ABSTRACT
In all known organisms, adenosine triphosphate (ATP) acts as the main energy-carrying molecule. e chemical energy is held in the energy-rich bonds of ATP and released when, during hydrolysis of the terminal phosphate bond of ATP to produce adenosine diphosphate (ADP), the phosphate is transferred to another molecule. e energy released is used for pumping ions against electrochemical gradients, the beating of cilia, biosynthetic reactions, the contraction of muscles and almost all other energy requiring processes in cells. At any one time, the ATP present in the cells of the body is only sucient for a few seconds of energy usage and it must be continuously re-synthesized from ADP. Synthesis of ATP involves oxidation of energy rich dietary substrates (carbohydrates, fatty acids and proteins). Some of these ATP-producing reactions require oxygen and some do not. ose that do not use oxygen (‘anaerobic’) can take place outside the mitochondria in the cytosol but are not very e- cient in terms of the amount of ATP produced from each molecule of substrate. For example, in anaerobic metabolism, the pyruvic acid formed by the initial step in the breakdown of glucose is further metabolised to lactic acid, producing just two molecules of ATP from each molecule of glucose. e overall reaction for this anaerobic glycolysis is:
C6H12O6 + 2 ADP + 2 Pi → 2 lactate + 2H+ + 2 ATP
When the oxygen supply is adequate, the electron transport chain in the mitochondria functions properly,
oxidative phosphorylation takes place and the glucose is completely oxidized to carbon dioxide and water producing many more molecules of ATP (about 38) per glucose molecule. e overall reaction in the presence of aerobic respiration is summarized by the equation:
C6H12O6 + 38 ADP + 38 Pi + 6O2 → 6 CO2 + 6 H2O + 38 ATP
Anaerobic glycolysis will occur whenever the mitochondria have an inadequate oxygen supply, for example, in skeletal muscle cells during heavy exercise. As the intensity of exercise increases, a point is reached where the exercising muscle cells farthest from the nearest capillary have an inadequate oxygen supply and lactate production begins. As work rate increases beyond this ‘anaerobic threshold’, more and more cells produce lactic acid, which accumulates in the muscle as well as passing into the bloodstream. In the heart it can be taken up and converted back to pyruvate, which enters the aerobic respiration pathways in the mitochondria. In the liver, especially during the recovery period aer exercise, lactic acid is converted to glucose and glycogen.
