Anaerobic respiration

In biology, anaerobic respiration is a way for an organism to produce usable energy, in the form of adenosine triphosphate, or ATP, without the involvement of oxygen; it is respiration without oxygen. This process is mainly used by prokayotic organisms (bacteria) that live in environments devoid of oxygen. Although oxygen is not used, the process is still called respiration because the basic three steps of respiration are all used, namely glycolysis, the citric acid cycle, and the respiratory chain, or electron transport chain. It is the use of the third and final step that defines the process as respiration. In order for the electron transport chain to function, a final electron acceptor must be present to take the electron away from the system after it is used. In aerobic orgainisms, this final electron acceptor is oxygen. Oxygen is a highly electronegative atom and therefore is an excellent candidate for the job. In anaerobes, the chain still functions, but oxygen is not used as the final electron acceptor. Other less electronegative substances such as sulfate (SO₄), nitrate (NO₃), and sulfur (S) are used. Oftentimes, anaerobic organisms are obligate anaerobes, meaning they can only respire using anaerobic compounds and can actually die in the presence of oxygen.

Anaerobic respiration is not the same as fermentation, which does not use either the citric acid cycle or the respiratory chain (electron transport chain) and therefore, cannot be classified as respiration.

Etymology

The term anaerobic comes from the Greek: "aer" means air and "bios" means life, while the prefix "an-" means without.

Examples of anaerobic respiration

glucose + 3NO₃^- + 3H₂O $\to$ 6HCO₃^- + 3NH₄⁺, ΔG⁰' = -1796 kJ

glucose + 3SO₄^2- + 3H⁺ $\to$ 6HCO₃^- + 3SH^-, ΔG⁰' = -453 kJ

glucose + 12S + 12H₂O $\to$ 6HCO₃^- + 12SH^- + 18H⁺, ΔG⁰' = -333 kJ

These very important terminal electron acceptors (nitrate NO₃^-, sulfate SO₄^2-, elemental sulfur S) have smaller reduction potentials than O₂, meaning that less energy is released per oxidized molecule of primary electron donor in the above reactions) than in aerobic respiration (i.e. it is less energetically efficient).

See also

• Anaerobic digestion
• Mechanical biological treatment

Bibliography

• Lawrence, Eleanor; Holmes, Sandra (1989), Henderson's dictionary of biological terms (10th ed.), University of Michigan: Wiley, ISBN 9780470214466
• Lane, Nick (2005), Power, sex, suicide: mitochondria and the meaning of life, Oxford University Press, ISBN 9780192804815
• Sparknotes Biology Study Guide Anaerobic Respiration