21.5 Metabolism without oxygen

In aerobic respiration, the final electron acceptor is an oxygen molecule, O ₂ . If aerobic respiration occurs, then ATP will be produced using the energy of high-energy electrons carried by NADH or FADH ₂ to the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD ⁺ for reuse as an electron carrier for the glycolytic pathway to continue. How is this done? Some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD ⁺ from NADH are collectively referred to as fermentation    . In contrast, some living systems use an inorganic molecule as a final electron acceptor. Both methods are called anaerobic cellular respiration    in which organisms convert energy for their use in the absence of oxygen.

Anaerobic cellular respiration

Certain prokaryotes, including some species of bacteria and Archaea, use anaerobic respiration. For example, the group of Archaea called methanogens reduces carbon dioxide to methane to oxidize NADH. These microorganisms are found in soil and in the digestive tracts of ruminants, such as cows and sheep. Similarly, sulfate-reducing bacteria and Archaea, most of which are anaerobic ( [link] ), reduce sulfate to hydrogen sulfide to regenerate NAD ⁺ from NADH.

Lactic acid fermentation

The fermentation method used by animals and certain bacteria, like those in yogurt, is lactic acid fermentation ( [link] ). This type of fermentation is used routinely in mammalian red blood cells and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue (that is, in muscles used to the point of fatigue). In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism. The chemical reactions of lactic acid fermentation are the following:

The enzyme used in this reaction is lactate dehydrogenase (LDH). The reaction can proceed in either direction, but the reaction from left to right is inhibited by acidic conditions. Such lactic acid accumulation was once believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this hypothesis. Once the lactic acid has been removed from the muscle and circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy.