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Photosynthesis Alternative Mechanisms of Carbon Fixation RuBisCO Image from: http://www.ncbi.nlm.nih.gov/books/NBK22344/ • RuBisCO is the most abundant protein on Earth. • It catalyzes the first reaction of the Calvin cycle. Photorespiration Image from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm • In the first reaction of the Calvin cycle, CO2 is “fixed”, forming 2 molecules of 3-PGA. • O2 competes with CO2 for the active site of RuBisCO. Photorespiration Image from: http://access.mmhs.ca/docs/Science/MMHS%20Web%20folder/Kamla/20102008.htm • When RuBP is oxidized, it produces only 1 molecule of 3-PGA. • This process is called photorespiration. Photorespiration • In warmer temperatures, RuBisCO’s affinity for O2 increases, resulting in less CO2 being fixed into G3P. • How have plants in warmer climates evolved to cope with RuBisCO’s affinity for O2? C4 Photosynthesis Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html • CO2 is combined with PEP to form 4-carbon oxaloacetate (enzyme PEP carboxylase) in mesophyll cells • oxaloacetate is converted to malate, then transported into bundle-sheath cells • malate is decarboxylated forming pyruvate, and CO2 enters the Calvin cycle C4 Photosynthesis Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html C4 Photosynthesis Image from: http://www.botany.hawaii.edu/faculty/webb/bot311/bot311-00/psyn/PsynDark2.htm C4 Plants CAM Plants • crassulacean acid metabolism (CAM), first discovered in Crassulacea family of plants • stomata are closed during the day, so CO2 is only absorbed at night • C4 pathway at night, C3 pathway during the day CAM Plants Image from: http://hyperphysics.phy-astr.gsu.edu/hbase/biology/calvin.html CAM plants Animation Narrated animation of C4 photosynthesis and CAM plants: http://www.uic.edu/classes/bios/bios100/le ctures/c4.htm