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274 Bioenergetics: How Do Organisms Acquire and Use Energy? Definition of life: “A living organism is characterized by the ability to effect a temporary and local decrease in entropy by means of enzyme-catalyzed chemical reactions.” —Isaac Asimov, 1962 Chapter opening photo Every living thing requires a constant supply of energy to stay alive. Plants use the energy of sunlight to make organic molecules. These hikers obtain energy from their food, which is made up of organic molecules produced by plants and other animals that ate plants. Overview Amid the fantastic array of living forms described in Chapter 9, one feature of life is so obvious that we rarely stop to consider it: Living things are distinct entities, separate from their environment. What is not so obvious is that distinction comes at a price, which is paid in the currency of energy. Every living organism requires a constant 10
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10-1 How Does Energy Behave in the Universe? 275 supply of energy to stay alive. That energy ultimately comes from the sun. If we fail to replenish the energy that we use, we die and slowly deteriorate until the differences between us and our surroundings disappear. For many centuries, it was believed that the energy of life was somehow different from other forms of energy in the universe. Life was defined and characterized by the existence of a special vital force . It was mistakenly believed that the vital force followed its own set of rules, different from the rules that govern the flow of energy in the inanimate world. We now recognize that energy, in all its various forms, is the same in both the living and nonliving worlds. The rules that govern energy apply universally. In this chapter, we will examine what those rules are and how the chemistry of life has evolved to capture and use energy without breaking the rules. 10-1 How Does Energy Behave in the Universe? Can you distinguish between something that is alive and something that is not? The question may seem trivial at first. Of course animals breathe, move, eat, and reproduce— all things that inanimate objects cannot do.When they cease to be alive, those activities also cease. Plants grow, reproduce, absorb sunlight, consume carbon dioxide, and turn it into complex carbohydrates.They, too, are clearly alive. But what of a virus? A virus is a particle of organic material—mainly protein and nucleic acid—that utterly depends upon a living host to reproduce. Some, including the authors of this book, do not con- sider viruses living. But others argue quite convincingly otherwise. Even more perplex- ing is the tiny nematode Aphelenchus , a microscopic worm that lives in seasonal ponds. In the winter, the worm crawls and wriggles, clearly alive. But in summer, when the pond dries up, the worm forms a dry coil that could turn into so much dust with a puff of air.
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