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Transcript
Lecture Outline 1. The Calvin Cycle fixes carbon makes reduced carbon compounds 2. Reactions of the Calvin Cycle – anabolic pathway input of NADPH + H+, input of ATP 3. Regulation of the Calvin Cycle 4. The problem with oxygen – Photorespiration 5. Tricks some plants use to limit photorespiration - C4 anatomy, C4 metabolism – division of labor - CAM plants, the difference is night and day Lecture 17 Photosynthesis Oct 10, 2005 II. Calvin Cycle 1 2 The purpose of the Carbon-fixation (Calvin Cycle) Reactions CO2 + NADPH + H+ + ATP DARK REACTIONS energy utilization C6H12O6 + NADP+ + ADP + Pi carbohydrate The Calvin Cycle 3 Note: synthesis of carbohydrate from CO2 is favorable only because coupled to very favorable reactions NADPH to NADP+ and ATP to ADP + Pi energy released is greater than it costs to make carbohydrate 4 Overview of Carbon-Fixation Reactions 24 CO2 – Carbon fixation – Reduction (energy input, reducing equiv input) – Regeneration of the CO2 acceptor (energy input – “priming step”) + Carbon Acid 2 3with one phosphate on it 5 Carbon Sugar with 2 phosphates on it ATP ADP + Pi ADP + Pi NADPH ATP NADP+ 12 with one phosphate on it 24 O 3 Carbon Aldehyde R-C-H = 20/24 5 Carbon Sugar with one phosphate on it 4/24 6 Carbon Sugar - Glucose 2 5 R-C-OH Reduction The Calvin cycle has three phases O = 12 Higher Energy Compound 6 with NO phosphate on it Carbon Fixation Carbon Fixation Priming Step Acid Rubisco -COOH Input of energy Reduction Aldehyde -C=O H Carried out by the enzyme “rubisco” (ribulose 1,5 bisphosphate carboxylase oxygenase) Do need to know this enzyme7 Key regulatory enzyme Regenerate What started with 8 Regulation of Rubisco Enzyme in Calvin Cycle Integration of Light-Dependent and Light-Independent Reactions 1st Substrate/Product availability Allosterically regulated by NADPH and ATP Very Narrow pH optimum They generally occur AT THE SAME TIME pH 8 / pH7 Light reaction Calvin cycle H2O CO2 Light NADP+ ADP +P1 pH 8 or above, inactive at 7 RuBP Photosystem II Electron transport chain Photosystem I Enzyme must be in reduced form ATP 3-Phosphoglycerate G3P Starch (storage) NADPH 9 Figure 10.21 Chloroplast Amino acids Fatty acids O2 PhotoRespiration - “ the OXYGEN PROBLEM” Some Plants Deal with this problem by a Oxygen is a competing substrate for the 1st enzyme DIVISION OF LABOR BETWEEN CELLS in the C3 cycle (Rubisco) CO2 + 5 Carbon Sugar with 2 phosphates on it O2 + 5 Carbon Sugar with 2 phosphates on it 3 Carbon Acid with one phosphate on it 3 Carbon Acid with one phosphate on it 2 Carbon Acid with one phosphate on it 3 Carbon Acid with one phosphate on it Net increase in material make glucose with “extra” Energy Wasted With NO synthesis 11 of glucose 10 Sucrose (export) C4 Plants Mesophyll cells perform “usual” noncyclic LightLight-Dependent Reactions make oxygen, oxygen, ATP and NADPH Do NOT perform the C3 (Calvin cycle) reactions Bundle Sheath cells perform “UN usual”” “UNusual cyclic LightLight-Dependent Reactions a lot of ATP but very little NADPH and very little O2 12 PERFORM the usual C3 (Calvin Cycle) reactions C4 leaf anatomy and the C4 pathway Photosynthetic cells of C4 plant leaf Mesophyll cell Mesophyll cell BundleBundlesheath cell PEP (3 C) NADPH used ADP Malate (4 C) ATP Malate (4 C) Oxaloacetate (4 C) Pyruate (3 C) NADPH regenerated CO2 C4 leaf anatomy Mesophyll CO CO 2 2 PEP carboxylase Oxaloacetate (4 C) Vein (vascular tissue) Mesophyll Cell C4 Metabolism Stomata BundleSheath cell Produce NADPH and ATP PEP carboxylase insensitive to O2 Malate brings across reducing equivalents Cyclic e- flow Little O2 CALVIN CYCLE Sugar Vascular tissue 13 Figure 10.19 Bundle Sheath - Cell C4 Metabolism OH NADPH / + C=O H+ CH2 H-C-O-H C=O \ NADP+ OH “carries” carries” malate Reducing 4C acid equivalents OH / C=O CH2 C=O C=O \ OH Oxaloacetate 4C acid Very low O2 Mesophyll cells provide a means for bundle sheath cells to acquire NADPH + H+ reducing power Mesophyll cells provide carbon dioxide to bundle sheath cells at higher concentration than in air Bundle Sheath cells not making oxygen, so very little competitor with C3 reactions CO2 Calvin Cycle Needs ATP Only Cyclic electron flow CH3 C=O C=O \ O-H pyruvate P PE ase CO2 xyl NADPH o b OH car + OH / CH3 H+ / C=O C=O C=O CH2 C=O CH2 C=O \ H-C-O-H C=O + Pi O-Pi C=O \ NADP+ “PEP” \ OH OH Oxaloacetate ADP malate 4C acid 4C acid CH3 ATP “carries” carries” C=O Reducing C=O equivalents Needs ATP and NAPDPH + H+ \ O-H NonNon-cyclic electron flow pyruvate 14 NADPH ATP ATP glucose 15 Costs more energy to do business this way… but has the advantage when CO2 is limiting (when stomates are closed - like on hot days) Who cares as long as the sun is shining ? 16 ATP is not limiting CAM Plants Cacti, pineapple Generally Slow growing – Open their stomata only at night, too hot during day – survive very adverse (dry) conditions NIGHT Perform PEP carboxylase reaction at night (CO2 assimilation) accumulate malate to high concentration in central vacuole use sugar oxidation/catabolism to power (NADH and ATP) carbon fixation DAY Perform “light” reactions during the day mostly cyclic e- flow to produce ATP (low O2) decarboxylate malate to yield CO2 and NADPH + H+ perform C3 reactions (Calvin Cycle) to produce 17 sugars and starch Summary 1. Photosynthetic “light” reactions produce ATP and reducing potential NADPH + H+ 2. Dark reactions use ATP and reducing potential to synthesize carbohydrates - powers reduction of 3-carbon acid to 3-carbon aldehyde - powers regeneration of starting material 5-carbon di-phosphate (priming step for CO2 fixation) 3. Rubisco enzyme regulated tightly by allosteric modulators pH, and reducing status of stroma 4. O2 interferes with carbon fixation by Rubisco enzyme 5. Metabolic “tricks” to avoid photorespiration - C4 metabolism - CAM metabolism 19 Pineapple Sugarcane Sunlight Powers Both phases (a) Figure 10.20 C4 Mesophyll Cell Organic acid Bundlesheath cell Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. CAM CO2 CO2 1 CO incorporated Organic acid 2 into four-carbon organic acids (carbon fixation) Night Day (b) CALVIN CYCLE Sugar 2 Organic acids release CO2 to Calvin cycle CALVIN CYCLE Sugar Sugar Oxidation Powers One Phase Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different18 times.