CONCEPTS IN BIOLOGY

PART I. INTRODUCTION

1. What Is Biology?

1.4. The Science of Biology

The science of biology is, broadly speaking, the study of living things. However, there are many specialty areas of biology, depending on the kind of organism studied or the goals a person has. Some biological studies are theoretical, such as establishing an evolutionary tree of life, understanding the significance of certain animal behaviors, or determining the biochemical steps involved in photosynthesis. Other fields of biology are practical—for example, medicine, crop science, plant breeding, and wildlife management. There is also just plain fun biology—fly-fishing for trout or scuba diving on a coral reef.

At the beginning of the chapter, we defined biology as the science that deals with life. But what distinguishes living things from those that are not alive? You would think that a biology textbook could answer this question easily. However, this is more than just a theoretical question. In recent years, it has become necessary to construct legal definitions of life, especially of when it begins and ends. The legal definition of death is important, too, because it may determine whether a person will receive life insurance benefits or if body parts may be used in transplants. In the case of a heart transplant, the person donating the heart may be legally “dead” but the heart certainly isn’t. It is removed while it is still alive, even though the person is not. In other words, there are different kinds of death. There is death of the whole living unit and death of each cell within the living unit. A person actually “dies” before every cell has died. Death, then, is the absence of life, but that still doesn’t tell us what life is.

Similarly, there has been much controversy over the question of when life begins. Certainly, the egg and the sperm that participate in fertilization are both alive, as is the embryo that results. However, from a legal and moral perspective, the question of when an embryo is considered a separate living thing is a very different proposition.

What Makes Something Alive?

Living things have abilities and structures not found in things that were never living. The ability to interact with their surroundings to manipulate energy and matter is unique to living things. Energy is the ability to do work or cause things to move. Matter is anything that has mass and takes up space. Developing an understanding of how living things modify matter and use energy will help you appreciate how living things differ from nonliving objects. Living things show five characteristics that nonliving things do not: (1) unique structural organization, (2) metabolic processes, (3) generative processes, (4) responsive processes, and (5) control processes. It is important to recognize that, although these characteristics are typical of all living things, they may not all be present in each organism at every point in time. For example, some individuals may reproduce or grow only at certain times. This section briefly introduces these basic characteristics of living things, which will be expanded on in the rest of the text.

Unique Structural Organization

The unique structural organization of living things can be seen at the molecular, cellular, and organism levels. Molecules such as DNA and proteins are produced by living things and are unique to each kind of living thing. Cells are the fundamental structural units of all living things. Cells have an outer limiting membrane and several kinds of internal structures. Each structure has specific functions. Some living things, such as people, consist of trillions of cells, whereas others, such as bacteria and yeasts, consist of only one cell. Nonliving materials, such as rocks, water, and gases, do not have a cellular structure.

An organism is any living thing that is capable of functioning independently, whether it consists of a single cell or a complex group of interacting cells (figure 1.12). Each kind of organism has specific structural characteristics, which it shares with all other organisms of the same kind. You recognize an African elephant, a redwood tree, or a sunflower as having certain characteristics, although other organisms may not be as easy to distinguish.

FIGURE 1.12. Structural Organization

Each organism, whether it is simple or complex, independently carries on metabolic, generative, responsive, and control processes. It also contains special molecules, a cellular structure, and other structural components. DNA is a molecule unique to living things. Some organisms, such as yeast or the protozoan Euplotes, consist of single cells, whereas others, such as orchids and humans, consist of many cells organized into complex structures.

Metabolic Processes

All the chemical reactions involving molecules required for a cell to grow, reproduce and make repairs are referred to as its metabolism. Metabolic properties keep a cell alive. The energy that organisms use is stored in the chemical bonds of complex molecules. Even though different kinds of organisms have different ways of metabolizing nutrients or food, we are usually talking about three main activities: taking in nutrients, processing them, and eliminating wastes.

Energy is expended when living things take in nutrients (raw materials) from their environment (figure 1.13). Many animals take in these materials by eating other organisms. Microorganisms and plants absorb raw materials into their cells to maintain their lives. Nutrient processing takes place once the nutrients are inside the organism or its cells. Most animals have organs that assist in processing nutrients. In all organisms, once inside cells, nutrients enter a network of chemical reactions. These reactions process the nutrients to manufacture new parts, make repairs, reproduce, and provide energy for essential activities. Waste elimination occurs because not all materials entering a living thing are valuable to it. Some portions of nutrients are useless or even harmful, and organisms eliminate these portions as waste. Metabolic processes also produce unusable heat energy, which can be considered a waste product. Microorganisms, plants, and many tiny animals eliminate useless or harmful materials through their cell surfaces, but more complex animals have special structures for getting rid of these materials.

FIGURE 1.13. Metabolism

The metabolic processes of this hummingbird include the intake of nutrients in the form of nectar from flowers.

Generative Processes

Generative processes are activities that result in an increase in the size of an organism—growth—or an increase in the number of individuals in a population—reproduction (figure 1.14). Growth and reproduction are directly related to metabolism, because neither can occur without gaining and processing nutrients.

FIGURE 1.14. Generative Processes

Generative processes as they relate to cells.

During growth, living things add to their structure, repair parts, and store nutrients for later use. In large organisms, growth usually involves an increase in the number of cells present.

Reproduction is also an essential characteristic of living things. Because all organisms eventually die, life would cease to exist without reproduction. Organisms can reproduce in two basic ways. Some reproduce by sexual reproduction, in which two individuals each contribute sex cells, which leads to the creation of a new, unique organism. Asexual reproduction (without sex) occurs when an organism makes identical copies of itself. Many kinds of plants and animals reproduce asexually when a part of the organism breaks off the parent organism and regenerates the missing parts.

Responsive Processes

Responsive processes allow organisms to react to changes in their surroundings in a meaningful way. There are three categories of responsive processes: irritability, individual adaptation, and evolution, which is also known as adaptation of populations.

Irritability is an individual’s ability to recognize that something in its surroundings has changed (a stimulus) and respond rapidly to it, such as your response to a loud noise, beautiful sunset, or bad smell. The response occurs only in the individual receiving the stimulus, and the reaction is rapid, because there are structures and processes already in place that receive the stimulus and cause the response. One-celled organisms, such as protozoa and bacteria, can sense and orient to light. Many plants orient their leaves to follow the sun. Animals use sense organs, nerves, and muscles to monitor and respond to changes in their environment.

Individual adaptation also results from an organism’s reaction to a stimulus, but it is slower than an irritability response, because it requires growth or some other fundamental change in an organism. For example, during the summer the varying hare has brown fur. However, the shortening days of autumn cause the genes responsible for the production of brown pigment to be “turned off” and new, white hair grows (figure 1.15). Plants also show individual adaptation to changing day length. Lengthening days stimulate the production of flowers and shortening days result in falling leaves. Similarly, your body will adapt to lower oxygen levels by producing more oxygen-carrying red blood cells. Many athletes like to train at high elevations because the increased number of red blood cells resulting from exposure to low oxygen levels delivers more oxygen to their muscles.

FIGURE 1.15. Individual Adaptation

The change in coat color of this varying hare is a response to changing environmental conditions.

Evolution involves genetic changes in the characteristics displayed within a population. It is a slow change in the genetic makeup of a population of organisms over many generations. Evolution enables a species (a population of a specific kind of organism) to adapt to long-term changes in its environment (figure 1.16). For example, between about 1.8 million and 11,000 years ago, the climate was cold and large continental glaciers covered northern Europe and North America. The plants and animals were adapted to these conditions. As the climate slowly warmed over the last 11,000 years, many of these species went extinct, whereas others adapted and continue in a modified form. For example, mammoths and mastodons were unable to adapt to the changing environment and became extinct, but some species, such as moose, elk, and wolves, were able to adapt to a warming environment and still exist today. Similarly, the development of the human brain and its ability to reason allowed our prehuman ancestors to craft and use tools. Their use of tools allowed them to survive and succeed in a great variety of environmental conditions.

FIGURE 1.16. Evolution

A principle that all scientists work with is the fact that things change over time. We know that chemicals react to become other kinds of substances, mountains crumble, rivers change course, and organisms reproduce and die. Evolution is also a change, but one that takes generations of time and results in descendents with a different genetic makeup than their ancestors. This sequence shows five species that illustrate that body size, leg structure, and food habits changed over time in horses.

Control Processes

Control processes are mechanisms that ensure an organism will carry out all metabolic activities in the proper sequence (coordination) and at the proper rate (regulation).

Coordination occurs within an organism at several levels. At the metabolic level, all the chemical reactions of an organism are coordinated and linked together in specific pathways. The control of all the reactions ensures efficient, stepwise handling of the nutrients needed to maintain life. The molecules responsible for coordinating these metabolic reactions are known as enzymes. Enzymes are molecules, produced by organisms, that are able to control the rate at which life’s chemical reactions occur. Enzymes also regulate the amount of nutrients processed into other forms. Enzymes will be discussed in detail in chapter 5.

Coordination also occurs at the organism level. When an insect walks, the muscles of its six legs are coordinated, so that orderly movement results. In plants, regulatory chemicals assure the proper sequence of events that result in growth in the spring and early summer, followed by flowering and the development of fruit later in the year.

Regulation involves altering the rate of processes. Many of the internal activities of an organism are interrelated and regulated, so that a constant internal environment is maintained. The process of maintaining a constant internal environment is called homeostasis. For example, when we begin to exercise we use up oxygen more rapidly, so the amount of oxygen in the blood falls. In order to maintain a constant internal environment, the body must obtain more oxygen. This requires more rapid contractions of the muscles that cause breathing and a more rapid and forceful pumping of the heart to get blood to the lungs. These activities must occur together at the right time and at the correct rate; when they do, the level of oxygen in the blood will remain normal while supporting the additional muscular activity (figure 1.17).

FIGURE 1.17. Control Process

Working the balance beam involves coordination of heart rate, breathing rate, and muscular activity in a controlled manner.

The Levels of Biological Organization and Emerging Properties

At this point you might be asking, “How can I possibly keep all this in my head?” Even biologists have difficulty keeping track of the vast amount of information being generated by researchers around the world. When you or biologists seek solutions to problems, it should be viewed at several levels at the same time. Doing this helps scientists create connections between different concepts. To be able to do this yourself, you must understand what these levels are. In order to help you, and biologists, conceptualize the relationships that exist at these various levels, this information has been organized into table 1.2. Return to this table as you move through the text to jog your memory and regain your perspective should you get confused.

TABLE 1.2. Levels of Organization for Living Things

Scientists recognize these levels as a ladder of increasing complexity from atoms to biosphere, each displaying new properties not seen on the previous step. These never-before-seen features that result from the interaction of simple components when they form much more complex substances are called emergent properties (figure 1.18). For example, when atoms on the first level interact to form molecules on the second level, new properties emerge that are displayed by the molecules (e.g., the ability to serve as genetic material). In turn, these molecules work together to form the parts of the next higher level, cells. Again, cells have a whole new set of emergent properties—all of life’s characteristics. Continuing on, cells become organized into tissues; tissues into organs; organs into organ systems; and organ systems into organisms. All of these levels of organization exist within you as an individual.

FIGURE 1.18. Emerging Properties

The properties you recognize as a car only become evident when the component parts are correctly assembled.

These levels continue to provide you with a biological context for the world around you. Organisms are grouped into populations on the basis of where they live. Several populations are defined as a community. Now, the levels of organization start to include nonliving environmental characteristics, too. Communities and their environment form ecosystems. Several ecosystems form biomes and, finally, several biomes form the biosphere of our planet. As before, novel properties emerge as you rise through the chart. At the highest level, some scientists begin to view our planet as a type of living entity that has unique emergent properties not found at lower levels of organization.

The Significance of Biology in Our Lives

To a great extent, we owe our high standard of living to biological advances in two areas: food production and disease control. Plant and animal breeders have modified organisms to yield greater amounts of food than did older varieties. A good example is the changes that have occurred in corn. Corn, a kind of grass, produces its seeds on a cob. The original corn plant had very small cobs, which were perhaps only 3 or 4 centimeters long. Selective breeding has produced varieties of corn with much larger cobs and more seeds per cob, increasing the yield greatly. In addition, plant breeders have created varieties, such as sweet corn and popcorn, with special characteristics. Similar improvements have occurred in wheat, rice, oats, other cereal grains and fruits (figure 1.19). The improvements in the plants, along with better farming practices (also brought about through biological experimentation), have greatly increased food production.

FIGURE 1.19. Biological Research Improves Food Production

(a) One food that has seen a vast increase in production and variation is the tomato. Tomatoes (Lycopersicon sp.) originated on the western coast of South America in Peru. Wild tomato species have tiny fruits, and only the red ones are edible. (b) Over the centuries, selective breeding and biotechnology have resulted in the generation of hundreds of varieties of this vegetable.

Animal breeders also have had great successes. The pig, chicken, and cow of today are much different animals from those available even 100 years ago. Chickens lay more eggs, beef cattle grow faster, and dairy cows give more milk. All these improvements increase the amount of food available and raise our standard of living.

Biological research has also improved food production by developing controls for the disease organisms, pests, and weeds that reduce yields. Biologists must understand the nature of these harmful organisms to develop effective control methods.

There also has been fantastic progress in the area of human health. An understanding that diseases such as cholera, typhoid fever, and dysentery spread from one person to another through the water supply led to the development of treatment plants for sewage and drinking water. Recognizing that diseases such as botulism and salmonella spread through food led to guidelines for food preservation and preparation that greatly reduced the incidence of these diseases. Many other diseases, such as polio, whooping cough, measles, and mumps, can be prevented by vaccinations (How Science Works 1.1). Unfortunately, the vaccines have worked so well that some people no longer bother to get them. Furthermore, we have discovered that adults need to be revaccinated for some of these diseases. Therefore, we see that some diseases, such as diphtheria, whooping cough, and chicken pox are reappearing among both children and adults.

They have not been eliminated, and people who are not protected by vaccinations are still susceptible to them. By helping us understand how the human body works, biological research has led to the development of treatments that can control chronic diseases, such as diabetes, high blood pressure, and even some kinds of cancer. Unfortunately, all these advances in health contribute to another major biological problem: the increasing size of the human population.

HOW SCIENCE WORKS 1.1

Edward Jenner and the Control of Smallpox

Edward Jenner (1749-1823) was born in Berkeley, Gloucestershire, in western England. He wanted to become a doctor, so he became an apprentice to a local doctor. This was the typical training for physicians at that time. After his apprenticeship, he went to London and studied with an eminent surgeon. In 1773, he returned to Berkeley and practiced medicine there for the rest of his life.

At that time in Europe and Asia, smallpox was a common disease, which nearly everyone developed, usually early in life. Many children died of it, and many who survived were disfigured by scars. It was known that people who had had smallpox once were protected from future infection. If children were deliberately exposed to smallpox when they were otherwise healthy, a mild form of the disease often developed, and they were protected from future smallpox infections. Indeed, in the Middle East, people were deliberately infected by scratching material from the pocks of an infected person into their skin. This practice was introduced to England in 1717 by Lady Mary Wortley Montagu, the wife of the ambassador to Turkey. She had observed the practice of deliberate infection in Turkey and had had her own children inoculated. This practice had become common in England by the early 1700s, and Jenner carried out such deliberate inoculations as part of his practice. He also frequently came into contact with individuals who had smallpox, as well as people infected with cowpox— a mild disease similar to smallpox.

In 1796, Jenner introduced a safer way to protect against smallpox as a result of his 26-year study of cowpox and smallpox. Jenner had made two important observations. First, many milkmaids and other farmworkers developed a mild illness, with pocklike sores, after milking cows that had cowpox sores on their teats. Second, very few of those who had been infected with cowpox became sick with smallpox. He asked the question "Why don't people who have had cowpox get smallpox?" He developed the hypothesis that the mild disease caused by cowpox somehow protected them from the often fatal smallpox. This led him to perform an experiment. In his first experiment, he took puslike material from a sore on the hand of a milkmaid named Sarah Nelmes and rubbed it into small cuts on the arm of an 8-year-old boy named James Phipps. James developed the normal mild infection typical of cowpox and completely recovered. Subsequently, Jenner inoculated James with material from a smallpox patient. (Recall that this was a normal practice at the time.) James did not develop any disease. Jenner's conclusion was that deliberate exposure to cowpox had protected James from smallpox. Eventually the word vaccination was used to describe the process. It was derived from the Latin words for cow (vacca) and cow- pox disease (vaccinae) (box figure).

When these results became known, public reaction was mixed. Some people thought that vaccination was the work of the devil. However, many European rulers supported Jenner by encouraging their subjects to be vaccinated. Napoleon and the empress of Russia were very influential and, in the United States, Thomas Jefferson had some members of his family vaccinated. Many years later, following the development of the germ theory of disease, it was discovered that cowpox and smallpox are caused by viruses that are similar in structure. Exposure to the cowpox virus allows the body to develop immunity against both the cowpox virus and the smallpox virus. In the mid-1900s a slightly different virus was used to develop a vaccine against smallpox, which was used worldwide. In 1979, almost 200 years after Jenner developed his vaccination, the Centers for Disease Control and Prevention (CDC) in the United States and the World Health Organization (WHO) of the United Nations declared that smallpox had been eradicated.

The painting depicts Edward Jenner vaccinating James Phipps.

Today, vaccinations (immunizations) are used to control many diseases that used to be common. Many of them were known as childhood diseases, because essentially all children got them. Today, they are rare in populations that are vaccinated. The following chart shows the schedule of immunizations recommended by the Advisory Committee on Immunization Practices of the American Academy of Pediatrics and American Academy of Family Physicians.

The Consequences of Not Understanding Biological Principles

A lack of understanding biological principles, and the inability to distinguish between valid scientifically obtained facts and personal opinions, can have significant consequences. Some people practice “selective acceptance of scientific evidence.” They have “faith” in the health products and procedures that have resulted from “good science” (e.g., antibiotics, heart transplants) but don’t “believe” or have “faith” in others (e.g., vaccinations, genetic engineering, stem cells).

Inability to See a Bigger Picture

There are some people who believe that you can get the flu by getting the vaccine in spite of scientific evidence to the contrary. While the vaccine may cause certain side effects (problems that occur in addition to the desired healing effect), it does not contain any viruses capable of causing infection. These people (1) confuse the side effects with actual flu symptoms; (2) don’t realize that the vaccine they received does not protect against other, related strains of influenza virus; or (3) may have already been infected before receiving the vaccine. In fact, by refusing to get vaccinated they jeopardize others in their community. By being vaccinated and becoming immune to the virus, they serve as a barrier to the spread of the virus, helping to prevent others from becoming infected. If enough people become immune as the result of immunization, there is less chance that others will get the illness.

Lack of Understanding the Interconnectedness of Ecological Systems

At one time, it was thought that the protection of specific land areas would preserve endangered ecosystems. However, it is now recognized that many activities outside park and preserve boundaries are also important. For example, although Everglades National Park in Florida has been well managed by the National Park Service, this ecosystem is experiencing significant destruction. Commercial and agricultural development adjacent to the park has caused groundwater levels in the Everglades to drop so low that the park’s very existence is threatened. Fertilizer has entered the park from surrounding farmland and has encouraged the growth of plants that change the nature of the ecosystem. In 2000, Congress authorized the expenditure of $1.4 billion to begin to implement a plan that will address the problems of water flow and pollution. The major goals are to reduce the amount of nutrients entering from farms and to increase the flow of water to the Everglades from Lake Okeechobee to the north.

The Damage Caused by Exotic Species

In North America, the introduction of exotic (foreign) species of plants and animals has had disastrous consequences in a number of cases (figure 1.20). Both the American chestnut and the American elm have been nearly eliminated by diseases that were introduced by accident. Another accidental introduction, the zebra mussel, has greatly altered freshwater lakes and rivers in the central and eastern parts of the United States. They filter tiny organisms from the water and deprive native organisms of this food source. In addition, they attach themselves to native mussels, often causing their death.

Other organisms have been introduced on purpose because of shortsightedness or a lack of understanding about biology. The European starling and the English (house) sparrow were both introduced into this country by people who thought they were doing good. Both of these birds have multiplied greatly and have displaced some native birds. Many people want to have exotic animals as pets. When these animals escape or are intentionally released, they can become established in local ecosystems and endanger native organisms. For example, Burmese pythons are commonly kept as pets. Today, they are common in the Everglades and kill and eat native species. Large pythons have even been observed attacking alligators. The introduction of exotic plants has also caused problems. At one time, people were encouraged to plant a shrub known as autumn olive as a wildlife food. The plant produces many small fruits, which are readily eaten by many kinds of birds and mammals. However, because the animals eat the fruits and defecate the seeds everywhere, autumn olive spreads rapidly. Today, it is recognized as an invasive plant needing to be controlled.

FIGURE 1.20. Exotic Animals

Exotic organisms such as starlings and zebra mussels have altered natural ecosystems by replacing native species.

Ethical Concerns

Advances in technology and our understanding of human biology have presented us with difficult ethical issues, which we have not been able to resolve satisfactorily. Major advances in health care have prolonged the lives of people who would have died if they had lived a generation earlier. Many of the techniques and machines that allow us to preserve and extend life are extremely expensive and are therefore unavailable to most citizens of the world. Many people lack even the most basic health care, while people in the rich nations of the world spend millions of dollars to have cosmetic surgery and to keep comatose patients alive with the assistance of machines.

Future Directions in Biology

Where do we go from here? Although the science of biology has made major advances, many problems remain to be solved. For example, scientists are seeking major advances in the control of the human population, and there is a continued interest in the development of more efficient methods of producing food.

One area that will receive more attention in the next few years is ecology. Climate change, pollution, and the destruction of natural ecosystems to feed a rapidly increasing human population are severe problems. We face two tasks: The first is to improve technology and our understanding about how things work in our biological world; the second, and probably the more difficult, is to educate people that their actions determine the kind of world in which future generations will live.

Another area that will receive much attention in the next few years is the relationship between genetic information and such diseases as Alzheimer’s disease, stroke, arthritis, and cancer. These and many other diseases are caused by abnormal body chemistry, which is the result of hereditary characteristics. Curing hereditary diseases is a big job. It requires a thorough understanding of genetics and the manipulation of hereditary information in all of the trillions of cells of the organism.

It is the intent of science to learn what is going on by gathering facts objectively and identifying the most logical courses of action. It is also the role of science to identify cause-and-effect relationships and note their predictive value in ways that will improve the environment for all forms of life—including us. Scientists should also make suggestions to politicians and other policy makers about which courses of action are the most logical from a scientific point of view.

1.4. CONCEPT REVIEW

12. Describe three advances that have occurred as a result of biology.

13. List three mistakes that could have been avoided had we known more about living things.

14. What is biology?

15. List five characteristics of living things.

16. What is the difference between regulation and coordination?

Summary

The science of biology is the study of living things and how they interact with their surroundings. Science can be distinguished from nonscience by the kinds of laws and rules that are constructed to unify the body of knowledge. Science involves the continuous testing of rules and principles by the collection of new facts. In science, these rules are usually arrived at by using the scientific method—observation, questioning, the exploration of resources, hypothesis formation, and the testing of hypotheses. When general patterns are recognized, theories and laws are formulated. If a rule is not testable, or if no rule is used, it is not science. Pseudoscience uses scientific appearances to mislead.

Living things show the characteristics of (1) a unique structural organization, (2) metabolic processes, (3) generative processes, (4) responsive processes, and (5) control processes. Biology has been responsible for major advances in food production and health. The incorrect application of biological principles has sometimes led to the destruction of useful organisms and the introduction of harmful ones. Many biological advances have led to ethical dilemmas, which have not been resolved. In the future, biologists will study many things. Two areas that are certain to receive attention are ecology and the relationship between heredity and disease.

Basic Review

1. Which one of the following distinguishes science from nonscience?

a. the collection of information

b. the testing of a hypothesis

c. the acceptance of the advice of experts

d. information that never changes

2. A hypothesis must account for all available information, be logical, and be _____.

3. A scientific theory is

a. a guess as to why things occur.

b. always correct.

c. a broad statement that ties together many facts.

d. easily changed.

4. Pseudoscience is the use of the appearance of science to _____.

5. Economics is not considered a science because

a. it does not have theories.

b. it does not use facts.

c. many economic predictions do not come true.

d. economists do not form hypotheses.

6. Reproduction is

a. a generative process.

b. a responsive process.

c. a control process.

d. a metabolic process.

7. The smallest independent living unit is the _____.

8. The smallest unit that displays characteristics of life is the _____.

9. An understanding of the principles of biology will prevent policy makers from making mistakes. (T/F)

10. Three important advances in the control of infectious diseases are safe drinking water, safe food, and _____.

11. If data are able to be justified and are on target with other evidence, scientists say that these data are

a. valid.

b. reliable.

c. expected.

d. appropriate.

12. Which is not a basic assumption in science?

a. There are specific causes for events observed in the natural world.

b. There are general rules or patterns that can be used to describe what happens in nature.

c. Events that occur only once probably have a single cause.

d. The same fundamental rules of nature apply, regardless of where and when they occur.

13. A variable that changes in direct response to how another variable is manipulated is known as

a. the dependent variable.

b. the independent variable.

c. the reliable variable.

d. a hypothesis.

14. Features that result from the interaction of simple components when they form much more complex substances are called

a. organizational properties.

b. emergent properties.

c. adaptive traits.

d. evolutionary traits.

Answers

1. b 2. testable 3. c 4. mislead 5. c 6. a 7. organism 8. cell 9. F 10. vaccination 11. a 12. c 13. a 14. B

Thinking Critically

The Scientific Method and Climate Change

One important trait that all scientists should share is skepticism. They should consistently and constantly ask the question, “Are you sure that’s right?” When considering the question of global warming, scientists might ask, “Is there a scientific basis that global warming is primarily due to greenhouse gases that are manmade?” The carbon dioxide content of the atmosphere is the highest it has been in millions of years and the rate of increase is unparalleled. What must scientists do to demonstrate a cause-and-effect relationship? As a scientist, how would you go about determining if this is simply a correlation and not a cause-and-effect relationship? How would you determine if there is a cause-and-effect relationship between the exponential increase in world human population in the last century and the increase in greenhouse gases? If the evidence ultimately points to a correlation, is it wise to ignore the potential risks associated with global warming?