In this chapter, we’ll go over some of the topics covered on the Science test—including the scientific method, cell theory and the origins of life, genetics, evolution and natural selection, plants, ecosystems and food chains, the human body and human health, and fossils.
Sometimes science teachers can be long on knowledge and short on teaching skills. You may have had one of these teachers, in which case you are probably convinced that science is tough and deadly dull. In fact, science can be fun if you learn to look in the right places. The GED® test writers say that they frequently get their topics and ideas from Discover, Science News, National Geographic, Popular Science, and American Health. If you want some added confidence for the GED® Science test, you may consider reading one of these magazines from time to time as you prepare for the GED® test. It will help you get into a “science head,” and besides, you may be astonished at how interesting you find these magazines. Not only that, but reading such articles may help you feel more comfortable with the topics tested on the science portion on the GED® test, and may therefore give you an important psychological advantage.
While it is impossible to predict exactly what you’ll see on the Science test, the folks who write the test do indicate that there are certain topics that they test regularly. In this chapter, we’ll review some of the test writers’ favorite life science topics. After each review, we’ll provide you with GED®-style practice questions based on those particular topics.
Here are the specific subjects we’ll cover in this chapter:
Scientific method
Cell theory and the origins of life
Genetics
Evolution and natural selection
Plants
Ecosystems and food chains
The human body and human health
Fossils
How did scientists learn anything about biology, the solar system, or the movement of continents in the first place? In each case, a scientist first came up with a hypothesis to explain an event that was not yet understood. A hypothesis is simply a possible explanation of an event or a phenomenon. Once she has her hypothesis, the scientist performs experiments to see whether the hypothesis is correct. These experiments must be carefully designed to make sure that the information they provide is accurate and that each experiment tests only one phenomenon at a time. For example, if you were testing whether aspirin alleviates headaches, you would have to make sure that the test subjects took only aspirin—nothing else. A good scientist would properly wonder whether it might be the water the subjects took to swallow the aspirin that actually alleviated the headache. As you refine your hypothesis, you are attempting to remove any confounding external forces or internal sources of error. By controlling for both internal and external factors, you are establishing a controlled sample that reduces the potential of encountering issues with validity and reliability.
Often, an experiment will be repeated many times to make sure that the same results occur. Sometimes the scientist will use what is called a control group—a group of test subjects that are not subjected to the phenomenon being tested. For example, a scientist who is studying the effect of microwaves on cancer cells (like the man in the passage you just read in the last chapter) may also study a second group of cancer cells that are not exposed to microwaves. This is called the control group. The scientist will watch both groups closely. After all, how will he know if the cells exposed to microwaves behave abnormally if he doesn’t know what normal is?
Only after other scientists have conducted the same experiments and obtained the same results is a hypothesis accepted as fact. And even then, scientists continue to reexamine their own thinking and the thinking of their predecessors. Sometimes accepted facts turn out to be wrong.
On the GED® Science test, you will occasionally be asked to evaluate the accuracy of information or the relevance of a method. These can be the most complicated questions on the test—and they almost always involve your thinking through the question based on the scientific method. Let’s look at an example.
Scientists conducted a study to determine whether drinking a small amount of alcohol each day reduces one’s chances of suffering from a heart attack. They tested 400 first-heart-attack survivors who maintained daily fitness routines and healthy diets by giving these individuals one alcoholic drink per day for 20 years. The results of the study are shown in the chart below, along with the incidence of second heart attacks among first-heart-attack survivors nationwide.
Incidence of Second Heart Attacks Among Study Participants
Incidence of Second Heart Attacks Among Average First-Heart-Attack Survivors Nationwide
1. Based on the results of the study, scientists concluded that drinking alcohol in small amounts lowers the risk of heart attack.
Which of the following represents a flaw in the study above?
A. The sample size of the study was too small to be effective.
B. The study was not performed over a long enough period of time to provide reliable data.
C. The exercise and diet routines of the subjects may have affected their incidence of heart attack.
D. The scientists did not perform the study on animals before attempting to study humans.
Here’s How to Crack It
As soon as you see the word “study,” you can be pretty sure that this passage will concern the scientific method. Not every evaluation question is about a study or experiment, of course, but clearly this one is. You’re asked to find a flaw in the study, so start by using Process of Elimination to get rid of some of the more clearly wrong answers first. Choice (A) would be a good answer if the number of study participants was small, but a sample size of 400 individuals seems large enough to be meaningful. Thus, (A) is incorrect. Similarly, the study was conducted over a period of 20 years, so the length of the study appears to be long enough to provide reliable data. Therefore, (B) is incorrect. Choice (D) seems unrealistic; even if testing animals before testing humans is preferable, you can’t recommend a new form of treatment for mass human consumption without first testing it on at least a few human subjects. Choice (C), however, describes a serious flaw in the study. This choice points out that the study observed the effects not just of a single variable—alcohol consumption—but of other variables—exercise and diet routines. In this case, the lower incidence of second heart attacks among study subjects might be due, not to alcohol consumption, but to better-than-average exercise and diet routines. As noted above, it’s much better to measure a single variable at a time than to attempt to measure multiple variables; otherwise, you don’t know which of the variables is really responsible for the results.
If you found this question challenging, it might help to remember that when the GED® test writers select a passage about an experiment, they like to see how much you know about the scientific method. Typical flaws in experiments include relying on a sample size that is too small, failing to include a control group, and allowing a sample to be tainted by other possible causes of the phenomenon in question (as was the case in this question).
control group—the group of people not being subjected to a phenomenon in a study
hypothesis—a possible explanation of an event or phenomenon
sample size—the number of subjects in a study
variable—an element that changes in a study
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 and 2 refer to the following passage.
In the year 79 A.D., Mount Vesuvius erupted and covered the town of Pompeii in about 5 m of ash, killing most of the inhabitants. However, much of the town was preserved under the volcanic debris which allowed researchers to learn a lot about the way of life in Pompeii when it was discovered 1,500 years later. Over the next two hundred years, it was determined that Pompeii must have had an intricate system for transporting water as well as an amphitheater that was used for entertainment and even a gymnasium for exercise.
Scientists found remains that also included many fossilized animals which have been used to better understand the wildlife that was present during that time period. Among the fossils was a bird that scientists believe to be part of a species that is now extinct. The bird had a slightly longer wing-to-body ratio than any known species that is alive today. It also has a stout and strangely pointed beak that may have given it a foraging advantage over competitors.
1. Based on the information in the passage, which of the following scientific conclusions is the most reasonable?
A. The volcanic eruption at Pompeii was responsible for the extinction of this species.
B. The bird did not live in Pompeii, but was on its way south for the winter.
C. No one survived the volcanic eruption at Pompeii.
D. The bird lived at the same time as the eruption.
2. Determine whether each of the following statements is a scientific observation or conclusion. For this drill, write the letters of the statements in the appropriate boxes.
All living organisms are made up of cells. Some organisms, such as amoebas, consist of single cells, while other organisms, such as humans, are composed of trillions of different cells. For instance, long tubular muscle cells, also known as myocytes, make up your muscle tissue, while your skin is comprised of skin cells. While the cells in your body may look different and have different functions, they are genetically identical.
There are two main types of cells: prokaryotic cells, which lack structure, and are generally found in single-cell organisms such as amoebas, and eukaryotic cells, which are very structured and which work together. Eukaryotic cells generally appear in more complex, multicellular living things.
All cells have a nucleus containing the essential genetic information of the organism of which they form a part. This information is made up of DNA and is contained in structures called chromosomes. Cells receive nourishment through diffusion, the process by which molecules spread from regions of high concentration to regions of low concentration. For example, the food that we eat is broken down and then transported in a watery solution throughout our bodies. The nutrients that individual cells need then diffuse through the permeable outer membrane of the cells, where the nutrients are converted to energy.
Most cells reproduce all by themselves in a process called mitosis, in which a cell splits into two new ones, each an exact copy of the original cell. Reproductive cells in animals divide by a different process called meiosis, in which the new cells contain only half the chromosomes of the original cell. Thus, when the sperm of a male unites with the egg of a female, together they make up the correct number of chromosomes, because the two “halves” (sperm cell and egg cell) make a “whole.”
Single-cell organisms can be as complex in structure as the plantlike algae and the animal-like amoeba or as relatively simple in structure as bacteria and viruses. If a microbial organism can cause disease, it is called a pathogen. Many bacteria and viruses are considered pathogens, although some kinds of bacteria can be helpful to humans. For example, certain kinds of bacteria in the small intestine help us digest food.
In addition to being composed of cells, complex organisms such as humans require organ systems to assist them in completing activities necessary for life. These essential functions of life include the following:
Circulation. This involves the movement of substances within an organism or its cells.
Excretion. This process involves removal of waste, whether that waste be in the form of carbon dioxide, urine, or sweat.
Growth. This is the process by which organisms increase the number of cells of which they are composed or increase in size.
Reproduction. A single organism may not need to reproduce in order to maintain its own health, but each species as a whole cannot survive unless its organisms reproduce. There are two types of reproduction: asexual reproduction, which involves only one parent, and which produces offspring identical to that parent, and sexual reproduction, which requires two parents, and which produces offspring that possess features that are a combination of both parents.
Movement. This simply involves a change in position of a living thing, and allows shifts in circulation, muscles, and other systems.
Nutrition. This is the process in which living things take in substances that allow them to grow and repair themselves. There are two types of nutrition: autotrophic nutrition, in which living organisms make their own food, and heterotrophic nutrition, in which organisms acquire food from their environments.
Synthesis. This is the process by which organisms take small, simple substances, and combine them to form larger, more complex substances.
Now that you know a few basics of cell theory, check out the question on the next page to discover how the GED® test writers might construct a question based on cell theory.
When a cell undergoes mitosis, it duplicates into two genetically exact replicas of itself, so that the new cells have exactly the same number of chromosomes as did the original cell. When a cell undergoes meiosis, it creates four cells, each with half the number of original chromosomes. Meiosis, unlike mitosis, takes place in two rounds.
Scientists conduct an experiment on three different cells, and track the number of resulting cells and chromosomes after each cell undergoes either meiosis or mitosis, according to the cell type. The results are shown below.
1. In trial 3, the cell undergoes 
and each resulting cell has 
chromosomes.
Here’s How to Crack It
Note that the final number of cells, after the cell in trial 3 divides, is 4. Thus, according to the information in the passage, the cell must have undergone meiosis, rather than mitosis. In meiosis, each resulting cell has half of the number of chromosomes of the original cell, so each resulting cell will have half of 46, or 23 chromosomes.
organelles—specialized structures within a living cell
cytoplasm—area between the membrane and nucleus that contains organelles
mitochondria—the powerhouses of the cell that supply energy and break down food and sugar into water
vacuole—large water-filled sac that stores food, water, and waste
plasma membrane—boundary between the cell and environment that regulates the products that enter and exit the cell
nuclear membrane—boundary between the nucleus and the rest of the cell that is permeable to molecules like DNA, RNA, and ATP
permeable membrane—a barrier that lets certain molecules through it
prokaryotic cells—cells that lack structure
eukaryotic cells—cells that are structured and work together
nucleus—part of a cell containing its essential genetic information
chromosomes—structures of DNA
diffusion—the process by which molecules spread from regions of high concentration to regions of low concentration
mitosis—the process in which cells divide into new ones, each of which is an exact copy of the original
meiosis—the process by which cells split into new ones, each of which contains only half as many chromosomes as the original cell
pathogens—microbial organisms that can cause disease
essential functions of life—processes that include circulation, excretion, growth, reproduction, movement, nutrition, and synthesis
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the following information.
Tuberculosis is caused by the mycobacterium, Mycobacterium tuberculosis, which kills approximately half of all infected patients. For many years, M. tuberculosis was susceptible to drug therapy; however, like many other disease-causing organisms, new drug-resistant strains of tuberculosis have emerged. Many theories have been postulated to explain the emergence of these resistant strains (also known as “superbugs”) but most healthcare professionals blame the over-prescription of antibiotics as well as their misuse by patients.
Today, before a tuberculosis patient can be treated, it is first necessary to find out which strain of bacterium is causing the disease and then to find a drug that will kill that particular strain. Fortunately, a new technique has been discovered that aids this process. Scientists can now insert luciferase, the enzyme that makes fireflies glow in the dark, into tuberculosis cells taken from the patient, which makes the tuberculosis cells glow in the dark (see Figure).
With their new glow-in-the-dark organisms, scientists can then test different drugs to see if they kill the bacteria (which stop glowing). This new test can be done in days, which is faster than previous tuberculosis tests which required up to five weeks.
1. If a tuberculosis cell with luciferase is treated with a drug but keeps on glowing, which of the following is most likely the case?
A. A higher dosage of the drug is needed.
B. The tuberculosis cell has been destroyed.
C. The cell is resistant to that particular drug.
D. Both A and C are correct.
2. What is likely to be the most important reason to save time on a tuberculosis test?
A. The patient can begin effective drug therapy that much sooner.
B. The scientists can perform twice as many tests and thus have more time for other experiments.
C. The enzyme luciferase lasts almost indefinitely.
D. Other types of bacteria have shown themselves less able to mutate.
3. The following chart depicts the dose response curve of M. tuberculosis to an antibiotic drug. If a scientist knows that an intensity score of 20 or less is considered acceptable (the remaining bacteria can be eradicated by the host’s immune response), click on the dose of antibiotic that the scientist should recommend for patients.
Genetics is the science of genes, and it involves the study of how parents pass on traits to their offspring. In order to begin learning the vocabulary of basic genetics, the first two genetics terms that you need to know are genotype and phenotype. A person’s genotype is the genetic code that he or she carries, and it provides information about particular traits. Phenotypes are the expressions of those traits. For example, your hair color, which is an expression of instructions carried in your genetic code, is a phenotype. Phenotypes often depend upon genotype, but they can also result from environmental factors.
You have inherited many of your traits from your parents; perhaps people tell you that you have your mother’s smile or your father’s eyes. Your parents passed along these traits through chromosomes, which were introduced in the previous section, and which are made up of wound-up DNA. Each human chromosome pair consists of one chromosome from the father and another from the mother, which is why you resemble, at least to some extent, both your father and mother. You could think of the genes, or sections of DNA, as sets of instructions for proteins, or traits. Of course, children inherit multiple traits from their parents, so it’s not surprising that human cells have not only more than one gene, but also more than one chromosome. In fact, human cells have 23 pairs of chromosomes, or 46 chromosomes each. Of those 23 pairs, one pair is made up of sex chromosomes. Male chromosomes are known as XY chromosomes, while female chromosomes are known as XX chromosomes. The remaining 22 pairs of chromosomes are called autosomes, and each autosome pair is a homologous pair, or a pair of chromosomes, one from the father and one from the mother, with the same structure.
If genes are instructions for traits, alleles are versions of the instructions for traits. For example, you might have an allele for attached earlobes, or you might have an allele for detached earlobes. Each allele is a different form of the same gene.
So how might GED® test writers test genetics concepts? Check out the following question.
Chromosomes come in pairs and contain genes, which code for inherited traits. If a gene is always expressed when present, it is called dominant and is represented by a capital letter. A dominant gene will be expressed whether an individual is pure and has two copies of the dominant gene or is hybrid and contains one dominant copy and one copy of another gene. In the case of hybrids, the dominant gene hides the expression of the recessive gene. A recessive gene is expressed only when it is pure, and it is represented by a lowercase letter.
In pea plants, the gene for round pea shape is dominant and the gene for wrinkled pea shape is recessive. A plant that produces wrinkled peas must be pure for the wrinkled gene, meaning it must contain two copies of the wrinkled gene. A plant that produces round peas can either be pure for the round gene or be hybrid and have one gene for round shape and one gene for wrinkled shape.
Punnett Squares are charts used to predict the odds of specific gene combinations in offspring. Below is a Punnett Square of a cross between a pea plant purebred for wrinkled peas and a pea plant that produces round peas.
1. Which of the following describes the offspring of this cross?
A. Half produce round peas and half produce wrinkled peas.
B. All produce wrinkled peas.
C. All produce round peas.
D. Half produce wrinkled peas and half produce both round and wrinkled peas.
Here’s How to Crack It
Half of the offspring (rr) possess only the genes for wrinkled peas, and will therefore produce wrinkled peas. The offspring in the other half of the square (Rr) possess one gene for wrinkled peas and one for round peas. Since the round gene is dominant over the wrinkled gene, these offspring will produce round peas. Thus, (A) is correct.
genotype—genetic code that a person carries and that provides information about particular traits
phenotype—expressions of genetic traits and traits that result from interactions of the genotype with the environment
genes—sections of DNA
autosomes—pairs of chromosomes other than the sex chromosomes
homologous pair—a pair of chromosomes, one from the father and one from the mother, with the same structure
alleles—versions of the instructions for genetic traits
hybrid—having one gene for one trait and another gene for a different trait
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the following information.
When an allele is represented by a capital letter, it is said to be dominant, and will always be expressed when coupled with another dominant allele. Recessive genes, which are represented by a lowercase letter, are expressed only when an individual is pure and has two copies of the allele.
In kittens, the gene for pointy ears is dominant and the gene for floppy ears is recessive. Kittens will express the pointy ear trait if the kitten is pure or has a hybrid of alleles. In order for floppy ears to be expressed on a kitten, the kitten must be pure for the recessive gene.
Punnett Squares are charts used to predict the odds of specific gene combinations in offspring. Below is a Punnett Square of a cross between two heterozygous parents.
1. What is the probability that an offspring will have pointy ears?
2. According to the information presented, what genotype should replace the “?” in the Punnett Square?
A. PP
B. Pp
C. pP
D. pp
3. The genotypic ratio of this Punnett Square is best expressed as 
It was Charles Darwin who first published the theory of evolution. This theory holds that species change slowly in response to factors in their environment. For example, if Earth’s climate started to get colder, dogs that happened to have thicker, warmer fur would tend to survive better than members of their species with thinner fur, thus allowing the survival of the fittest. The surviving dogs would pass on the thicker fur gene to their puppies, and so this trait would gradually become a prevailing characteristic of their species. This process in which traits that help an organism survive gradually triumph over traits that don’t is called natural selection.
The other way evolution operates is genetic drift. Genetic drift is the accumulation of changes in the frequency of alleles (versions of a gene) over time due to sampling errors—changes that occur as a result of random chance. For example, in a population of owls there may be an equal chance of a newly born owlet having long talons or short talons, but due to random breeding variances a slightly larger number of long-taloned owlets are born. Over many generations, this slight variance can develop into a larger trend, until the majority of owls in that population have long talons. These breeding variances could be a result of a chance event—such as an earthquake that drastically reduces the size of the nesting population one year. Small populations are more sensitive to the effects of genetic drift than large, diverse populations.
Just as new species are formed by natural selection and genetic drift, other species may become extinct. Extinction occurs when a species cannot adapt quickly enough to environmental change and all members of the species die.
Darwin theorized that all species on Earth descended from one or two very simple organisms that gradually evolved into different kinds of more complex organisms. Thus, according to Darwin, human beings are thought to have evolved from simple sea creatures that gradually changed over millions of years into a common ape-like ancestor that in turn gradually evolved into Homo sapiens.
Today, animals are divided into two categories: invertebrates and vertebrates. Invertebrates do not have a backbone—they include worms, jellyfish, and insects. Vertebrates do have a backbone and include most of the animals you know, including fish, birds, and mammals. Mammals are animals that nurse their young. The whale is the largest mammal. Homo sapiens (otherwise known as human beings) are also mammals.
Ready to try a question on evolution and natural selection? Work the problem below.
1. Based on the information in the chart above, which of the following animals are vertebrates?
A. crustaceans
B. amphibians
C. insects
D. round worms
Here’s How to Crack It
Vertebrates are animals with backbones, so look for an animal that appears on the right-hand side of the chart. Crustaceans, insects, and round worms are all animals that appear on the left-hand side of the chart, under “Animals without Backbones,” so (A), (C), and (D) are incorrect. Amphibians appear under the right-hand side of the chart, under “Animals with Backbones,” so (B) is the correct answer.
survival of the fittest—another name for natural selection
adaptation—short-term changes in a species
evolution—long-term changes in a species
genetic drift—the accumulation of changes in the frequency of versions of a gene over time due to changes that occur as a result of random chance
invertebrates—animals without backbones
vertebrates—animals with backbones
mammals—animals that nurse their young
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 and 2 refer to the following passage.
Nearly 75% of plant and animal species on Earth, including all non-bird dinosaurs, went extinct during the Cretaceous-Paleogene (or Cretaceous-Tertiary) extinction event. Scientists have spent years speculating about what precipitated this extinction. Various hypotheses have been presented to account for the disappearance of these huge creatures. The prevailing theory is that a large asteroid collided with Earth and its impact raised a cloud of dust that prevented sunlight from reaching the planet’s surface for many years. Temperatures fell rapidly, and without sunlight, much of the plant life on the planet’s surface disappeared. Without warmth or food, the dinosaurs, as well as many other species, became extinct.
In support of this theory, scientists have found a thin layer of sediment in marine and terrestrial rocks that contains metals that are rare on Earth but abundant in asteroids. Additionally, a large crater was found in the Gulf of Mexico that must have been formed when an asteroid hit Earth at roughly the same time as the extinctions took place.
Amid the destruction was opportunity for evolution. There were countless vacated ecological niches and the surviving organisms rapidly adapted and developed in order to occupy these niches. It is believed that several of the mammals, including primates, that are alive today developed during this time.
1. If the preceding information is true, which of the following is the most likely explanation for the fact that there is still life on Earth today?
A. Some forms of life were able to survive in spite of the lack of sunlight.
B. The dinosaurs went into hibernation.
C. Sunlight is unnecessary for the survival of plant forms.
D. There was no food at all on the planet’s surface during these years.
2. If Charles Darwin read this passage, would he agree with the statements made in the last paragraph?
A. No. According to Darwin’s theory of natural selection, organisms were destined to fulfill specific niches only when it was best for their evolutionary fitness.
B. No. Darwin would not have considered a tragedy to be an opportunity.
C. Yes. Darwin believed that slight differences between organisms can be selected for through natural selection and that this would be exacerbated in a time when there were many potential ecological niches.
D. Yes. Darwin believed that individual organisms were capable of evolution.
Most plants are made up of many cells. Plants make their own food through a fascinating process called photosynthesis. Plant cells contain a chemical called chlorophyll, which gives plants their green color. When a plant’s leaves absorb sunlight, the chlorophyll in the leaves converts the sunlight into energy. Meanwhile, the roots of the plant have been drawing water from the ground all the way up into the leaves. The plant uses the energy it has just converted to split the water (H2O) into its two components—hydrogen and oxygen. The hydrogen is used by the plant, along with carbon dioxide from the air, to create sugar and starches for its own nutrition. The oxygen is released into the atmosphere—incidentally making it possible for humans to go on breathing.
The growth process of plants is dependent on the seasons. Some plants last only one growing season and die during the winter. To carry on the species, these plants produce seeds, which lie fallow during the winter months and then germinate in the spring. Other plants, called perennials, live for many years. The upper shoots of a perennial wither and die during the winter, but the roots live through the winter and produce new shoots in the spring.
Trees are also a kind of plant whose stalks have become woody. Trees survive the winter because their trunks protect them from the cold. If you were to cut down a tree and look at a horizontal cross section inside, you would find many circular rings. This is because in the spring and summer, the tree grows more rapidly, producing wider pores. During the winter, the growth is slower, producing denser wood. By counting the rings of a tree, you can tell how old it is.
Check out the passage on plants below, and answer the question that follows.
When plants respond to an environmental stimulus by growing in a particular way, that response is called a tropism. There are five kinds of tropism:
phototropism—a response in which plants turn toward the light
geotropism—a response in which roots grow toward the earth and the shoots and flowers grow toward the sky
thigmotropism—a response in which plants curl around any object they touch
hydrotropism—a response in which plants’ roots grow toward a source of water
chemotropism—a positive attraction of roots toward the presence of certain chemicals
A student decides to study the way in which a particular plant responds to sunlight over a six-month period. At the beginning of the study, the student moves the plant, which originally bent toward a nearby window, so that plant bends directly away from window and the angle between the direction in which the plant bends and the window is 180𠂰. The graph below shows the changes in the way that the plant bends as the year progresses.
1. The plant’s movements toward the window are an expression of which of the following?
A. phototropism
B. geotropism
C. thigmotropism
D. chemotropism
Here’s How to Crack It
This question is not about the roots but about the visible part of the plant above the soil. Based on that information, only two choices are possible: phototropism and thigmotropism. Because the question does not mention curling, and according to the graph, the plant bends toward the window at the end of the study, the correct answer to this question must be (A).
stomates—the microscopic openings through which gases diffuse into and out of leaves during photosynthesis
pistil—female reproductive organs of a flower
stamen—male reproductive organs of a flower
photosynthesis—the process through which plants make their own food
chlorophyll—the chemical which provides plants with their green color
germinate—begin to grow or put out shoots
perennials—plants that live for many years
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the following passage.
If you cut a horizontal cross section through the trunk of a tree, you can see the tree’s growth rings (also known as tree rings). Each ring inside the trunk of a tree represents the annual growth of the tree. There are two parts of a growth ring. The inner part is formed during the early growth season (spring and early summer in North America) and is relatively less dense. The outer part of each ring is denser and is formed during the late summer and autumn.
The growing conditions of a particular year will affect the size of a ring. In ideal growing conditions (such as adequate moisture and a relatively long growing season), a ring may be larger because the tree was able to grow more during that time. However, a cold summer or a drought can cause the ring size to be smaller than normal.
Researchers can use tree rings to learn about the growing conditions that were present during a particular time period. The tree depicted below was cut down at the end of the growing year in 2000.
1. Based on the figure, which of the following years was the best growing season?
A. 2000
B. 1999
C. 1998
D. 1997
2. How old is the tree represented in the figure?
A. 6 years
B. 7 years
C. 8 years
D. 9 years
3. The following graph compares tree ring size to the average amount of monthly rainfall.
In the graph above, average monthly rainfall is the 
variable, while tree ring size is the Select 
variable.
An ecosystem is a place where many organisms live and depend on other organisms for survival. For example, grass uses the energy of the sun to grow, a rabbit eats the grass, a hawk eats the rabbit. Every animal is eating something else to get energy, except for the grass, which gets its energy from sunlight through photosynthesis.
In any given ecosystem, the total number of resources is limited. A given region, for example, can support only so many rabbits before the rabbits begin to run out of grass to eat and their population begins to dwindle. When this happens, the hawk population will also begin to dwindle. The carrying capacity of a particular ecosystem is the maximum population size of a given species that can exist in that ecosystem given the food, water, and other resources available in the region.
To balance an ecosystem, all of the different organisms, including plants and animals, have certain jobs. Let’s look at the different roles that plants and animals have in an ecosystem.
Plants get energy from the sun to make food. Plants are producers because they make (produce) their own food. Animals, unlike plants, need to eat to get energy. Animals are consumers because they must eat (consume) other living things for food. Primary consumers eat only producers. Rabbits and cows are examples of primary consumers because they eat only plants. A consumer that eats another consumer is called a secondary consumer. Frogs are secondary consumers because they eat flies, grasshoppers, and other consumers.
All primary consumers eat plants, and the scientific term for these consumers is herbivores. Secondary consumers that eat only animals and not any plants are called carnivores. Many consumers eat both plants and animals, depending on what is available. These consumers are called omnivores.
There are other organisms in an ecosystem besides producers and consumers. These organisms, like earthworms, bacteria, and fungi, eat dead organisms and return nutrients back to the soil for plants to use. These organisms are called decomposers because they break down other organisms to get energy.
The food chain is the process by which organisms pass energy from one to another. The first step in a food chain occurs when a green plant absorbs energy from the sun and converts it to chemical energy through photosynthesis. The second step occurs when an animal eats the plant, converting the energy contained in the plant into energy that the animal can store in its cells. Following this, a larger animal then eats the first animal, and stores the energy from that animal in its own cells. This process may continue through several steps. Eventually, at the top of the food chain, a fungus or bacterium breaks down decaying organic matter. The figure on the next page shows an example of a food chain.
Food Chain
An alternative way to show how animals transfer energy to one another is by using a food web. This is a slightly more complicated chart that takes into account the fact that there is often more than one animal vying for a particular kind of food.
Food Web
Not all relationships between animals are predatory. Some are mutualistic, meaning that both organisms in a relationship benefit from each other. For example, insects that eat nectar from a flower are also helping the flower. The flower’s pollen sticks to the insects as they eat, and then the pollen drops off as they fly to another flower.
Some are parasitic, meaning that one organism benefits while another organism suffers. Think about fleas on a dog. The fleas bite the dog to get energy from its blood and in return, the poor dog gets itchy welts and possible diseases. In this kind of relationship, the flea is called a parasite and the dog is called a host. Tapeworms, ticks, and lice are other examples of parasites.
Some exhibit commensalism, meaning that one organism is helped while the other organism is neither helped nor harmed. A great example of commensalism is the relationship between a remora and a whale. The remora has a sucker on the top of its head that attaches to the whale’s top, mouth, or underside. Through this attachment, the remora can travel around more easily and eat any debris that falls from the whale. The whale is left unharmed.
1. A food web in an ecosystem is shown below:
Select the producer from the list below, and drag it into the box.
Select the herbivores from the list below, and drag them into the box.
Select the carnivores from the list below, and drag them into the box.
Here’s How to Crack It
Plants make their own food, and are therefore producers. Thus, you should select plants as the producers. Herbivores eat plants, and based on the food web, deer, rabbits, and mice eat plants, so you should select these as the herbivores. Carnivores are animals that eat other animals. Coyotes eat deer and rabbits; hawks eat rabbits, mice, and snakes; and snakes eat mice. Therefore, coyotes, hawks, and snakes are carnivores.
producers—green plants and algae that take the sun’s energy and convert it into chemical energy
consumers—animals that eat the producers and/or sometimes each other
decomposers—organisms such as bacteria that live on dead or decaying organic matter, breaking it down into its basic ingredients and beginning the life process all over
ecosystem—a place where many organisms live and depend upon other organisms for survival
carrying capacity—the maximum population size of a given species that a certain ecosystem can support
herbivores—primary consumers, or living things that eat plants
carnivores—secondary consumers, or animals that eat only animals
omnivores—animals that eat both plants and animals
food chain—the process by which organisms pass energy from one to another
food web—a chart that shows the way that organisms pass energy from one to another, and that takes into account the fact that there is often more than one animal vying for a particular kind of food
mutualistic relationships—relationships in which organisms benefit from helping each other
parasitic relationships—relationships in which one organism benefits while another suffers
commensalism—a class of relationships in which one organism benefits while another is neither helped nor harmed
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the food web below.
1. Which of the following organisms in the food web above are eaten by both land and sea creatures?
A. minnows
B. wolves
C. salmon
D. groupers
2. The absence of on the food web above would most devastate the rest of the web.
A. minnows
B. wolves
C. salmon
D. algae
3. Drag and drop the organisms from the list below into the following categories.
The human body is truly remarkable. Eleven different systems work together to keep your heart pumping, air in your lungs, and the rest of your body functioning. You’ll need to know something about these systems in order to successfully tackle the GED® Science test, so you may find it helpful to familiarize yourself with them.
First, the circulatory system allows blood to circulate through your body and deliver oxygen, nutrients, and hormones to your cells, and carry away wastes such as carbon dioxide. It helps you to stay nourished, maintain a constant body temperature, and fight disease. Your lymphatic system is actually part of your circulatory system, and is composed of lymphatic vessels that carry lymph, a clear fluid, toward your heart. Each day, approximately 17 of your 20 liters of body’s plasma are reabsorbed into your blood vessels, but 3 liters get left behind. The lymphatic system creates an alternative way for this plasma to return to your blood.
Next, the integumentary system is composed of your skin, hair, and nails, and helps to protect your body from damage from its environment. The urinary system, otherwise known as the renal system, includes your bladder, kidneys, ureters, and urethra. This system is responsible for eliminating waste from your body, regulating blood pH, controlling your electrolyte and metabolite levels, and regulating your blood pressure and volume. Your lungs make up your respiratory system, moderate your intake of oxygen and expulsion of carbon dioxide. Your muscular system consists of your skeletal, cardiac, and smooth muscles, and allows you to move, remain upright, and circulate blood through your body. It works closely with your nervous system, which controls your muscular system and regulates both voluntary and involuntary motions by transmitting signals to various parts of your body. Your bones—all 270 of them at birth—make up your skeletal system, which supports your body and allows it to move, produces blood cells, regulates endocrine, and stores ions. Your digestive system includes your mouth, esophagus, stomach, intestines, anus, and rectum. It helps you break down food so that it can be absorbed into your body, and it eliminates waste. The reproductive system, which is made up of the sex organs, as well as fluids, pheromones, and hormones, allows humans to have offspring. Your endocrine system, which consists of glands that secrete hormones into your circulatory system, is made up of glands that contribute to actions such as sweating and salivating.
Why do you need so many systems? One reason is that your body needs to maintain homeostasis, or stable internal conditions. For example, you need to maintain a constant internal temperature, a constant pH, and constant blood glucose levels. Your body’s systems work together to maintain homeostasis.
Of course, nutrition and environment also strongly contribute to your health. Poor nutrition may cause diseases such as diabetes, scurvy (a disease that results from a lack of vitamin C), and cardiovascular disease, among others.
Try out the following human health and human body question.
The human body maintains itself through several systems. For example, the integumentary system, which is composed of skin, hair, and nails, helps to protect the body from harm. The endocrine system consists of glands that secrete hormones into your circulatory system, such as the mammary, pituitary, salivary, and sweat glands. Lungs make up the respiratory system, and moderate intake of oxygen and expulsion of carbon dioxide. The muscular system consists of skeletal, cardiac, and smooth muscles, and allows humans to move and remain upright, and circulates blood through the body. These, along with seven other systems, maintain homeostasis, or stable internal conditions. When the body is no longer in homeostasis, these systems act to bring the body back to equilibrium.
1. On a hot summer day, a runner quickly becomes overheated. In order to bring down his internal temperature, the runner’s body releases sweat.
This is an example of the 
system glands being activated to 
.
Here’s How to Crack It
According to the passage, “the endocrine system consists of glands that secrete hormones into your circulatory system, such as the mammary, pituitary, salivary, and sweat glands.” Thus, sweat glands are a part of the endocrine system, so the answer to the first question is “endocrine.” The passage also states that the bodily systems listed, “along with seven other systems, maintain homeostasis, or stable internal conditions. When the body is no longer in homeostasis, these systems act to bring the body back to equilibrium.” When a runner is overheated, he or she no longer has stable internal conditions, so the body’s systems act to lower that temperature. Thus, the correct answer to the second question is “return the body to homeostasis.”
systems of the human body—the circulatory system, the lymphatic system, the integumentary system, the urinary system, the respiratory system, the muscular system, the nervous system, the skeletal system, the digestive system, the reproductive system, and the endocrine system
homeostasis—stable internal conditions
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the following information.
The human body maintains equilibrium through the use of multiple bodily systems. For example, the muscular system, which circulates blood through your body and allows you to move, is comprised of skeletal, cardiac, and smooth muscles. Working closely with the muscular system, the nervous system controls your muscular system and regulates both voluntary and involuntary motions by sending signals to parts of the body. Likewise, the skeletal system works in harmony with the muscular system, providing internal organs with protection and working with muscles to enable movement. Along with the other bodily systems, the muscular, nervous, and skeletal systems work together to keep the body in equilibrium.
1. Which of the following bodily systems from the list below play a role in movement? Choose all that apply.
Bodily Systems that Play a Role in Movement
2. Susan suffered from a heart attack in February, which primarily impacted the 
muscles within the 
system.
3. Franklin is walking through the desert when he comes upon a poisonous scorpion. Franklin’s “flight or fight” response is initiated by his endocrine system and he then reacts by running in the opposite direction of the scorpion. Which system is responsible for triggering Franklin’s movement?
A. skeletal
B. endocrine
C. muscular
D. nervous
When we think of a disease, we often think of an illness (which can have a variety of symptoms and effects) that negatively impacts humans, animals, and plants. While you won’t need to know the scientific names of every disease on the GED® test, you should have a general idea about the way in which disease spreads, the impact of disease on a population, and how to combat disease.
Bacteria are single-celled, living organisms. Despite their small size, bacteria have ribosomes and a single loop of DNA, which contains approximately 5,000 genes. Bacteria are also capable of reproduction through a process called binary fission, which occurs when the DNA replicates and the bacterial cell splits into identical daughter cells. So, how can you catch a bacterial infection? A bacterial infection can be transmitted through the air, tainted food or water, an insect bite, or direct contact with open sores on the body. Consider the bubonic plague, which was transmitted both through the air via fleabites, and dysentery, which is contracted when an individual consumes contaminated food or water. Thankfully, most harmful bacteria can be killed with chemicals or boiling water, and many bacterial infections can be treated with antibiotics, which are substances that can destroy or inhibit the growth of infectious bacteria by interfering with their cellular processes. It should be noted that not all bacteria are harmful. There are a number of “friendly” bacteria, such as L. acidophilus, that aid in the digestive process.
A virus is a piece of DNA or RNA encapsulated by a protein coat, or capsid. While a complex virus can have a hundred genes, all viruses are extremely small and no virus is alive. Viruses can’t reproduce independently, don’t perform any biological functions, and rely on living cells for replication through a process called lysis, or the lytic cycle. In the lytic cycle, a virus attaches to a host cell, injects its DNA or RNA into the host cell, and makes hundreds of copies of itself. Eventually, the host cell bursts, releasing the viral copies into the organism. Once replicated, viruses can be transmitted through either air or direct contact. For example, a child could contract the chicken pox through inhalation, but could contract AIDS only through direct contact with infected bodily fluids. Unlike bacterial infections, however, viral infections cannot be treated with antibiotics. Instead, the most effective preventative treatment against contracting a viral infection is immunization or vaccination, the process of stimulating the immune system to produce defenses against harmful viruses by introducing harmless, yet similar, viruses. Unfortunately, vaccines must be continually adapted, as many viruses (such as the common cold) are capable of rapid mutation.
Viral and bacterial pathogens, or disease-causing agents, can have devastating consequences on human, animal, and plant populations. During the Middle Ages, the bubonic plague spread throughout Europe, killing over one-third of the continent’s population. Referred to as the Black Death, victims of the plague suffered from diarrhea, vomiting, chills, and blood-filled boils. Due to the lack of antibiotics and unsanitary living conditions during this time period, tens of millions had died by the time the plague ran its course. If a disease is particularly brutal, it can have a severe impact on the population demographics. In animal populations, for example, a virulent pathogen could cause the species to become extinct. While such fatal bacterial infections, such as the bubonic plague, sometimes occur today in pre-industrialized countries, modern medicine and sanitation practices have greatly reduced mortality rates due to bacterial infections. Conversely, modern sanitation and medicine have not been able to mitigate the consequences of many viral infections.
Acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV) and spread via bodily fluids, is perhaps the most notorious viral disease today. While cells in the immune system fight pathogens in a healthy individual, cells in the immune system are infected and attacked by HIV in an individual with AIDS. In turn, the individual’s immune system is destroyed and the person becomes more susceptible to illness. Unfortunately, the sheer number of varieties and the rapidly mutating nature of HIV make the disease extremely hard to treat. AIDS can have a particularly devastating impact on populations of intravenous drug users or those that cannot afford, or do not emphasize, the use of birth control.
AIDS—Acquired immune deficiency syndrome (AIDS) is a disease caused by HIV that is characterized by a greatly increased susceptibility to infection due to compromised cellular immunity. An individual with AIDS can develop a number of health conditions including pneumonia, tuberculosis, toxoplasmosis, and fungal infections. There is currently no cure for AIDS.
bacteria—Typically single-celled, prokaryotic organisms that exist in large groups. Bacteria can be either beneficial, such as bacteria that decompose organic matter, or harmful, such as bacteria that cause infections.
binary fission—A type of asexual reproduction most common in prokaryotes. Bacterial binary fission is the process utilized by bacteria to carry out cell division, reproduce, and increase the bacterial population.
capsid—A protein sheath that surrounds the genetic material of a virus.
disease—An atypical condition that compromises the normal bodily functions of an organism, typically associated with negative symptoms and feelings of pain and weakness
HIV—Human immunodeficiency virus (HIV) is a virus that attacks the immune system by destroying T-helper white blood cells, leaving an individual with a damaged immune system. HIV can be transmitted through the exchange of bodily fluids such as blood and semen. There is currently no cure for HIV and without antiretroviral treatment, HIV can progress to AIDS.
immunization—The process by which an organism is made immune or resistant to an infectious disease. Generally, immunization occurs via the administration of a vaccine.
infection—A condition in which a pathogen invades and multiplies within the tissues and organs of a living organism. An infection can cause the immune system to be activated, at which point symptoms and disease will present themselves.
lytic cycle—A portion of the viral reproduction cycle during which the virus replicates itself, leading to the destruction of both the infected cell and its membrane.
pathogen—A bacterium, virus, fungus, or other microorganism that can cause disease.
vaccination—A process that allows an organism to achieve a state of immunity to a particular pathogen through artificial means. In active immunization, a weakened or inactive form of a pathogen is introduced to an organism that causes the immune system to respond and create immunological memory, or the ability to remember pathogens for future infections. Passive immunization involves the administration of specific antibodies that provide immediate, temporary protection against a pathogen.
virus—A small, nonliving, infectious agent that is composed of both nucleic acid and a protein coat. While a virus cannot reproduce on its own, a virus can invade an organism’s cell, reproduce within the cell, and then destroy the cell.
You should now have a firm grasp of concepts related to disease transfer that will be tested on the GED® test. Let’s try some sample questions.
You can check your answers in Part VIII: Answer Key to Drills.
Questions 1 through 3 refer to the following information.
Maria is a doctor at the World Health Organization who analyzes bacterial infections, which can be cured with antibiotics that disrupt the living cellular structures of the diseased cells, and viral infections, for which vaccines can be developed. Maria also analyzes the way in which the disease is transferred among individuals, determining whether an infection is spread through the air, contaminated water, animal bites, or direct contact.
Maria has been sent to Ezralia, a small, pre-industrialized state, where the majority of the population lives in squalor, with most homes lacking indoor plumbing and other innovations related to modern sanitation. Over the past six weeks, the people of Ezralia have been suffering from an unidentified disease, known as Mooshika, that is decimating the population. Maria interviews a number of individuals suffering from Mooshika and finds that the majority of the individuals who have suffered from Mooshika drink water from the well on the north side of town, and over 85% of the victims support themselves by raising goats and selling them at the local market.
1. After analyzing the disease, Maria determines that the infection is alive. Therefore, the disease is 
and can be addressed with 
.
2. Based on Maria’s findings, how could Mooshika be transferred? Choose all that apply.
3. All of the following are potential ways to treat and prevent viral infections EXCEPT
A. vaccination.
B. quarantining infected animals.
C. antibiotics.
D. avoiding contact with infected persons.