Photorespiration: RuBisCO's Oxygenase Activity
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RuBisCO sometimes binds oxygen instead of carbon dioxide, starting photorespiration, which spends ATP and releases fixed carbon without producing sugar.
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On a hot day with stomata partly closed, oxygen outpaces carbon dioxide inside the leaf, so RuBisCO binds oxygen more often.
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Photorespiration wastes energy and forfeits carbon already fixed, rather than merely reversing the Calvin cycle.