Thermodynamically, biological oxidation of organic substrates is comparable to nonbiological oxidations, such as the burning of wood. The total free energy released is the same, whether the source is a biological substance, such as glucose, or the oxidation of a compound in a wood fire, calorimeter, or cell.

Biological oxidations, however, are far more complex processes than combustion. When wood is burned, all of the energy is released as heat; that is, useful work cannot be performed, except through the action of a device such as a steam engine. In biological oxidations, by contrast, oxidation reactions occur without a large increase in temperature and with capture of some of the free energy as chemical energy.

Metabolic energy capture occurs largely through the synthesis of ATP, a molecule designed to provide energy for biological work. The capture of energy is quite efficient. In the catabolism of glucose, for example, about 40% of the 2870 kJ/mol of energy released is used to drive the synthesis of ATP from ADP and Pi.

Unlike the oxidation of glucose by oxygen (as in a fire), most biological oxidations do not involve direct transfer of electrons from a substrate directly to oxygen. Instead, a series of coupled oxidation-reduction reactions occurs, with the electrons passed to intermediate electron carriers such as NAD⁺ before they are finally transferred to oxygen.

Because the potential energy stored in the organic substrate is released in small increments, it is easier to control oxidation and capture some of the energy as it is released-small energy transfers waste less energy than a single large transfer.

Not all metabolic energy comes from oxidation by oxygen. Substances other than oxygen can serve as terminal electron acceptors. For example, some microogranisms growing anaerobically (in the absence of oxygen) generate energy by transferring electrons to inorganic substances, such as sulfate ion or nitrate ion. Other microorganisms, like the lactic acid bacteria, reduce organic substances, such as pyruvate, to form lactate. Most of these organisms derive their energy from fermentations, which are energy-yielding catabolic pathways that proceed with no net change in the oxidation state of the products as compared with that of the substrates.

Because metabolic energy comes primarily from oxidative reactions, the more highly reduced a substrate, the higher its potential for generating biological energy. Thus, combustion of fat provides more heat energy than combustion of an equivalent mass of carbohydrate.

Reducing equivalents can be defined as 1 mole of hydrogen atoms (one proton and one electron per H atom). For example, two reducing equivalents are used in the reduction of one half mole of oxygen to water

¹/₂ O2 + 2e^- + 2H⁺ -> H2O

Remember that the breakdown of complex organic compounds yields both energy and reducing equivalents, but the biosynthesis of such compounds utilizes both.