Living Systems use various forms of energy, which are Interconvertible.
There are two principal forms of energy: kinetic and potential. Kinetic energy is the energy of movement—the motion of molecules, for example. The second form of energy, potential energy, or stored energy, is more important in the study of biological or chemical systems.
Interconvertibility of All Forms of Energy
According to the first law of thermodynamics, energy is neither created nor destroyed, but can be converted from one form to another. In photosynthesis, for example, as we have just seen, the radiant energy of light is transformed into the chemical potential energy of the covalent bonds between the atoms in a sucrose or starch molecule. In muscles and nerves, chemical potential energy stored in covalent bonds is transformed, respectively, into kinetic and electric energy. In all cells, chemical potential energy, released by breakage of certain chemical bonds, is used to generate potential energy in the form of concentration and electric potential gradients. Similarly, energy stored in chemical concentration gradients or electric potential gradients is used to synthesize chemical bonds, or to transport other molecules “uphill” against a concentration gradient. This latter process occurs during the transport of nutrients such as glucose into certain cells and transport of many waste products out of cells. Because all forms of energy are interconvertible, they can be expressed in the same units of measurement, such as the calorie or kilocalorie
Oxidation occurs in over one-quarter of the known chemical reactions catalyzed by enzymes in living cells. In many cases this is accomplished by the transfer of hydrogen atoms or electrons from one molecule (hydrogen or electron donor) to another (the acceptor). Reactions of this type are the major source of energy for life processes. In other cases molecular oxygen is involved directly in the oxidation reaction. In all cases the enzymic oxidation reaction involves the participation of a cofactor which may merely serve as a second substrate (known as a coenzyme) or which may be an integral part of the enzyme, acting as a carrier of reducing equivalents (known as a prosthetic group). The principal sources of reducing equivalents are the numerous specific metabolic breakdown products of the major foodstuffs: carbohydrates, fats, and proteins. Energy release from these metabolites occurs in a stepwise series of hydrogen and electron transfers to molecular oxygen.