Photorespiration

Article By:

Sharkey, Thomas D. Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.511500

Light-dependent carbon dioxide release and oxygen uptake in photosynthetic organisms caused by the fixation of oxygen instead of carbon dioxide during photosynthesis. When the reactions of photosynthesis use oxygen instead of carbon dioxide, phosphoglycolate is formed and there is a loss of carbon from the photosynthetic pathway. Phosphoglycolate inhibits photosynthesis if it is allowed to accumulate in the plant. The reactions of photorespiration breakdown phosphoglycolate and recover 75% of the carbon for the photosynthetic reaction sequence. The remaining 25% of the carbon is released as carbon dioxide. Photorespiration reduces the rate of photosynthesis in plants in three ways: carbon dioxide is released; energy is diverted from photosynthetic reactions to photorespiratory reactions; and competition between oxygen and carbon dioxide reduces the efficiency of the important photosynthetic enzyme ribulose-bisphosphate (RuBP) carboxylase. There is no known function of the oxygenation reaction; most scientists believe it is an unavoidable side reaction of photosynthesis, but it is possible that photorespiration improves nitrate reduction. See also: Carbon dioxide; Oxygen; Photosynthesis

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