The CO 2 -fixing enzyme is called ribulose 1,5-bisphosphate carboxylase/oxygenase ( rubisco ). • Rubisco is found in all photosynthetic organisms that use the Calvin cycle to fix carbon, and is thought to be the most abundant enzyme on Earth. • Rubisco is inefficient because although it does catalyze the addition of CO 2 to RuBP, it also catalyzes the addition of O 2 to RuBP. Photorespiration • Oxygen and carbon dioxide compete at the enzyme’s active sites, which slows the rate of CO 2 reduction. • When O 2 and RuBP react in rubisco’s active site, one of the products undergoes a process called photorespiration . – Photorespiration “undoes” photosynthesis because it consumes energy and releases fixed CO 2 . • When photorespiration occurs, the rate of photosynthesis declines drastically. • Carbon fixation is favoured over photorespiration when a cell’s CO 2 concentration is high and O 2 concentration is low. Carbon Dioxide Enters/Leaves Through Stomata • Stomata are leaf structures where gas exchange occurs. They consist of two guard cells that change shape to open or close. • When a leaf’s CO 2 concentration is low during photosynthesis, stomata open to allow atmospheric CO 2 to diffuse into the leaf and its cells’ chloroplasts. • A strong concentration gradient favouring entry of CO 2 is maintained by the Calvin cycle, which constantly uses up the CO 2 in chloroplasts.
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