Science 101: Biology Chapter One The Mystery of Life Almost every aspect of life is mysterious. How do birds fly? What makes a heart beat? Why are redwood trees tall, and why does a child resemble her parents? How does disease spread? Why are humans different from other animals? What makes the moss on a rock different from the rock? How did living things get here? What is life? A good way to begin answering questions like these is to understand some of the basic concepts of biology. Biologists have learned to recognize key characteristics that often are found in living things, including reproduction, growth, metabolism, and adaptation, to answer the question, "What is life?" Living things also can be understood as concentric layers of organization, from subatomic particles to the biosphere itself, the total region of Earth where life exists. It is also important to understand persistent themes in the history of life. These include continuity and change (evident in the process known as evolution) and a strong relationship between structures and the functions they perform. Such concepts are a useful beginning to probing the mystery of life. What Is Life? Life is not easy to define, evenfor biologists, the people who study it as a profession. The average person can usually tell a living thing from a nonliving one, but articulating what makes them different is difficult. Is the difference that an ant moves but a pebble does not? No: a pebble moves under certain conditions, such as rolling down a slope. Is it that a flower grows but a crystal doesn't? But crystals do grow under the right conditions. There may not be a comprehensive one-line definition of life, but life can be defined in terms of several characteristics that are typically present in living things. These include reproduction, growth, metabolism, irritability, and adaptation. Reproduction and Growth Living things make more of themselves--in two senses. Through reproduction, they make more individuals of their own species, or kind. Through growth, they make their own bodies larger in an orderly way, with all parts increasing in size. Both processes depend on division of the most basic biological unit: the cell. A cell splits to form two new cells. In single-celled organisms, such as bacteria, this is enough to bring about reproduction: two organisms now exist instead of one. In organisms that have many cells and reproduce sexually, the process is more complicated. Simple cell division will result in growth of tissues--for example, when the muscle cells in a weightlifter's arms divide, the arms thicken.But reproduction requires the union of two sex cells, one each from a male and a female. The result is an embryo that will grow into a new individual. Using and Finding Food To stay alive, living things must take in energy and raw materials, use them, and discard the waste products. They get their energy in many ways--a meadow absorbing sunlight, bees collecting pollen and nectar from flowers, or a tiger eating its prey. But they all need the energy and raw materials known as food to fuel their reactions, maintain their bodies, and grow. The sum of the chemical processes by which organisms perform these activities is known as metabolism. To succeed in finding food--and in the equally important task of avoiding becoming food--living things must be responsive to their environments. They must have the property of irritability, or sensitivity, the ability to sense and respond to stimuli. The sense apparatus may be as simple as the light-sensitive eyespot of a one-celled algae or as complex as an insect's compound eye. Sensitivity usually sparks some kind of movement, such as the sudden flight of birds when a person draws near, or the bending of a plant toward sunlight. To survive, living things must also fit themselves, or adapt, to their environments. They do so in the long term when the most successful members of a population--the fastest sharks or the tallest oaks--pass on to their descendants those characteristics that made them successful. This kind of adaptation contributes to the process of evolution through which species change and new species originate. Adaptation happens in the short term when an organism adjusts to passing changes, as when a person's pupils shrink on leaving a dark room and entering into sunlight. This and other adaptations are part of the arsenal that living things draw upon in order to metabolize, grow, and reproduce--the basic characteristics of life. A Tendency Toward Order Although life is different from nonlife, it is not completely different. Living things exist in a nonliving universe and depend on it in many ways, from plants absorbing energy from sunlight to bats finding shelter in caves. Indeed, living things are made of the same tiny particles--subatomic particles--that make up nonliving things. What makes organisms different from the materials that compose them is their level of organization. Living things exhibit not just one but many layers of biological organization. This tendency toward order is sometimes modeled in a pyramid of life. The Pyramid of Life In this pyramid, each level has structures that are larger and more complex than those below it. The structures on each level contain those below it, but do not contain any above it. For example, an organ contains tissues, which contain cells, but organs do not contain organisms; rather, it is the organism that contains organs. So the pyramid of life is a pyramid of increasing complexity until one reaches the top, the entire biosphere, or the region of Earth that contains life. The pyramid levels are: 1. subatomic particle--A unit of matter, such as a proton,electron, or neutron that can compose atoms. 2. atom--A larger unit of matter that can compose even larger units called molecules. 3. molecule--A molecule is the smallest part of a substance that still has the chemical identity of the substance. For example, a water molecule still behaves like water, but if broken into its constituents, one oxygen and two hydrogen atoms, it will not. Science 101: Biology . Copyright © by George Ochoa. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold. Excerpted from Biology by George Ochoa All rights reserved by the original copyright owners. 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