Types of Respiration

Hydrogen atoms removed during glucose metabolism are accepted by NAD+, which is reduced to NAD+H+. NAD+ must be regenerated from NADH+ in order to pick up more hydrogen atoms. If this did not happen glycosis would stop, resulting in the death of the organism. Living organisms have various for disposing off hydrogen from NADH+H+. The three main types of respiration are based on the manner in which organisms oxidize NADH+H+ to NAD+. In all these three types of respiration glycolysis is the common feature. The fate of pyruvic acid and the method of hydrogen disposal determine the types of respiration, which are (1) fermentation (2) anaerobic respiration by some bacteria, and (3) aerobic respiration.

(1) Fermentation takes place under anaerobic conditions, and is of two main types, alcoholic fermentation and l atic acid fermentation (muscle glycolysis). Alcoholic fermentation is carried on by yeasts, some bacteria, and even green plants under lack of oxygen. Pyruvic acid is first decarboxyplated to acetaldehyde, which is then reduced to ehthyl alcohol by NADH, and NAD+ is regenerated.



Lactic acid fermentations (muscle glycolysis) is characteristic of animals and some forms of bacteria. Pyruvic acid formed as a result of glycolysis acts as a hydrogen acceptor and yields lactic acid.

NAD+ is regenerated. Both types of fementation yield only two mole cules of ATP.

(2) In anaerobic respiration of some bacteria, hydrogen is passed on to oxygen which is not in the free form. It is found as a part an inorganic compound such as sulphate or nitrate.

(3) Aerobic respiration takes place in the presence of free oxygen. As usual, pyruvic acid is first formed glucose by glycolysis. Pyruvic acid is completely oxidized through the Kerbs citric acid cycle and the hydrogen transport system to yield carbon dioxide and water. Oxygen serves as the final hydrogen acceptor. For each molecule of glucose completely oxidized to carbon dioxide and water, 38 molecules of ATP are generated.




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