Energy Production in Biological Systems
Our society depends on energy derived from burning fuels of various kinds in "engines" designed to release, capture, and deliver that energy in a useful form. Typically, wood, oil or coal are burned in the presence of oxygen (air) to liberate energy with the production of CO2 and water. These combustion processes typically occur quite rapidly and generally require relatively high temperatures.
Biological systems operate on the basis of similar energetic principles. There are three types of fuel consumed by a living cell, carbohydrate, fat, and protein and living cells derive large amounts of energy from the "burning" (oxidation) of these fuels. These biological fuels are closely related to those used in mechanical engines and their consumption produces the same waste products, CO2 and water. Carbohydrate and wood are very similar (starch and cellulose are both composed entirely of glucose), fat and oil are both hydrocarbons, and protein is converted to a complex set of carbon based molecules.
While the overall energetics are similar, the molecular processes of energy production in living cells are profoundly different. Unlike mechanical engines, living cells can produce energy very efficiently at temperatures of less than 100 degrees F while capturing that energy and coupling it to systems that carry out useful work with very little waste or loss.
A detailed treatment of the reactions involved in energy production is not
necessary for our purposes here. However, it is useful to note that all biological fuels ultimately are converted to one of a small number of intermediates in what is known as the Tricarboxylic Acid Cycle (TCA cycle; sometimes called the Citric Acid Cycle.) The name TCA was given to this cycle because a key intermediate is citric acid, a molecule that contains three carboxylic acid (COOH) groups.
The TCA cycle can be viewed as starting with a reaction in which a four carbon molecule (oxaloacetate) combines with a two carbon molecule (acetyl- CoA) to form a six carbon molecule (citrate). The following steps of the cycle involve a series of reactions in which two carbons are released as carbon dioxide (CO2) and the remaining four carbons are used to reform the original four carbon starting material, oxaloacetate. With every turn of the cycle, two carbons are oxidized to CO2 and the material
(oxaloacetate) needed for another turn of the cycle is regenerated.
The oxidation of carbon to CO2 is coupled to the reduction of other compounds. These in turn are oxidized (donate electrons) by a series of reactions coupled to a final step that consumes molecular oxygen to produce water. These reactions are all carried out in a small subcellular "organelle," the mitochondrion, and taken together lead to the production of a great deal of useful energy.