4.3: As energy is captured and converted, the amount of energy available to do work decreases.

Every minute of every day—even on cloudy ones—the sun shines brightly, releasing tremendous amounts of energy. Organisms on earth cannot capture every bit of the sun’s energy that makes it to the surface of the earth; indeed, most plants capture only a tiny fraction of the available energy. What happens to the rest? This unused energy does not simply disappear. Accountants would love to monitor the flow of energy because, as in a good accounting ledger, all of the energy numbers add up perfectly. All energy from the sun can be accounted for. Some (probably less than 1%) is captured and transformed into usable chemical energy by organisms through photosynthesis (FIGURE 4-5). The rest of the energy from the sun is reflected back into space (probably about 30%) or is absorbed by land, the oceans, and the atmosphere (about 70%) and mostly transformed into heat. Heat is not easily harnessed to do work, however, and is therefore a much less useful form of energy than the energy transformed into chemical energy in plants (and stored as carbohydrates).

The same accounting also exists on a smaller level. If you eat a bowl of rice, some portion of the chemical energy stored in the bonds of the molecules that make up the rice grains is transformed into usable energy that can fuel your cells’ activities. All the rest is transformed into heat and is ultimately lost into the atmosphere.

The fact that energy can change form but never disappear is an important feature of energy in the universe, whether we are looking at the sun and the earth or a human and her rice bowl. Just as energy can never disappear or be destroyed, energy can never be created. All the energy now present in the universe has been here since the universe began, and everything that has happened since then has occurred by the transformation of one form of energy into another. In all our eating and growing, driving and sleeping, we are simply transforming energy. The study of the transformation of energy from one type to another, such as from potential energy to kinetic energy, is called thermodynamics, and the first law of thermodynamics states that energy can never be created or destroyed. It can only change from one form to another.

Because plants capture less than 1% of the sun’s energy, it might seem like they are particularly inefficient. But we humans are also rather inefficient at extracting the chemical energy of plants when we eat them. These inefficiencies occur because every time energy is converted from one form to another, some of the energy is converted to heat. When a human converts the chemical energy in a plate of spaghetti into the kinetic energy of running a marathon, or when a car transforms the chemical energy of gasoline into the kinetic energy of forward motion, some energy is converted to heat, the least usable form of kinetic energy. In automobiles, for example, about three-quarters of the energy in gasoline is lost as heat (FIGURE 4-6).

The second law of thermodynamics states that every conversion of energy is not perfectly efficient and invariably includes the transformation of some energy into heat. Although heat is certainly a form of energy, it is almost completely useless to living organisms for fueling their cellular activity because it is not easily harnessed to do work. Put another way, the second law of thermodynamics tells us that although the quantity of energy in the universe is not changing, its quality is. Little by little, the amount of energy that is available to do work decreases. Now that we understand that organisms on earth cannot capture every single bit of energy released by the sun—and that energy conversions are inefficient—we can look at the chief energy currency of the cell: ATP.