Summary b i o l o g yToday and Tomorrow With Physiology

4 Introduction The Secret Life of Earth In this era of satellites and global positioning systems, submarines and sonar, could there possibly be any more places on Earth that we have not explored? Well, yes, actually. In 2005, for instance, helicopters dropped a team of explor- ers into the middle of a vast and otherwise inaccessible Indonesian cloud forest. Within minutes, the explorers realized that their landing site, a dripping, moss- covered swamp, was home to plants and animals that had been unknown to science. Over the next month, they discovered dozens of new species there, including a giant treetop rhododendron with flowers the size of plates and a frog the size of a pea. They also came across hun- dreds of species that are on the brink of extinction in other parts of the world, some that supposedly were extinct, and one that had not been seen in so long it had been forgotten. The animals in the forest had never learned to be afraid of humans, so they could be approached and even picked up (Figure 1.1). A few new species were discovered as they casually wandered through the campsite. Team member Bruce Beehler remarked, “Everywhere we looked, we saw amazing things we had never seen before. I was shouting. This trip was a once-in-a- lifetime series of shouting experiences.” How do we know whether a particular organism belongs to a new species? What is a species, anyway, and why should dis- covering a new one matter to anyone other than a scientist? You will find the answers to such questions in this book. They are part of the scientific study of life, biology, which is one of many ways we humans try to make sense of the world around us. Trying to understand the immense scope of life on Earth gives us some perspective on where we fit into it. For example, we routinely discover hundreds of species every year, but about 20 species become extinct every minute in rain forests alone. The current rate of extinctions is about 1,000 times faster than nor- mal, and human activities are responsible for the acceleration. At this rate, we will never know about most of the species that are alive on Earth today. Does that matter? Biologists think so. Whether or not we are aware of it, we humans are intimately connected with the world around us. We are profoundly changing the entire fabric of life on Earth. The changes are, in turn, affecting us in ways we are just beginning to fathom. Ironically, the more we learn about nature, the more we realize we have yet to learn. But don’t take our word for it. Find out what biologists know, and what they do not, and you will have a solid foundation upon which to base your own opinions about our place in this world. By reading this book, you are choosing to learn about the human connection—your connection—with all life on Earth. Ironically, the more we learn about nature, the more we realize we have yet to learn. 1.1Impacts/Issues: FIGURE 1.1 Biologist Kris Helgen holds a rare golden-mantled tree kangaroo he and his colleagues found in a cloud forest in the Foja Mountains of New Guinea. Chapter 1 Invitation to Biology 5 1.2Life’s Levels of Organization If you are reading this book, you are starting to explore how a subset of scientists—biologists— think about nature. Nature is every substance and energy in the universe except what humans have manufactured. It includes flowers, water, animals, rocks, thunder, and so on. Biologists study the parts of nature that have to do with life, past and present. Through their work, we glimpse a great pattern of organization (Figure 1.2). The pattern starts with atoms, which are basic building blocks of all matter 1 . At the next level of organization, atoms join as mol- ecules 2 . Only living cells make the molecules of life—complex carbohydrates and lipids, pro- teins, and nucleic acids—in nature. The pattern crosses the threshold to life when many mol- ecules organize as a cell 3 . A cell is the smallest unit of life that can survive and reproduce on its own, given information in its DNA, energy, and raw materials. An organism is an individual that consists of one or more cells. In larger multicelled organisms, trillions of cells may be organized as tissues, organs, and organ systems that interact to keep the individual’s body working properly 4 . At the next level of organization, a population is a group of individuals of the same type, or species, living in a given area 5 . An example would be all of the lake trout living in Lake Tahoe, California. At the next level, a community consists of all populations of all spe- cies in a given area 6 . An underwater ocean community, for example, may include many kinds of organisms that make their home in or on a particular reef. The next level of organization is the ecosystem: a community interacting with its environment 7 . The most inclusive level, the biosphere, encompasses all regions of Earth’s crust, waters, and atmosphere in which organisms live 8 . Remember that life is more than the sum of its individual parts. In other words, some emergent property occurs at each successive level of life’s orga- nization. An emergent property is a characteristic of a system that does not appear in any of a system’s component parts. For example, the molecules of life are themselves not alive. Considering them separately, no one would be able to predict that a particular arrangement of molecules would form a living cell. Life—an emergent property—appears first at the level of the cell. How does “life” differ from “nonlife”?
The building blocks—atoms—that make up all living things are the same ones that make up all nonliving things.
Atoms join as molecules. The unique properties of life emerge as certain kinds of molecules become organized into cells.
Higher levels of organization include multicelled organisms, populations, com- munities, ecosystems, and the biosphere. Take-Home Message atom Fundamental building block of all matter. biology Systematic study of life. biosphere All regions of Earth where organisms live. cell Smallest unit of life. community All populations of all species in a given area. ecosystem A community interacting with its environment. emergent property A characteristic of a system that does not appear in any of a system’s component parts. molecule An association of two or more atoms. nature Everything in the universe except what humans have manufactured. organism Individual that consists of one or more cells. population Group of individuals of the same species that live in a given area. FIGURE 1.2 Animated! Levels of organization in nature, from simpler to more complex. 1 Atoms 2 Molecules 3 Cells 4 Organisms 5 Populations 6 Communities 7 Ecosystems 8 The biosphere 1 2 3 4 5 6 7 8 6 Introduction 1.3Overview of Life’s Unity “Life” is not easy to define: It is just too big, and it has been changing for billions of years. Even so, we know that all living things have similar character- istics. All living things require energy and raw materials; they sense and respond to change; and they reproduce with the help of DNA. Energy Sustains Life’s Organization Eating supplies your body with energy and nutrients that keep it organized and functioning. Energy is the capacity to do work. A nutrient is a substance that an organism needs for growth and survival, but cannot make for itself. All organisms spend a lot of time acquiring energy and nutrients, although different species get them from different sources. The differences allow us to classify organisms into one of two broad categories: producers and consumers. Pro- ducers make their own food using energy and simple raw materials they get directly from their environment. Plants are producers that use the energy of sunlight to make sugars from water and carbon dioxide (a gas in air). Consumers cannot make their own food; they get energy and nutrients indirectly—by feeding on other organisms. Animals are con- sumers. So are decomposers, which feed on the wastes or remains of other organisms. The leftovers of their meals end up in the environment, where they serve as nutrients for pro- ducers. Said another way, nutrients cycle between producers and consumers. Energy, however, is not cycled. It flows through the world of life in one direction: from the environment to organisms. This flow maintains the organization of individual organisms. It is also the basis of how organisms interact with one another and their environment. The flow is one-way, because with each transfer, some energy escapes as heat. Cells do not use heat to do work. Thus, the energy that enters the world of life eventually leaves it, permanently (Figure 1.3). Organisms Sense and Respond to Change An organism senses and responds to change both inside and outside of its body by way of receptors. A receptor is a molecule or cellular structure that responds to a spe- cific form of stimulation (Figure 1.4). A receptor that has been stimulated can trigger changes in an organism’s activities. For example, after you eat, the sugars from your meal enter your blood- stream. The added sugars in your blood bind to molecular receptors on cells of the pancreas (an organ). Binding sets in motion a series of events that causes cells throughout the body to take up sugar faster, so the sugar level in your blood quickly falls. The interplay of processes keeps your blood sugar level within a cer- tain range, which in turn helps keep your cells alive and your body functioning. The fluid in your blood is part of your body’s internal environment, which consists of all body fluids outside of cells. Unless the internal environment is kept within certain ranges of composition, temperature, and other conditions, your body cells will die. By sensing and adjusting to change, you and all other organisms keep conditions in the internal environment within a range that PRODUCERS plants and other self-feeding organisms B Nutrients that become incorporated into the cells of producers and consumers are eventually released by decomposition. Some cycle back to producers. A Producers harvest energy from the environment. Some of that energy flows from producers to consumers. C All energy that enters the world of life eventually flows out of it, mainly as heat. sunlight energy CONSUMERS animals, most fungi, many protists, bacteria FIGURE 1.3 Animated! The one-way flow of energy and the cycling of materials in the world of life. Below, a consumer eating a producer. Chapter 1 Invitation to Biology 7 favors cell survival. Homeostasis is the name for this process, and it is a defin- ing feature of life. Organisms Grow and Reproduce Individuals of every natural population are alike in certain aspects of their body form, function, and behav- ior, but the details of such traits often differ from one individual to the next. For example, humans characteristically have two eyes, but those eyes occur in a range of color among individuals. Eye color and most other traits are the outcome of information encoded in DNA, or deoxyribonucleic acid. DNA is the signature molecule of life. It carries information that guides growth—increases in cell number, size, and volume—and development, the process by which the first cell of a new individual becomes a multicelled adult. Only multicelled species undergo development, but all organisms inherit DNA from parents. Lions look like lions and not like peas because they inherited lion DNA, which differs from pea DNA in the information it carries. Inheritance refers to the transmission of DNA from parents to offspring. Such transmission occurs by processes of reproduction, which produce new individuals. consumer Organism that gets energy and carbon by feeding on tissues, wastes, or remains of other organisms. development Multistep process by which the first cell of a new individual becomes a multicelled adult. DNA Deoxyribonucleic acid; molecule that carries heredi- tary information about traits. energy The capacity to do work. growth Increases in the number, size, and volume of cells in multicelled species. homeostasis Set of processes by which an organism keeps its internal conditions within tolerable ranges. inheritance Transmission of DNA from parents to off- spring. nutrient Substance that an organism needs for growth and survival, but cannot make for itself. producer Organism that makes its own food using energy and simple raw materials from the environment. receptor Molecule or structure that responds to a specific form of stimulation. reproduction Process by which parents produce offspring. How are all living things alike?
A one-way flow of energy and a cycling of nutrients sustain life’s organization.
Organisms sense and respond to change. They make adjustments that keep conditions in their internal environment within a range that favors cell survival, a process called homeostasis.
Organisms grow, develop, and reproduce based on information encoded in their DNA, which they inherit from their parents. Take-Home Message FIGURE 1.4 Organisms have receptors that allow them to sense and respond to stimuli such as the mechanical energy of a bite. 8 Introduction 1.4Introduction to Life’s Diversity Each time we discover a new species, or kind of organism, we give it a two-part name. The first part of the name specifies the genus (plural, genera), which is a group of species that share a unique set of features. It designates one species when combined with the second part, the species name. Individuals of a species share one or more heritable traits, and they can interbreed successfully if the spe- cies is a sexually reproducing one. Genus and species names are always italicized. For example, Panthera is a genus of big cats. The lions in Figure 1.4 are of the species Panthera leo. Tigers, or P. tigris, are a different species in the same genus. Note that the genus name may be abbreviated after it has been spelled out once. We use various classification systems to organize and retrieve informa- tion about species. Most systems sort species into groups on the basis of their traits. Figure 1.5 shows a common system in which all species are grouped into three domains: Bacteria, Archaea, and Eukarya. Protists, plants, fungi, and animals make up domain Eukarya. We return to systems of naming and grouping organisms in more detail in Chapter 12. All bacteria (singular, bacterium) and archaeans are single-celled organ- isms. All of them are prokaryotes, which means that their DNA is not contained within a nucleus. A nucleus is a membrane-enclosed sac that pro- tects a cell’s DNA. As a group, prokaryotes are the most diverse organisms. Different kinds are producers or consumers that inhabit nearly all of the bio- sphere, including extreme environments such as frozen desert rocks, boiling sulfur-clogged lakes, and nuclear reactor waste. The first cells on Earth may have faced similarly hostile challenges to survival. Cells of eukaryotes have a nucleus. Structurally, protists are the sim- plest kind of eukaryote. Different protist species are producers or consumers. Many are single cells that are larger and more complex than prokaryotes. Some of them are tree-sized, multicelled seaweeds. Cells of fungi, plants, and animals are also eukaryotic. Most fungi (singular, fungus), such as the types that form mushrooms, are multicelled. Many are decomposers, and all secrete enzymes that digest food outside the body. Their cells then absorb the released nutrients. Plants are multicelled species that live on land or in freshwater environ- ments. Most are producers. By a process called photosynthesis, they harness the energy of sunlight to drive the production of sugars from carbon dioxide and water. Besides feeding themselves, plants and other photosynthesizers serve as food for most of the other organisms in the biosphere. Animals are multicelled consumers that ingest tissues or juices of other organisms. Herbivores graze, carnivores eat meat, scavengers eat remains of other organisms, and parasites withdraw nutrients from the tissues of a host. Animals grow and develop through a series of stages that lead to the adult form, and most kinds actively move about during at least part of their lives. How do living things differ from one another?
Organisms differ in their details; they show tremendous variation in traits.
Classification systems by which species are grouped according to traits help us organize information about species. One common system sorts all species into three domains: Bacteria, Archaea, and Eukarya. Take-Home Message Bacteria Archaea FIGURE 1.5 Animated! Three-domain classifica- tion system with a few representatives of life’s diversity. Eukarya Chapter 1 Invitation to Biology 9 1.5The Nature of Scientific Inquiry Most of us assume that we do our own thinking—but do we, really? You might be surprised to find out just how often we let others think for us. For instance, a school’s job, which is to impart as much information as possible to students, meshes with a student’s job, which is to acquire as much knowledge as possible. In this rapid-fire exchange of information, it is easy to forget about the quality of what is being exchanged. If you accept information without question, you allow someone else to think for you. Critical thinking means judging information before accepting it. “Critical” comes from the Greek kriticos (discerning judgment). When you think this way, you move beyond the content of information. You look for underlying assump- tions, evaluate the supporting statements, and think of possible alternatives. How does the busy student manage this? Be aware of what you intend to learn from new information. Be conscious of bias or underlying agendas in books, lectures, or online. Decide whether ideas are based on opinion or evi- dence. Question authority figures (respectfully). Consider what you want to believe, and realize that those biases influence your learning. Such practices will help you decide whether to accept or reject information. The Scope—and the Limits—of Science Because each of us is unique, there are as many ways to think about the natural world as there are people. Science, the systematic study of nature, is one way. It helps us be objec- tive about our observations of nature, in part because of its limitations. We limit science to a subset of the world: only that which is observable. Science does not address some questions, such as “Why do I exist?” Most answers to such questions are subjective—they come from within as an inte- gration of the personal experiences and mental connections that shape our consciousness. This is not to say subjective answers have no value, because no human society can function for long unless its individuals share standards for making judgments, even if they are subjective. Moral, aesthetic, and philosophi- cal standards vary from one society to the next, but all help people decide what is important and good. All give meaning to what we do. Also, science does not address the supernatural, or anything that is “beyond nature.” Science does not assume or deny that supernatural phenomena occur, but scientists may still cause controversy when they discover a natural explana- tion for something that was thought to be supernatural. Such controversy often arises when a society’s moral standards have become interwoven with its under- standing of nature. For example, Nicolaus Copernicus studied the planets centuries ago in Europe, and concluded that Earth orbits the sun. Today his conclusion seems obvious, but at the time it was heresy. The prevailing belief was that the Creator made Earth—and, by extension, humans—as the center of the universe. Galileo Galilei, another scholar, found evidence for the Copernican model of the solar system and published his findings. He was forced to publicly recant his publica- tion, and to put Earth back at the center of the universe. Exploring a traditional view of the natural world from a scientific perspec- tive might be misinterpreted as questioning morality even though the two are not the same. As a group, scientists are no less moral, less lawful, or less spiri- tual than anyone else. As you will see in the next section, however, their work follows a particular standard: Their explanations must be testable in ways that others can repeat. animal Multicelled consumer that develops through a series of embryonic stages and moves about during part or all of the life cycle. archaean A member of the prokaryotic domain Archaea. bacterium A member of the prokaryotic domain Bacteria. critical thinking Mental process of judging information before accepting it. eukaryote Organism whose cells characteristically have a nucleus. fungus Type of eukaryotic consumer that obtains nutrients by digestion and absorption outside the body. genus A group of species that share a unique set of traits. plant A multicelled, typically photosynthetic producer. prokaryote Single-celled organism in which the DNA is not contained in a nucleus. protist Diverse group of simple eukaryotes. science Systematic study of nature. species A type of organism. 10 Introduction 1.6 Science helps us communicate our experiences without bias, so it may be as close as we can get to a universal language. We are fairly sure, for example, that laws of gravity apply everywhere in the universe. Intelligent beings on a distant planet would likely understand the concept of gravity. We might well use gravity or another scientific concept to communicate with them, or anyone, anywhere. The point of science, however, is not to communicate with aliens. It is to find common ground here on Earth. How Science Works Observations, Hypotheses, and Tests To get a sense of how science works, consider this list of common research practices: You might hear someone refer to these practices as “the scientific method,” as if all scientists march to the drumbeat of a fixed procedure. They do not. There are different ways to do research, particularly in biology (Figure 1.6). Some biologists survey and observe without making hypotheses. Others make hypoth- eses and leave the testing to others. A few stumble onto valuable information they are not even looking for. Regardless of the variation, one thing is constant: Scientists do not accept information simply because someone says it is true. They evaluate the supporting evidence and find alternative explanations. Does this practice sound familiar? It should—it is critical thinking. Theories and Laws Suppose a hypothesis has not been disproven even after years of tests. It is consistent with all evidence gathered to date, and it has helped us to make successful predictions about other phenomena. When a hypothesis meets these criteria, it is considered a scientific theory (Table 1.1). For 1. Observe some aspect of nature. 2. Frame a question about your observation. 3. Read what others have discovered concerning the subject, then propose a hypothesis, a testable answer to your question. 4. Using the hypothesis as a guide, make a prediction—a statement of some con- dition that should exist if the hypothesis is not wrong. Making predictions is called the if–then process: “if” is the hypothesis, and “then” is the prediction. 5. Devise ways to test the accuracy of the prediction by conducting experiments or gathering information. Tests may be performed on a model, or similar system, if testing an object or event directly is not possible. 6. Assess the results of the tests or observations—the data. Data that confirm the prediction are evidence in support of the hypothesis. Data that disprove the pre- diction are evidence that the hypothesis may be flawed. 7. Report all the steps of your work, along with any conclusions you drew, to the scientific community. Science helps us communicate our experiences without bias. Take-Home Message What is science?
Science is the study of the observable—those objects or events for which objective evidence can be gathered. It does not address the supernatural. Chapter 1 Invitation to Biology 11 example, scientists no longer spend time testing the hypothesis that all matter is composed of atoms, for the compelling reason that no one has ever detected matter composed of anything else. This hypothesis is now called atomic theory. A law of nature describes a phenomenon that has been observed to occur in every circumstance without fail, but for which we currently do not have a complete scientific explanation. The laws of thermodynamics, which describe energy, are examples. We know how energy behaves, but not why it behaves the way it does. Most scientists carefully avoid the word “truth” when discussing science, preferring instead to use “accurate” in reference to data. In science, there is only evidence that supports a hypothesis because an infinite number of tests would be necessary to confirm that a theory holds under every possible circumstance. A single observation or result that is not consistent with a theory would open it to revision. For example, if someone discovered a type of matter not composed of atoms, the atomic theory would be revised. The theory of evolution, which states that change occurs in a line of descent over time, still holds after a cen- tury of testing and scrutiny. As with all other scientific theories, we cannot be sure that it will hold under all possible conditions, but we can say it has a very high probability of not being wrong. If evidence turns up that is inconsistent with evolution, biologists will revise the theory. You may hear people apply the word “theory” to a speculative idea, as in the phrase “It’s just a theory.” Speculation is opinion or belief, a personal conviction that is not necessarily supported by evidence. A scientific theory is not an opin- ion: By definition, it must be supported by a large body of evidence. Unlike theories, many beliefs and opinions cannot be tested. Ideas that can- not be tested cannot be disproven. Personal conviction has tremendous value in our lives, but it is not the same as scientific theory. hypothesis Testable explanation of a natural phenomenon. law of nature Generalization that describes a consistent natural phenomenon for which there is incomplete scientific explanation. model System similar to an object or event that cannot itself be tested directly. prediction A statement, based on a hypothesis, about a condition that should exist if the hypothesis is not wrong. scientific theory Hypothesis that has not been disproven after many years of rigorous testing, and is useful for making predictions about other phenomena. Table 1.1 Examples of Scientific Theories Theory Main Premises Cell theory All organisms consist of one or more cells, the cell is the basic unit of life, and all cells arise from existing cells. Atomic theory All substances consist of atoms. Evolution Change occurs in inherited traits of a population over generations. Germ theory Microorganisms cause many diseases. Global warming Human activities are causing Earth’s average temperature to increase. Plate tectonics Earth’s crust is cracked into pieces that move in relation to one another. Big Bang The universe originated with an explosion and continues to expand. FIGURE 1.6 Making observations for a field study is one of many ways of doing science. How does science work?
Scientific inquiry involves making observations and asking questions about some aspect of nature; formulating hypotheses; making predictions and testing them using real or model systems; and reporting the results and conclusions. Take-Home Message 12 Introduction 1.7The Power of Experiments Careful observations are one way to test predictions that flow from a hypothesis. So are experiments. Experiments are tests that can support or falsify a predic- tion. They are usually designed to determine the effects of a single variable, which is a characteristic or event that differs among individuals. Biological sys- tems are complex, with many interacting variables. It can be difficult to study one variable separately from the rest. Thus, biology researchers often test two groups of individuals, side by side. An experimental group is a set of individu- als that have a certain characteristic or receive a certain treatment. This group is tested side by side with a control group, which is identical to the experimental group except for one variable—the characteristic or the treatment being tested. Thus, any differences in experimental results between the two groups should be an effect of changing the variable. We will look at two different experiments to see how these groups are used. Potato Chips and Stomachaches In 1996 the FDA approved Olestra ®, a fat replacement manufactured from sugar and vegetable oil, as a food additive. Potato chips were the first Olestra- containing food product on the market in the United States. Controversy soon raged. Some people complained of intestinal problems after eating the chips and concluded that the Olestra was at fault. Two years later, researchers at Johns Hopkins University School of Medicine designed an experiment to test the hypoth- esis that this food additive causes cramps. They predicted that if Olestra causes cramps, then people who eat Olestra will be more likely to get cramps than people who do not. To test the prediction, they used a Chicago theater as a “labora- tory.” They asked 1,100 people between the ages of thirteen and thirty-eight to watch a movie and eat their fill of potato chips. Each person got an unmarked bag that contained 13 ounces of chips. The individuals who got Olestra-containing potato chips were the experimental group, and individuals who got regular chips were the control group. The variable consisted of the presence of Olestra in the chips. Afterward, the researchers contacted all of the people and tabulated any reports of gastrointestinal problems. Of 563 people making up the experimental group, 89 (15.8 percent) com- plained about cramps. However, so did 93 of the 529 people (17.6 percent) making up the control group—who had munched on the regular chips! This experiment disproved the prediction that people who eat Olestra are more likely to get cramps than people who do not (Figure 1.7). Butterflies and Birds Consider the peacock butterfly, a winged insect that was named for the large, colorful spots on its wings. In 2005, researchers published a report on their tests to identify factors that help peacock butter- flies defend themselves against insect-eating birds. The researchers made two observations. First, when a peacock butterfly rests, it folds its wings, so only the dark underside shows. Second, when a butterfly sees a predator approaching, it repeatedly flicks its paired forewings and hindwings open and closed. At the same time, each forewing slides over the hindwing, which produces a hissing sound and a series of clicks. Eats regular potato chips Eats Olestra potato chips Olestra ® causes intestinal cramps. People who eat potato chips made with Olestra will be more likely to get intestinal cramps than those who eat potato chips made without Olestra. 89 of 563 people get cramps later (15.8%) 93 of 529 people get cramps later (17.6%) Percentages are about equal. People who eat potato chips made with Olestra are just as likely to get intestinal cramps as those who eat potato chips made without Olestra. These results do not support the hypothesis. Control Group Experimental Group Hypothesis Prediction Experiment Results Conclusion A B C D E FIGURE 1.7 Animated! The steps in a sci- entific experiment to determine if Olestra causes cramps. A report of this study was published in the Journal of the American Medical Association in January of 1998.