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Photorespiration Photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 Respiration (mitochondrial respiration) C6H12O6 + 6O2 6CO2 + 6H2O Photorespiration C6H12O6 + 6O2 6CO2 + 6H2O Sunflower leaves The relative rate of photosynthesis is a function of temperature. Hew, Krotkov, Canvin, Plant Physiol 44:671, 1969 The relative rate of photosynthesis is a function of temperature. The relative rate of photosynthesis is a function of temperature. Why is Rubisco called a “carboxylase-oxygenase? - Alternative activity at high temperatures: HO The metabolism of phosphoglycolate involves enzymes In the peroxisome and the mitochondrion, as well as the chloroplast The metabolism of phosphoglycolate involves enzymes In the peroxisome and the mitochondrion, as well as the chloroplast The metabolism of phosphoglycolate involves enzymes In the chloroplast, peroxisome and the mitochondrion, as well as the chloroplast 2 RuBP + 2 O2 2 P-glycolate + 2 PGA O2 ATP 2P PGA CO2 2 RuBP + 3 O2 + ATP 3 PGA + CO2 + 2 Pi 3 PGA Why does high temperature promote photorespiration? CO2 vs O2 solubilities An increase in temperature lowers the concentration of CO2 in the cytoplasm relative to the concentration of O2. Is the formation of P-glycolate by rubisco an “evolutionary mistake,” which occurred because the primordial atmosphere had a low O2 concentration? If high temperatures increase photorespiration, will global warming result in a decrease in plant productivity? If high temperatures increase photorespiration, will global warming result in a decrease in plant productivity? Or will an increase in atmospheric CO2 produce a general increase in plant productivity? The high temperatures excluded gymnosperms, especially from low latitudes. Some plants show better net photosynthesis at high temperatures than others. Some plants show better net photosynthesis at high temperatures than others. Apparent photosynthesis = Gross photosynthesis - Photorespiration - Mitochondrial Respiration What accounts for the difference in optimum temperature between Spartina and Leucopoa? Summary •The apparent rate of photosynthesis is a function of temperature, with most plants showing a maximum at about 25oC. •Rubisco can react with O2, to produce P-glycolate and P-glycerate. •The metabolism of phosphoglycolate results in O2 uptake and CO2 production: photorespiration. •High temperature promotes photorespiration by increasing O2/CO2. •Some plants show maximum photosynthesis at 35-45oC.