photorespiration

photorespiration [¦fōd·ō‚res·pə′rā·shən]

(biochemistry)

Respiratory activity taking place in plants during the light period; CO₂is released and O₂is taken up, but no useful form of energy, such as adenosinetriphosphate, is derived.

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Photorespiration

Light-dependent carbon dioxide release and oxygen uptake in photosynthetic organisms caused by the fixation of oxygen instead of carbon dioxide during photosynthesis. This oxygenation reaction forms phosphoglycolate, which represents carbon lost from the photosynthetic pathway. Phosphoglycolate also inhibits photosynthesis if it is allowed to accumulate in the plant. The reactions of photorespiration break down phosphoglycolate and recover 75% of the carbon to 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. See Photosynthesis

The rate of photosynthesis can be stimulated as much as 50% by reducing photorespiration. Since photosynthesis provides the material necessary for plant growth, photorespiration inhibits plant growth by reducing the net rate of carbon dioxide assimilation (photosynthesis). Plants grow faster and larger under nonphotorespiratory conditions, in either low oxygen or high carbon dioxide atmospheres. Most of the beneficial effects on plant growth achieved by increasing CO₂ may result from the reduced rate of photorespiration. See Plant growth

There are some plants that avoid photorespiration under certain conditions by actively accumulating carbon dioxide inside the cells that have ribulose-bisphosphate carboxylase/oxygenase. Many cacti do this by taking up carbon dioxide at night and then releasing it during the day to allow normal photosynthesis. These plants are said to have crassulacean acid metabolism (CAM). Another group of plants, including corn (Zea mays), take up carbon dioxide by a special accumulating mechanism in one part of the leaf, then transport it to another part of the leaf for release and fixation by normal photosynthesis. The compound used to transport the carbon dioxide has four carbon atoms, and so these plants are called C₄ plants. Plants that have no mechanism for accumulating carbon dioxide produce the three-carbon compound phosphoglycerate directly and are therefore called C₃ plants. Most species of plants are C₃ plants. See Plant respiration