biology

What Makes a Thing “Living”?

Biology is the many-faceted study of living things. But what, exactly, is a living thing? Some of our definition comes directly from the work of Pasteur, Mendel, and others. Their work demonstrated that life can come only from other life. That means that all living things must reproduce. They pass genetic information on to their offspring—and all living things do this with DNA. This genetic information helps determine the physical structure of the offspring.

As Mendel and Morgan observed, an offspring’s physical structure can vary. This variation, sometimes called diversity, may have fortunate or unfortunate effects for the individual, but it helps the species survive. If a given population has numerous variations in its gene pool (all of the genes present in the population), the population is more likely to have at least some members that can survive an environmental change. Over time, these individual variations accumulate, reshaping the population in new ways. This is the nature of evolution. Because the species can only continue living via reproduction, the species is continuous over time.

Living things also detect and respond to stimuli. A stimulus is a change, event, or substance that causes an organism to act. In animals and human beings, stimuli can be detected with eyes, ears, nose, touch, or taste buds. For example, you step outside and feel drops of rain on your head. You go back inside and get an umbrella. The raindrops are the stimulus. They were detected by your sense of touch. You responded by getting an umbrella. Pain is another example of a stimulus. If you stub your toe, the pain causes you to move backward or flinch. Drugs, chemicals, and electricity can also be stimuli. The important factor is that stimuli provoke responses.

What else do all living things have in common? For every individual, from a bacterium to a blue whale, life begins and ends. In the stretch of time between those two phenomena, the living thing must, in essence, work to stay alive. Whether it feeds on insects or sunlight, every living thing must consume enough energy that its cells can carry out all their internal processes. These processes include building new cells for growth, removing dead cells and waste matter, helping the individual reproduce, and so forth. These processes are chemical; they depend on chemical reactions

that can take place only under certain circumstances. Therefore, a living thing’s body must maintain those circumstances. These circumstances are often called the life form’s internal environment. Maintaining that environment—in other words, working to live—is called homeostasis.

All living things do the following:

■ Reproduce.

■ Pass along their traits through DNA.

■ Consume energy sources and expel waste products to maintain homeostasis.

■ Respond to their environments.

■ Respond to stimulus.

■ Change over time.

■ Can differ as individuals while still being part of a species.

■ Consist of biomolecules arranged into cells, which have definite internal structures and functions and in the more complicated life forms, are organized into tissues and organs that also have definite structures and functions.

Using these criteria, compare a rock and a snail. Both contain large quantities of carbon. Both change over time. The rock may be weathered by its environment, but it does not respond to stimuli. A snail does respond to stimuli, such as the chemicals that identify the presence of a food source. If a predator is nearby, the rock is unaffected, but the snail’s instincts are in a panic. Similarly, the rock may break into smaller pieces, but it does not reproduce. It does not eat or produce waste. Its internal molecules do not change. The snail, on the other hand, must consume and expend quite a bit of energy simply to go about the business of being a snail. Its body is constantly working to find food, to process that food into energy and new cell tissue, to survive the threat of predators and environmental change, and finally to create other living copies of itself that will carry on the snail species when this individual dies.

What if these criteria were used to compare the rock to something else—say, a tree? Like a rock, the tree does not move on its own. A tree does not stalk and eat its prey. Remember, though, what Leeuwenhoek discovered: there is much more going on than the human eye can see. A tree in the sun is eating. It is metabolizing the sunlight into energy and releasing oxygen as waste. Why? To maintain its internal functions. A tree grows or withers depending on its environment. When the seasons change, the tree responds, budding out new leaves or shedding the old ones. The tree also reproduces. It might hide its seeds in fruit like an apple. It might send seeds spinning down like a maple in the form of a samara (winged fruit). It might encase its seeds in a cone like a pine. It might simply send out a long root to a new location like an aspen. The tree, it turns out, has far more in common with the snail than with the rock.

Biology involves an immense variety of life forms. Because these life forms all live on Earth, they are interrelated. During the past couple of centuries, people have begun to recognize the impact of environmental change on living things. Some environmental changes—earthquakes, tsunamis, or wildfires—are relatively easy to observe. Others are more elusive, such as chemical changes in the ocean or shifts in the atmosphere. Sometimes, a change in one species represents an environmental change for another species, as when a disease in frogs causes a drop in the population of the birds that eat those frogs.

Environmental change also includes the effects of human activity. The extent to which people affect the world’s climate, along with the responsibility human beings bear for attempting to reverse some of these changes, is still a topic of major debate. However, you can look around your neighborhood and see the human effects on microclimates. The eaves of your house provide a habitat for spiders; a building’s steam vent creates a warm area in which lichen thrive; trash bins sustain thriving communities of rats, birds, squirrels, raccoons, bacteria, and bugs. Likewise, all these living things affect how people live. The creatures in the trash might be doing us a favor by reducing our quantity of waste. They could be carrying diseases that might

drastically reduce our population. By catching mosquitoes and flies, the spider might be preventing disease and keeping your food supply safe. The lichen might eventually decompose into soil whose nutrient patterns allow certain rare plants to grow. These plants, in turn, attract different insects and foraging animals and reshape a tiny part of your world.