Calvin Cycle

Calvin cycle - aka C3 photosynthesis 

• creates organic molecules from CO2
• uses ATP (from cyclic/noncyclic photophosphorylation) to power endergonic reactions
• uses reducing power of NADPH to attach H to C atoms
• carbon fixation - CO2 binds to ribulose 1,5-biphosphate (RuBP)
  • RuBP - 5-carbon sugar made from reassembling bonds of fructose 6-phosphate and glyceraldehyde 3-phosphate
  • forms 2 molecules of 3-phosphoglycerate (PGA)
  • process catalyzed by rubisco (ribulose biphosphate carboxylase/oxygenase)
• 3 CO2 + 9 ATP + 6 NADPH + water >> glyceraldehydes 3-phosphate + 8 P + 9 ADP + 6 NADP+
• w/ 3 turns of Calvin cycle, 3 CO2 enters, 3 RuBP regenerated, 1 glyceraldehyde 3-phosphate created
• uses enzymes that functions best under light
• glyceraldehydes 3-phosphate - 3-carbon sugar that can be converted to fructose 6-phosphate and glucose 1-phosphate in cytoplasm w/ reversed glycolysis reactions
• glucose 1-phosphates combined into insoluble polymer as starch when there’s high levels of glyceraldehydes 3-phosphate

energy cycle - metabolisms of chloroplasts/mitochondria are related 

• photosynthesis uses products of respiration as starting substrates
• respiration uses products of photosynthesis as starting substrates
• Calvin cycle uses part of glycolytic pathway, in reverse, to make glucose
• enzymes used in both processes similar or the same

photorespiration - releases CO2 by attaching O2 to RuBP, reversing Calvin cycle 

• rubisco can oxidize RuBP, undoing the Calvin cycle
• CO2/O2 compete for same active site on rubisco enzyme
• at 25°C, rate of carboxylation 4x that of oxidation (20% of fixed carbon lost)
• higher temperature >> stomata close to conserve H2O >> CO2 can’t go in >> favors photorespiration
• 25-50% of photosynthetically fixed carbon lost through photorespiration

C4 photosynthesis - phosphoenolpyruvate (PEP) carboxylated to make 4-carbon compound  

• uses PEP carboxylase enzyme (attracts CO2 more than rubisco)
• no oxidation activity in 4-carbon compound >> no photorespiration
• minimalizes photorespiration when 4-carbon compound decarboxylates to contribute CO2 into the system

C4 pathway - used by plants in much warmer environments 

• C4 photosynthesis conducted in mesophyll, Calvin cycle conducted in bundle-sheath cells
• phosphoenolpyruvate (3-carbon) carboxylated to form oxaloacetate (4-carbon)
• oxaloacetate turned into malate in C4 plants
• malate decarboxylated into pyruvate in bundle-sheath cells, releasing CO2
• bundle-sheath cells retain CO2 for Calvin cycle
• pyruvate goes back to mesophyll, where it turns back to phosphoenolpyruvate
• requires 30 ATP (C3 photosynthesis needs 18), but more advantageous in hot climate

crassulacean acid metabolism ( CAM) - used by succulent (water-storing) plants 

• stomata close during the day, open at night (reverse of what happens in most plants)
• makes organic compounds at night, decarboxylates them to have high CO2 levels during the day
• uses both C4/C3 pathways in the same cells (C4 plants use C4/C3 pathways in different cells)