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Photorespiration is a respiratory process in many higher plants by which they take up oxygen in the light and give out some carbon dioxide, contrary to the general pattern of photosynthesis.

Photorespiration occurs when the CO2 levels inside a leaf become low. This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss.

Photorespiration begins in the chloroplast, when rubisco attaches oxygen to RuBP in its oxygenase reaction. Two molecules are produced in this reaction: a three-carbon compound, 3-PGA, and a two-carbon compound, phosphoglycolate.

Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates. RuBISCO, the enzyme which fixes carbon dioxide during the Calvin cycle, is also responsible for oxygen fixation during photorespiration.

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