” is not the same as “ACTAAGGCT.” According to the prevailing theory, each gene specifies the construction of one protein molecule. This is called the one gene, one protein model. If the nucleotides are out of order, the protein molecule they make will probably be useless, and the organism’s functioning will likely be impaired, a little or a lot.
Every model used to explain cell biology, including the one gene, one protein model, is subject to change as more information becomes available. But the changes are likely to be minor for understanding basic anatomy and physiology.
Pairing at the molecular level
Remember nucleotide complementary pairs? (If not, see the section “Nucleic acids and nucleotides” earlier in the chapter.) So if a nucleotide is in place on one strand of DNA, and each type of nucleotide binds with only its complementary partner (A with T, C with G, and so on), what do you think is on the other DNA strand? Right! The other nucleotide of the complementary pair. Wherever there’s a G on one strand, there’s a C on the other, and the pair is attached in the middle. And if the two strands become separated (which they do), what do you think will happen? The nucleotides on each strand will attract and hold other molecules of their complementary partners, thus creating two new double-strands identical to the original double-strand. This is how DNA replication occurs.
Synthesizing protein
A gene that’s active sends messages to its own cell or to other cells, ordering them to produce molecules of its particular protein, the only one it’s capable of making. The message is sent from DNA through the intermediary of mRNA (messenger RNA), which places an order at the protein factory of the cell and stays around for a while to supervise production. The first part of the process, where DNA “writes the order” in the form of a sequence of nucleotides on mRNA, takes place in the nucleus and is called transcription (“writing across”). The next part of the process, where RNA places the order at the factory, is called translation (“carrying across”). The last part, where the amino acid monomers are sorted out and assembled into the polypeptide, starts in the ribosome. Figure 3-5 shows this process. The finishing touches are put on the protein molecule in the endoplasmic reticulum and the Golgi body. The process from transcription to the last finishing touch on the protein molecule is called gene expression.
Keep in mind the relationship between the genome and gene expression. Your entire genome is contained in identical DNA molecules in the nucleus of every one of your cells; it remains unchanged through your lifetime. Any given gene may be expressed in only a few cells, or only occasionally in a few cells, or only under certain physiological conditions, or possibly never. The totality of gene expression changes every second throughout your lifetime, as quickly as nerves transmit impulses and cells react.
The terms gene expression, protein synthesis, and transcription-and-translation are essentially the same in meaning, but they’re used in different contexts in cell biology and physiology.
The Cell Cycle
The life cycle of an individual cell is called the cell cycle. The moment of cell cleavage, when a cell membrane grows across the “equator” of a dividing cell, is considered to be the end of the cycle for the mother cell and the beginning of the cycle for each of the daughter cells.
Typically, but by no means universally, interphase is the longest period of the cell cycle. Interphase comes to an end when the cell divides in the process of mitosis. We discuss both these periods of the cell cycle in the following sections.
Cells that divide, cells that don’t
All cells arise from the division of another cell, but not all cells go on to divide again:
Zygote: This is the diploid cell that comes into existence when the sex cells (ovum and sperm, both haploid) fuse at conception. Almost immediately, the zygote divides into two somatic cells.
Somatic cells: These include all the cells of the body except the sex cells — in other words, all the diploid cells of the body. Somatic cells may be relatively differentiated (somewhat specialized), terminally differentiated (they never divide again), or stem cells.
Stem cells: These are special kinds of rather “generic” somatic cells that divide to produce one new stem cell and one new somatic cell that goes on to differentiate into a particular type of cell in a particular type of tissue. The embryo (organism in the very early stages of development) has very special stem cells, called pluripotent (“many powers”) stem cells, which have the ability to give rise to just about any kind of cell an organism needs, given the right chemical environment. When an organism has developed beyond the embryo stage, embryonic stem cells disappear, and other types of stem cells, called multipotent or adult stem cells, arise in particular tissue types and specialize in producing new cells for that tissue. (Turn to Chapter 9 for a description of how stem cells in the bone marrow give rise to many types of blood cells.)
Sex cells (gametes): These form when specialized somatic cells in the reproductive system divide by a process called meiosis. Meiosis is the only cellular process in the human life cycle that produces haploid cells. See Chapter 14 for more details on sex cells and the processes of meiosis.
Table 3-2 summarizes how different types of cells behave when it comes time to divide.
TABLE 3-2 Dividing Behavior of Different Cell Types
* Some somatic cells go on to terminal differentiation and never divide again.
** Sex cells arise from meiosis of certain somatic cells. They are haploid cells and never divide again.
Interphase
Interphase begins when the cell membrane fully encloses the new cell and lasts until the beginning of mitosis or meiosis. The duration of interphase may be anywhere from minutes to decades. Generally speaking (there are always exceptions in cell biology), cells do most of their differentiating and most of their routine metabolizing during interphase. Stem cells grow in size and duplicate organelles during interphase, in preparation for mitosis (more on mitosis in a minute). Some other cells enter mitosis after an extended period of steady-state metabolism. Sometimes, a cell remains in interphase, carrying out its physiological function for years and years until it dies.
DNA replication
DNA replication is an early event in cell division, occurring during interphase, just prior to the beginning of mitosis or meiosis but within the protected space in the nuclear envelope. Maintaining the integrity of the DNA code is absolutely vital.
During DNA replication, the double helix must untwist and “unzip” so that the two strands of DNA are split apart. As shown in Figure 3-6, each strand becomes a template for building the new complementary strand. This process occurs a little at a time along a strand of DNA. The entire DNA strand doesn’t unravel and split apart all at once. When the top part of the helix is open, the original DNA strand looks like a Y. This partly open/partly closed area where replication is happening is the replication fork.
In Figure 3-6, the symbols 5′ and 3′ (read five prime and three prime) indicate the direction in which DNA replication is occurring. The template strand is read in the 3′-to-5′ direction. The bases that are complementary to the template strand are added in the 5′-to-3′ direction.
Mitosis
A cell enters a process of mitosis (division) in response to signals from the nucleus. As shown in Figure 3-7, mitosis is a multistage process, proceeding in the following stages:
Prophase: The nuclear envelope is dismantled, and the duplicated DNA, in the form of chromatin, thickens and coils into chromosomes. Each duplicated chromosome is composed of two identical strands of DNA referred to as chromatids. The chromatids are held together by a protein mass called the centromere. (Note: When the chromatids separate, each is considered a new chromosome.) Cellular structures called centrioles and spindle fibers form and move to the poles of the cell.
Metaphase: The chromosomes line up to form a perfect row at the center of the cell. The spindle fibers, attached to the centrioles at one end, attach to the chromosomes. At this point, 92 chromatids are in a double set of 46 chromosomes.
Anaphase: The centromere is split by enzymatic activity, and the chromatids are pulled apart by the spindle fibers toward one of the centrioles: 46 to one, 46 to the other. Following this movement, the chromosomes are referred to as daughter chromosomes, and the set at one pole is identical to the set at the opposite pole. But the cell isn’t quite ready to divide yet.
Telophase: A fresh nuclear membrane is reassembled around each set of chromosomes. The spindles dissolve, which frees the daughter chromosomes.
Illustration by Kathryn Born, MA
FIGURE 3-7: The stages of mitosis: prophase, metaphase, anaphase, and telophase.
At this point, when each of the two identical nuclei is at one pole of the cell, mitosis is technically over. However, the cell’s cytoplasm still has to actually split apart into two masses, a process called cytokinesis. The center of the mother cell indents and squeezes the cell membrane across the cytoplasm until two separate cells are formed. The two daughter cells are then in interphase and go on to differentiation or not, depending on the instructions to the cell from the genome.
Organizing Cells into Tissues
A tissue is an assemblage of cells, not necessarily identical but from the same origin, that together carry out a specific function. As we discuss in Chapter 1, tissue is the second level of organization in organisms, above (larger than) the cell level and below (smaller than) the organ level.
Like just about everything else in anatomy, tissues are many and various, and they’re grouped into a reasonable number of “types” to make talking about them and understanding them a little simpler. The tissues of the animal body are grouped into four types: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. All body tissues are classified into one of these groups.
Connecting with connective tissue
Connective tissues connect, support, and bind body structures together and are the most abundant tissue by weight. Generally, connective tissue is made up of cells that are spaced far apart within a gel-like, semisolid, solid, or fluid matrix. (A matrix is a material that surrounds and supports cells. In a chocolate chip cookie, the dough is the matrix for the chocolate chips.)
Connective tissue has many functions, and thus many forms; it is the most varied of all the tissue groupings. In some parts of the body, such as the bones, connective tissue supports the weight of other structures, which may or may not be directly connected to it. Other connective tissue, like adipose tissue (fat pads), cushions other structures from impact. You encounter lots of connective tissue in the chapters to come because every organ system has some kind of connective tissue.
We discuss the specialized connective tissues bone and cartilage in some detail in Chapter 5, and we discuss the important connective tissue blood in detail in Chapter 9. (What? Blood is a tissue? A connective tissue? Yes, and you’ll see why.)
The other types of connective tissue, or proper connective tissues, are all means of connections. Just as we have different kinds of tapes and glues, we have different varieties of connective tissue. They contain varying proportions of fibrous proteins of two types: collagenous and elastic. Collagenous fibers are made of collagen, a bulky protein, and serve to provide structure. Elastic fibers are made of elastin, a thin protein, and serve to provide stretch.
The following are the proper connective tissues found in the human body:
Areolar (a type of loose connective tissue): This tissue surrounds and separates structures in every part of the body. It forms a thin membrane with ample space for blood vessels to pass through. Both collagenous and elastic fibers are prevalent in its gel-like matrix.
Dense regular connective tissue: This tissue is characterized by its tightly packed collagenous fibers arranged parallel to each other, making it very strong. There are very few cells present and there is very little blood flow. Tendons and ligaments are made of dense regular tissue (see Chapter 5).
Dense irregular connective tissue: Very similar to dense regular, the fibers in this tissue are disorganized, leaving more space for blood flow. The dermis is a typical tissue of this type (see Chapter 4).
Adipose tissue (a type of loose connective tissue): Composed of fat cells, adipose tissue provides fuel storage and insulation as well as support and protection to its underlying structures.
Reticular tissue (a type of loose connective tissue): This type of tissue uses a thinner variety of collagenous fibers to form a net. It creates the framework of such organs as the spleen, the lymph nodes, and the liver.
Elastic connective tissue: Collagenous fibers are bundled in parallel with bands of elastic fibers sandwiched between (like lasagna). This makes elastic tissue strong but stretchy — perfectly suited for the walls of hollow organs and arteries.
Continuing with epithelial tissue
Epithelial tissue forms the epidermis of the integument (the skin and its accessory structures; see Chapter 4), covers all your internal organs, and forms the lining of the internal surfaces of the blood vessels and hollow organs.
Epithelial tissues create coverings and linings; they’re always bordered by “empty space” on one side. The other side is a basement membrane that allows resources to diffuse up into the tissue from the connective tissue below (epithelial tissues have no blood flow).
Epithelial tissue comes in eight types that are defined by the way epithelial cells are combined and shaped (see Figure 3-8).
Simple squamous epithelium: A single layer of flat cells, this tissue functions in rapid diffusion and filtration. The lining of the alveoli (small sacs) of the lung is a typical tissue of this type.
Simple cuboidal epithelium: A single layer of cuboidal cells, this tissue functions in absorption and secretion. This tissue is typically found in glands. The cuboidal cells have the capacity to produce and modify the glandular product (for example, sweat, oil, or milk).
Simple columnar epithelium: A single layer of cells that are elongated in one dimension (like a column). Like simple cuboidal epithelium, this tissue functions in secretion and absorption. This type of tissue is primarily found lining portions of the digestive tract.
The cells may also be ciliated, possessing a type of organelle called cilia — hairlike structures that act to move substances along in waves. Ciliated simple columnar epithelium can be found lining the uterine tube.
Pseudostratified columnar epithelium: A single layer of columnar cells. Note that the prefix pseudo means “false.” The tissue appears stratified, or layered, because the cells’ nuclei don’t line up in a row, as they do in simple columnar epithelium. Other than that, they’re the same, and have similar functions of absorption and secretion. This type of tissue lines ducts in testicular structures.
More commonly, this tissue type is ciliated. It is present in the linings of the respiratory tract, functioning in a more or less identical way to the simple columnar ciliated epithelium.
Stratified squamous epithelium: This tissue consists of several layers of cells: squamous epithelial cells on the outside with deeper layers of cuboidal or columnar epithelial cells. It’s found in areas where the outer layer is subject to wear and needs to be replaced continuously. The epidermis of the skin is an example of a specific type called keratinized stratified squamous epithelial tissue.
Stratified cubiodal epithelium: Several layers of cuboidal cells that line the ducts associated with the sweat, mammary, and salivary glands.
Stratified columnar epithelium: Several layers of elongated cells mainly act as protection in such structures as the conjunctiva of the eye and the pharynx.
Transitional epithelium: The cells of this tissue can change (or transition) from cuboidal when relaxed to squamous when stretched, as needed by the tissue. This tissue is found in the lining of the bladder, where having a little room to stretch is sometimes handy. See a diagram of transitional epithelium in Chapter 12.
Illustration by Kathryn Born, MA
FIGURE 3-8: The cellular composition of epithelial tissue.
Mixing it up with muscle tissue
Muscle tissue comes in three types: skeletal muscle, smooth muscle, and cardiac muscle. We discuss the similarities and differences in cellular composition in these three types of tissue in Chapter 6. We also discuss in Chapter 6 the anatomy and physiology of the large organ system called the muscular system, of which skeletal muscle tissue is a major component. In Chapter 9, we discuss the function of cardiac muscle in the context of the cardiovascular system as well as the role of smooth muscle in blood circulation. We also cover smooth muscle’s role in the digestive system in Chapter 11.
Getting nervous about nervous tissue?
Don’t be. Nervous tissue is relatively simple in one way: Your body has only one type of nervous tissue, and it’s made mostly of only one type of cell, the neuron. You can get lots more information about the nervous system in Chapter 7, if you get the impulse to find out more.
Finding out what cells do
The process of mitosis occurs only in diploid cells — cells that have two copies of the DNA. That is, all cells except the mature sex cells, which are haploid — they have only one copy of the organism’s DNA.
For the purposes of anatomy and physiology, remember that all cells have certain very important features in common, but they differentiate into a vast array of shapes and sizes, containing vastly diverse structures, and having different functions and life cycles.
