Unit two. The Living Cell
5. Energy and Life
5.2. The Laws of Thermodynamics
Running, thinking, singing, reading these words—all activities of living organisms involve changes in energy. A set of universal laws we call the laws of thermodynamics govern these and all other energy changes in the universe.
The First Law of Thermodynamics
The first of these universal laws, the first law of thermodynamics, concerns the amount of energy in the universe. It states that energy can change from one state to another (from potential to kinetic, for example) but it can never be destroyed, nor can new energy be made. The total amount of energy in the universe remains constant.
A lion eating a giraffe is in the process of acquiring energy. Rather than creating new energy or capturing the energy in sunlight, the lion is merely transferring some of the potential energy stored in the giraffe’s tissues to its own body (just as the giraffe obtained the potential energy stored in the plants it ate while it was alive). Within any living organism, this chemical potential energy can be shifted to other molecules and stored in chemical bonds, or it can be converted into kinetic energy, or into other forms of energy such as light or electrical energy. During each conversion, some of the energy dissipates into the environment as heat energy, a measure of the random motions of molecules (and, hence, a measure of one form of kinetic energy). Energy continuously flows through the biological world in one direction, with new energy from the sun constantly entering the system to replace the energy dissipated as heat.
Heat can be harnessed to do work only when there is a heat gradient—that is, a temperature difference between two areas. This is how a steam engine functions. In old steam locomotives like you see in figure 5.2, heat was used to move the wheels. First, a boiler (not shown) heats up water to create steam. The steam is then pumped into the cylinder of the steam engine, where it moves the piston to the right. The moving of this piston then does the work of the steam engine by moving a lever that turns the wheel. Cells are too small to maintain significant internal temperature differences, so heat energy is incapable of doing the work of cells. Thus, although the total amount of energy in the universe remains constant, the energy available to do useful work in a cell decreases, as progressively more of it dissipates as heat.
Figure 5.2. A steam engine.
In a steam engine, heat is used to produce steam. The expanding steam pushes against a piston that causes the wheel to turn.
The Second Law of Thermodynamics
The second law of thermodynamics concerns this transformation of potential energy into heat, or random molecular motion. It states that the disorder in a closed system like the universe is continuously increasing. Put simply, disorder is more likely than order. For example, it is much more likely that a column of bricks will tumble over than that a pile of bricks will arrange themselves spontaneously to form a column. In general, energy transformations proceed spontaneously to convert matter from a more ordered, less stable form, to a less ordered, more stable form. Without an input of energy from the teenager (or a parent), the ordered room in figure 5.3 falls into disorder.
Figure 5.3 Entropy in action.
As time elapses, a teenager's room becomes more disorganized. It takes energy to clean it up.
Entropy is a measure of the degree of disorder of a system, so the second law of thermodynamics can also be stated simply as “entropy increases.” When the universe formed 10 to 20 billion years ago, it held all the potential energy it will ever have. It has become progressively more disordered ever since, with every energy exchange increasing the entropy of the universe.
Key Learning Outcome 5.2. The first law of thermodynamics states that energy cannot be created or destroyed; it can only undergo conversion from one form to another. The second law states that disorder (entropy) in the universe tends to increase. Life converts energy from the sun to other forms of energy that drive life processes; the energy is never lost, but as it is used, more and more of it is converted to heat.