photorespiration

The chemical combination of carbohydrates with oxygen in plants with the release of carbon dioxide. Photorespiration requires the presence of light, is catalyzed in the chloroplasts by the same enzymes that catalyze the combination of carbohydrates with carbon dioxide during photosynthesis, and occurs when oxygen concentrations in the cell are high. Photorespiration typically takes place during conditions of high light intensity, dryness, and heat (often resulting in the closure of stomata), when the amount of carbon dioxide entering the plant is reduced, and the amount of oxygen produced by photosynthesis accumulates. Photorespiration thus acts to produce carbon dioxide when it is unavailable and acts as a check on photosynthesis and on the productivity of the plant. Unlike cellular respiration, photorespiration does not produce any ATP or NADH, and so consumes chemical energy rather than produces it. Many angiosperms have a supplemental method of carbon-dioxide uptake that minimizes losses from photorespiration.