Key Terms
system consumer population
photosynthesis homeostasis natural selection
producer adaptation evolution
With life's many levels of organization and great diversity of organisms, biologists have a huge subject to study. And it gets bigger every year as researchers continually make new discoveries. How can anyone make sense of all this information? Fortunately, there are some basic ideas, or themes, that apply to biology at all levels and for all organisms. The ten themes described here will help you connect the many things you'll learn as you explore life.
1. Biological Systems
Have you ever heard the saying "The whole is greater than the sum of its parts"? This saying captures the importance of how a combination of parts can form a more complex organization called a system. A system has properties that are based on the arrangement and interactions of its parts. For example, a bicycle is a mechanical system you can use for exercise or transportation. But just try to get around on a box full of bicycle parts!
Your body, like that of any organism, is a living system. You make use of the interactions among its parts when you type on a keyboard or click a computer mouse. The joints in your fingers and wrist give your hand a wide range of movements. But your bones themselves cannot move. Movement depends on contractions of the muscles attached to the bones. Muscles are coordinated by signals from the brain, carried by nerves. Finally, blood vessels supply all of these parts with oxygen and food. Together, the parts of your body enable you to work the computer. You are certainly more than the sum of your parts, and so are all biological systems.
An ecosystem such as a forest is also a biological system. Like your body, an ecosystem has properties that depend on how its parts interact. For example, the organisms in the ecosystem require a steady supply of certain chemicals to live. Plants obtain most of their necessary chemicals from the soil, water, and air. Animals acquire most of the chemicals they need by eating plants or other animals. Chemicals are returned to the soil by bacteria and fungi that decompose the wastes and remains of organisms. You could say that such interactions of
organisms with each other and with the nonliving environment "put the system in ecosystem." The biological systems theme applies to all levels of life, from the biosphere all the way down to the interactions of molecules in cells.
2. The Cellular Basis of Life
In most multicellular organisms, cells are organized into higher levels of organization
(Figure 1-10). Beginning with the cellular level, the next level is a tissue, which is a
group of similar cells that together perform a specific function. For example, nerve tissue
consists of many nerve cells organized into a complex network. Several types of tissue
together may make up a structure called an organ. The brain is an organ that consists of
nerve tissue and other types of tissues. Finally, several organs that together carry out a
major body function make up an organ system. In this example, the brain, spinal cord,
and nerves make up the organ system called the nervous system.
Figure 1-10
The human body, like most multicellular organisms, consists of many levels of organization.
A multicellular organism's development and survival are based on the functions and interactions of its many cells. This cellular basis of life is a theme you will encounter often as you explore the living world.
3. Form and Function
Which is the better tool: a hammer or a screwdriver? The answer depends on what you want to do. You probably would not choose a hammer to loosen a screw or a screwdriver to pound in a nail. The heavy head of a hammer is suited to driving in nails, and the thin, flat edge of a screwdriver is suited to turning screws. How something works is related to its structure. In other words, form fits function.
4. Reproduction and Inheritance
"Like begets like" is an old saying that describes the ability of organisms to reproduce their own kind. For example, a Japanese macaque monkey and her baby resemble each other. The baby macaque in turn may grow up and produce similar-looking offspring of its own.
What explains the similarity between parents and their offspring? You read earlier in the chapter that offspring inherit units of information called genes from their parents. Genes are responsible for family resemblance.
Recall also that genes are made of information-rich molecules called DNA. Each cell in
your body contains a copy of all the DNA that you inherited from your mother and father.
When a cell divides, it copies its DNA and passes this genetic information on to each of
the two cells it produces. How is this information passed from parent organisms to
offspring? In humans, an egg cell from the mother fuses with a sperm cell from the father
(Figure 1-13). The result is a fertilized cell containing a combination of DNA from both
parents. The inherited DNA directs the transformation of the fertilized egg into a person,
with his or her own eye color, facial features, and other characteristics.
Figure 1-13
When an egg cell and sperm cell fuse, DNA from each parent is combined in the fertilized egg. The inherited DNA directs the eventual transformation of the fertilized egg into a person.
This brief introduction to DNA suggests several questions.
How does DNA store information?
How do cells copy and pass along this information?
How does the inherited DNA bring about such traits as the color of eyes or the shape of a nose?
5. Interaction With the Environment
No organism is completely isolated from its surroundings. As part of an ecosystem, each organism interacts continuously with its environment. For example, a plant obtains water and nutrients from the soil, carbon dioxide gas from the air, and energy from sunlight. The plant uses these three "inputs" from its environment for photosynthesis—the process by which plants make food.
But the plant also has an impact on its surroundings. For example, as a plant grows, its roots break up rocks and release acids that change the soil. This affects the types of organisms that can live in the soil. Plants also release oxygen as a byproduct of photosynthesis. Other organisms as well as plants use this oxygen for their own survival.
The transfer of chemicals between organisms and their environments is a key process in any ecosystem. Think about your own chemical exchanges with the outside world. You breathe air, drink water, eat food, and get rid of waste products. Living requires a daily balance of such "inputs" and "outputs."
In addition to chemical exchange, there are many other ways you interact with your environment. If you go outside on a bright summer day, the sun may cause you to squint. Perhaps the bark of an approaching dog causes you to turn your head quickly. Just as you are constantly sensing and responding to changes in your environment, so are all other organisms. For example, a
specialized leaf of the Venus' flytrap senses the light footsteps of a soon-to-be-digested green bottle fly. The plant responded to this environmental stimulus by rapidly folding the leaf together. You will discover many such examples of organism-environment interactions as you explore life.
6. Energy and Life
Moving, growing, reproducing, and other activities of life require organisms to perform work. Work depends on a source of energy. You obtain your energy in chemical form—in the sugars, fats, and other "fuel-like" molecules in your food. Your cells use this energy for all their work. You "burn" fuel to move, to think, and even to keep your heart beating when you are asleep.
On a bigger scale, you can trace energy through an ecosystem. Energy flows into an ecosystem as sunlight and exits in the form of heat. Figure 1-15 is a simplified diagram of this energy flow through a forest ecosystem. Note how the ecosystem's organisms convert one form of energy to another. For example, in the process of photosynthesis, plants convert light energy to the chemical energy stored in sugars and other foods. Plants and other photosynthetic organisms are an ecosystem's producers, so named because they produce the food upon which the entire
ecosystem depends. The plants use some of the food they produce for their own fuel and building material. A portion of the stored energy reaches consumers, which are animals and other
organisms that eat (consume) the food made by the producers.
What happens to the chemical energy stored in the food consumers eat? It is converted to other forms of energy as the organism carries out its life activities. Moving, thinking, breathing, seeing, and everything else you do requires your cells to convert some of the chemical energy of food into other forms of energy. You can compare this energy conversion to a car converting the chemical energy stored in gasoline to the mechanical energy of moving wheels. Whenever an organism or a car performs work, it converts some of its energy supply to heat. The heat is released to the environment. Even when you are sitting still in class, you produce about as much heat as a 100-watt light bulb. Because all organisms lose energy in the form of heat, an ecosystem cannot recycle energy. Life on Earth depends on a continuous supply of energy from the sun.
7. Regulation
Another theme you will encounter frequently in your study of biology is the ability of organisms to regulate their internal conditions. For example, you have a "thermostat" in your brain that reacts whenever your body temperature varies slightly from 37°C (about 98.6°F). If this internal thermostat detects a slight rise in your body temperature on a hot day, your brain signals your skin to produce sweat. Sweating helps cool your body.
Panting is another example of a cooling mechanism. You've probably seen a dog pant on a hot day, but did you know that some birds also pant? Panting causes moisture on the large surface of the animal's lungs to evaporate, cooling the body as a result.
The ability of mammals and birds to regulate body temperature is just one example of homeostasis, or "steady state." Mechanisms of enable organisms to regulate their internal environment, despite changes in their external environment.
8. Adaptation and Evolution
Can you find the three animals in Figure 1-18 on page 17 of your book? These organisms are three species of insects called mantids. Their shapes and colors enable them to blend into their backgrounds. This camouflage makes the mantids less visible to animals that feed on insects. It also makes them less visible to the insects the mantids feed on!
Adaptations The unique characteristics that camouflage each mantid species are examples of adaptations. An adaptation is an inherited trait that helps the organism's ability to survive and reproduce in its particular environment.
How do mantids and other organisms adapt to their environments? Part of the answer is the variation among individuals in a population. A population is a localized group of organisms belonging to the same species. Just as you and your classmates are not exactly alike, individuals of all populations, including mantids, also vary in some of their traits. These variations reflect each individual's particular combination of inherited genes. And this hereditary variation is the raw material that makes it possible for a population to adapt to its environment. If a particular variation is helpful, individuals with the variation may live longer and produce more offspring than those that do not have it. This process is called natural selection because it works by the natural environment "selecting" certain inherited traits.
color is determined by its genes. Now suppose that the soil has recently been blackened
by a fire. For birds that eat the beetles, it is easiest to spot the beetles that are lightest in
color. On average, the darker beetles have a better chance of surviving and reproducing,
passing their genes for dark color on to their offspring. In contrast, the lighter beetles are
captured more easily, and fewer survive to produce offspring. After many generations,
most of the beetles in the population are dark. This abundance of dark color is an
adaptation of the beetle population to its environment.
Figure 1-19
In this hypothetical example of natural selection, darker beetles are more likely to survive longer and reproduce, passing their genes on to more offspring.
Evolution Natural selection is the mechanism by which evolution occurs. The term
evolution means "a process of change." Biologists use the word evolution specifically to mean a generation-to-generation change in the proportion of different inherited genes in a population. For example, in the beetle example, genes for dark color are becoming more common and genes for light color are becoming less common over the generations of beetles. The beetle population is said to be undergoing evolution, or evolving.
9. Biology and Society
More than ever before, modern biology is changing humans' everyday lives. New findings about DNA affect such fields as medicine and agriculture. Research on the nervous system is improving the treatment of certain mental illnesses. The study of evolution is helping health professionals understand how disease-causing bacteria become resistant to antibiotic drugs. Environmental issues such as water and air pollution are changing how people think about their relationship to the biosphere. If you watch the evening news or read a newspaper for a week, it's likely you'll hear about many issues that relate to biology, such as stem cell research, animal cloning, environmental issues, genetically modified crops, or new ways to treat diseases. The concepts you study this year will help you to have informed opinions about the impact of biology's rapid progress.
10. Scientific Inquiry
Biology is a science and, as such, relies on certain processes of inquiry. Scientific inquiry involves asking questions about nature and then using observations or experiments to find possible answers to those questions.
