4.1–4.4: Energy flows from the sun and through all life on earth.

Sunlight shines through the redwoods.

Imagine that you are on a long road trip. Your car’s fuel gauge is nearing empty, so you pull off the highway. Instead of driving into a gas station, however, you head to the back of a fast-food restaurant and fill the fuel tank with used cooking grease. You head back to the highway, ready to drive several hundred miles before needing another pit stop.

Fast food for your car? Yes! The idea isn’t as far-fetched as it sounds. In fact, on the roads of America today, many vehicles run on biofuels, fuels produced from plant and animal products (FIGURE 4-1). Most vehicles, however, run on fossil fuels such as gasoline. These fuels (which also include oil, natural gas, and coal) are produced from the decayed remains of plants and animals modified over millions of years by heat, pressure, and bacteria.

Figure 4.1: Biofuel technology. Biofuels are chemically similar to fossil fuels.

It turns out that biofuels, fossil fuels, and the food fuels that supply energy to most living organisms are chemically similar. This fact is not surprising because energy from the sun is the source of the energy stored in the chemical bonds between the atoms in all these fuels. Let’s investigate how fuels provide energy.

When we burn gasoline, long chains of carbon and hydrogen atoms are broken down. As the bonds linking those atoms are broken and molecules with lower-energy bonds are formed, carbon dioxide (CO₂) and water are produced, and a lot of energy that was stored in the chemical bonds holding each gasoline molecule together is released. (An interesting fact: the energy released in burning one gallon of gas is equivalent to the caloric content of 15 large cheese pizzas.) In an automobile engine, some of this released energy is harnessed to push pistons, spin a crankshaft, turn wheels, and move the car.

Animal fats and the oils in many plants—such as those used to cook french fries—share an important chemical feature with gasoline. Like gasoline, these fats and oils contain chains of carbon and hydrogen atoms bound together, and just as with gasoline, breaking these bonds and forming new, lower-energy bonds releases large amounts of energy (and water and CO₂). If this released energy can be captured efficiently, it, too, can be used to push pistons and turn car wheels.

Cars that run on biofuels are more than just a technological trick. The production of biofuels requires only the plant or animal source, sunlight, air, water, and a relatively short amount of time—a few months or years, depending on the source. On the other hand, the production of fossil fuels such as coal or crude oil requires plant and animal remains and millions of years. This difference gives biofuels an important advantage over fossil fuels: they are a renewable resource. For this reason, biofuels point the way toward a future of reduced dependence on fossil fuels, the supplies of which are dwindling and the combustion of which has many harmful consequences, such as increasing global warming and releasing cancer-causing particles into the atmosphere.

Are we at the point yet where all our cars can run on biofuels? Not quite. There are several significant drawbacks to the increased use of biofuels, chief among them the destruction of forests, wetlands, and other ecologically important habitats resulting from the increased use of land to grow these fuels. (And fossil fuel and water are used in the process.) This is why the search for better fuels continues.

In this chapter we explore how plant, animal, and other living “machines” run on energy stored in chemical bonds. Nearly all life depends on capturing energy from the sun and converting it into forms that living organisms can use. This energy capture and conversion occurs in two important processes that mirror each other: (1) photosynthesis, the process by which plants capture energy from the sun and store it in the chemical bonds of sugars and (2) cellular respiration, the process by which all living organisms release the energy stored in the chemical bonds of food molecules and use it to fuel their lives (FIGURE 4-2). The sun to you in just two steps!

Figure 4.2: Photosynthesis and cellular respiration. The sun to you in just two steps!