The first enzyme of Calvin cycle, rubisco, as the name indicates, has both carboxylase and oxygenase activity. It means that rubisco catalyzes the fixation of both C02 and 02.
When C02 is fixed by the carboxylation reaction with RuBP, the product is 2 molecules of 3-PGA. But when 02 is fixed by the oxygenation reaction with RuBP, one molecule of 3-PGA and one molecule of phosphoglycolic acid are produced.
Whether rubisco performs as carboxylation or oxygenation reaction depends upon the ratio of C02/02. High ratio of C02/02 fixes C02 while low ratio fixes 02. Under normal atmospheric conditions of COo and 02, some amount of 0″ is always fixed by rubisco reaction producing phosphoglycolic acid in the chloroplast. Phosphoglycolic acid is subsequently metabolized in a series of reactions in the peroxisome and mitochondrion that result in the release of C02 and recovery of the remaining carbon of phosphoglycolic acid by the Calvin cycle.
Thus during active photosynthesis in light, apart from fixing C02 by the Calvin cycle, the leaves also consume O and release CO similar to that happens during cellular respiration. This light-dependent CO evolution and Photorespiratory pathway consumption is termed as photorespiration.
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The conventional respiration (cellular respiration), in order, to distinguish it from photorespiration, is often named as the dark respiration, even though it takes place both in light and in darkness. Unlike dark respiration, which takes place in the cytoplasm and mitochondrion, photorespiratory metabolism is carried out involving three organelles such as chloroplast, peroxisome and mitochondrion. Photorespiration is prevalent in C3 plants and by releasing C02 it decreases the net fixation of C02 in light during photosynthesis. Therefore, photorespiration is considered as a wasteful process in C3 plants.
It can be recalled here that in C. plants the product of carboxylation reaction in mesophyll cells is transported to bundle sheath cells where it is decarboxylated releasing C02. These reactions serve as a C02 concentrating mechanism in bundle sheath cells. Since rubisco is present only in the bundle sheath cells, high C02 concentration in these cells ensure that rubisco functions as a carboxylase minimizing oxygenase activity.
Thus in C4 plants photorespiration does not occur or is suppressed. For this reason, the productivity of C4 plants is higher than the C3 plants.