Mader Biology 8th Ed Mader Biology 8th Ed Calvin Cycle Reactions Calvin cycle is a series of reactions that produce carbohydrates before returning to the starting point again Utilizes atmospheric carbon dioxide to produce carbohydrates Includes Carbon dioxide fixation Carbon dioxide reduction RuBP Regeneration Mader Biology 8th Ed Mader Biology 8th Ed Calvin Cycle Reactions Step 1 Carbon Dioxide Fixation CO2 is attached to RuBP Ribulose 1 5 bis phosphate The result is a 6 carbon molecule which splits into two 3 carbon molecules Rubisco Ribulose 1 5 bisphosphate carboxylase oxygenase speeds up this reaction Rubisco is the most abundant enzyme Mader Biology 8th Ed C5 C3 C3 Mader Biology 8th Ed Calvin Cycle Reactions Step 2 Reduction of Carbon Dioxide Mader Biology 8th Ed Calvin Cycle Reactions Step 2 Reduction of Carbon Dioxide PGAP Mader Biology 8th Ed Calvin Cycle Reactions Step 3 Regeneration of RuBP C3 C5 Mader Biology 8th Ed Mader Biology 8th Ed Calvin Cycle Reactions Carbon Budget 3 x C5 3 x C 1 I CO2 Fixation 3 x C6 I 6 x C3 C3 Photosynthesis I PGA Tour CO2 Reduction I 6 x C3 1 x C3 PGAL Glucose I 5 x C3 I 3 x C5 RuBP Regeneration Mader Biology 8th Ed Calvin Cycle Reactions Energy Budget 3 x C5 3 x C 1 I 3 x C6 I 6 x C3 I PGA Tour requires 6 ATP 6 NADPH I 6 x C3 1 x C3 PGAL Glucose I 5 x C3 I RuBP Regeneration 3 x C5 requires 3 ATP Mader Biology 8th Ed Importance of Calvin Cycle PGAL glyceraldehyde 3 phosphate product of the Calvin cycle C 3 exported to the cytosol out of the chloroplast can be converted to a variety of organic molecules fatty acids and glycerol combined in plant oils sugars glucose fructose starch cellulose amino acid synthesis Mader Biology 8th Ed Mader Biology 8th Ed When Things Go Wrong Photorespiration Rubisco is not 100 specific for CO2 It can fix O2 Photorespiration Rubisco evolved when atmospheric CO2 concentration was much higher Normally no problem Gas exchange keeps O low and CO high 2 2 During water shortage Stomata close to save water O increases and CO decreases 2 2 Rubisco starts to fix O2 Mader Biology 8th Ed When Things Go Wrong Photorespiration Don t get confused it s not aerobic respiration Aerobic respiration Mitochondria Breakdown of food uses O2 releases CO2 Generates energy ATP GOOD Photorespiration Rubisco malfunctions and fixes O instead 2 of CO2 uses O2 releases CO2 Products are difficult to salvage Wastes energy BAD Mader Biology 8th Ed C4 Photosynthesis C4 plants Different leaf anatomy Bundle sheath cells and mesophyll cells contain chloroplasts Mesophyll cells are arranged concentrically around the bundle sheath cells Kranz anatomy Kranz is German for wreath Different mechanism to fix CO2 They can avoid photorespiration In hot dry climates net photosynthetic rate of C4 plants is about 2 3 times that of C3 plants Mader Biology 8th Ed Leaf Anatomy C3 vs C4 Mader Biology 8th Ed C4 Photosynthesis CO2 is fixed in mesophyll cells PEP Phosphoenolpyruvate PEP carboxylase Malate C 4 Mader Biology 8th Ed C4 Photosynthesis Malate is moved to bundle sheath cells CO2 is released and refixed in Calvin cycle Spatial separation C4 requires more energy 5 ATP per CO2 than C3 3ATP per CO2 but it avoids photorespiration Mader Biology 8th Ed Mader Biology 8th Ed Carbon Dioxide Fixation in C3 and C4 Plants Mader Biology 8th Ed CAM Photosynthesis special form of C 4 Crassulacean Acid Metabolism C4 plants partition carbon fixation in space while CAM partitions by time During the night CAM plants fix CO2 forming C4 molecules which are stored in large vacuoles During the day C molecules release 4 CO2 to Calvin cycle when NADPH and ATP are available Water Conservation Mader Biology 8th Ed Mader Biology 8th Ed Review Autotrophic organisms plants Photosynthetic Pigments Photosynthesis Light Reactions Noncyclic Cyclic Calvin Cycle Reactions C 4 CAM Mader Biology 8th Ed Mader Biology 8th Ed