There are two principal forms of energy: kinetic and potential. Kinetic energy is the energy of movement—the motion of molecules, for example. Potential energy is stored energy—the energy stored in covalent bonds, for example. Potential energy plays a particularly important role in the energy economy of cells.

Thermal energy, or heat, is a form of kinetic energy—the energy of the motion of molecules. For heat to do work, it must flow from a region of higher temperature—where the average speed of molecular motion is greater—to one of lower temperature. Although differences in temperature can exist between the internal and external environments of cells, these thermal gradients do not usually serve as the source of energy for cellular activities. The thermal energy in warm-blooded animals, which have evolved a mechanism for thermoregulation, is used chiefly to maintain constant organismal temperatures. This is an important homeostatic function because the rates of many cellular activities are temperature dependent. For example, cooling mammalian cells from their normal body temperature of 37 °C to 4 °C can virtually “freeze” or stop many cellular processes (e.g., intracellular membrane movements).

Radiant energy, the kinetic energy of photons, or waves of light, is critical to biology. Radiant energy can be converted to thermal energy, for instance, when light is absorbed by molecules and the energy is converted to molecular motion. Radiant energy absorbed by molecules can also change the electronic structure of the molecules by moving electrons into higher-energy orbitals, whence it can later be recovered to perform work. For example, during photosynthesis, light energy absorbed by pigment molecules such as chlorophyll is subsequently converted into the energy of chemical bonds (see Chapter 12).

Mechanical energy, a major form of kinetic energy in biology, usually results from the conversion of stored chemical energy. For example, changes in the lengths of cytoskeletal filaments generate forces that push or pull on membranes and organelles (see Chapters 17 and 18).

Electric energy—the energy of moving electrons or other charged particles—is yet another major form of kinetic energy, one with particular importance to membrane function, as in electrically active neurons (see Chapter 22).

Several forms of potential energy are biologically significant. Central to biology is chemical potential energy, the energy stored in the bonds connecting atoms in molecules. Indeed, most of the biochemical reactions described in this book involve the making or breaking of at least one covalent chemical bond. In general, energy must be expended to make covalent bonds in typical biomolecules, and energy is released when those bonds are broken. For example, the high potential energy in the covalent bonds of glucose can be released by controlled enzymatic combustion in cells (see Chapter 12). This energy is harnessed by the cell to do many kinds of work.

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A second biologically important form of potential energy is the energy in a concentration gradient. When the concentration of a substance on one side of a barrier, such as a membrane, is different from that on the other side, a concentration gradient exists. All cells form concentration gradients between their interior and the external fluids by selectively exchanging nutrients, waste products, and ions with their surroundings. Furthermore, the fluids within organelles in cells (e.g., mitochondria, lysosomes) frequently contain different concentrations of ions and other molecules than the cytoplasm; the concentration of protons within a lysosome, as we saw in the last section, is about 500 times that in the cytoplasm. Concentration gradients of protons across membranes are an important driver of energy production in mitochondria.

A third form of potential energy in cells is an electric potential—the energy of charge separation. For instance, there is a gradient of electric charge of about 200,000 volts per centimeter across the plasma membranes of virtually all cells. We discuss how concentration gradients and electric potential gradients are generated and maintained in Chapter 11 and how they are converted to chemical potential energy in Chapter 12.