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DARK REACTIONS Energy Utilization the Calvin Cycle

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DARK REACTIONS Energy Utilization the Calvin Cycle 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 Oct 10, 2005 Photosynthesis II. Calvin Cycle1 2 The purpose of the Carbon-fixation (Calvin Cycle) Reactions + CO2 + NADPH + H + ATP + DARK REACTIONS C6H12O6 + NADP + ADP + Pi carbohydrate energy utilization Note: synthesis of carbohydrate from CO2 is favorable only because coupled to very favorable reactions The Calvin Cycle NADPH to NADP+ and ATP to ADP + Pi energy released is greater than it costs to make carbohydrate 3 4 Overview of Carbon-Fixation Reactions O 12 = 24 R-C-OH The Calvin cycle has three phases CO2 + 2 3 Carbon Acid – Carbon fixation with one phosphate on it – Reduction (energy input, reducing equiv input) 5 Carbon Sugar with 2 phosphates on it ATP – Regeneration of the CO2 acceptor Reduction (energy input – “priming step”) ADP + Pi ADP + Pi NADPH ATP NADP+ 12 24 O= 5 Carbon Sugar 20/24 3 Carbon Aldehyde R-C-H with one phosphate on it with one phosphate on it Higher 4/24 Energy Compound 5 6 Carbon Sugar - Glucose 2 6 with NO phosphate on it Carbon Acid -COOH Carbon Fixation Fixation Input of energy Reduction Rubisco Priming Step Aldehyde -C=O H Carried out by the enzyme “rubisco” (ribulose 1,5 bisphosphate carboxylase oxygenase) Regenerate What started Do need to know this enzyme7 8 Key regulatory enzyme with Regulation of Rubisco Integration of Light-Dependent 1st Enzyme in Calvin Cycle and Light-Independent Reactions Substrate/Product availability They generally occur AT THE SAME TIME Allosterically regulated by NADPH and ATP Light reaction Calvin cycle H O 2 CO2 Light Very Narrow pH optimum pH 8 / pH7 NADP+ ADP + P pH 8 or above, inactive at 7 1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I Enzyme must be in ATP G3P NADPH Starch reduced form (storage) Figure 10.21 Amino acids 9 Fatty acids 10 Chloroplast O2 Sucrose (export) 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) C4 Plants 3 Carbon Acid Net increase CO2 Acid + with one phosphate on it in material Mesophyll cells perform “usual” make noncyclic Light-Dependent Reactions 3 Carbon Acid 5 Carbon Sugar glucose with make oxygen, ATP and NADPH with one phosphate on it “extra” with 2 phosphates on it Do NOT perform the C3 (Calvin cycle) reactions Bundle Sheath cells perform “UNusual” Carbon O 2 Acid Energy cyclic Light-Dependent Reactions 2 with one phosphate on it + Wasted a lot of ATP but With NO very little NADPH 5 Carbon Sugar 3 Carbon Acid synthesis and very little O2 with one phosphate on it with 2 phosphates on it of glucose11 12 PERFORM the usual C3 (Calvin Cycle) reactions C4 leaf anatomy and the C4 pathway Mesophyll Cell C4 Metabolism P PE se Mesophyll yla CO2 NADPH ox cell Mesophyll OH rb cell cell a Photosynthetic COCO c PEP carboxylase 2 2 OH + cells of C plant 4 / leaf Bundle- H+ CH / H C=O 3 sheath C=O cell C=O cell Oxaloacetate (4 C) PEP (3 C) CH2 ADP CH C=O NADPH 2 C=O Vein used H-C-O-H \ Malate (4 C) ATP C=O + P (vascular tissue) i O-P (vascular tissue) C=O + i C4 leaf anatomy Malate (4 C) NADP \ Oxaloacetate (4 C) Pyruate (3 C) \ “PEP” NADPH OH regenerated CO 2 OH Oxaloacetate Stomata Bundle- ADP Mesophyll Sheath malate Cyclic 4C acid cell CALVIN 4C acid e- flow Produce NADPH and ATP CYCLE 4C acid Little O2 PEP carboxylase insensitive to O CH3 2 “carries” C=O ATP Malate brings across reducing equivalents Sugar Reducing C=O Vascular equivalents + \ tissue Needs ATP and NAPDPH + H 13 O-H 14 Figure 10.19 Non-cyclic electron flow Non-cyclic electron flow pyruvate Bundle Sheath - Cell C4 Metabolism CH3 C=O OH OH Mesophyll cells provide a means for bundle sheath NADPH C=O / / cells to acquire NADPH + H+ reducing power C=O + \ + C=O H O-H CH2 CH Mesophyll cells provide carbon dioxide to bundle sheath 2 pyruvate H-C-O-H C=O cells at higher concentration than in air C=O C=O \ NADP+ \ Bundle Sheath cells not making oxygen, so very little OH “carries” OH competitor with C3 reactions malate Reducing Oxaloacetate 4C acid CO equivalents 4C acid 2 NADPH Costs more energy to do business this way… but has the advantage when CO is limiting Needs ATP Only Calvin ATP 2 Cycle (when stomates are closed - like on hot days) Cyclic electron flow ATP Who cares as long as the sun is shining ? Very low O 15 16 2 glucose ATP is not limiting CAM Plants Cacti, pineapple Generally – Open their stomata only at night, too hot during Slow day – survive very adverse (dry) conditions growing NIGHT Sugarcane Pineapple Perform PEP carboxylase reaction at night (CO2 assimilation) Sugar accumulate malate to high concentration in central vacuole Sunlight C4 CAM Oxidation use sugar oxidation/catabolism to power (NADH and ATP) Powers CO2 CO2 Mesophyll Cell Powers carbon fixation Both Night Organic acid 1 Organic acid CO2 incorporated One phases Bundle- into four-carbon organic acids Phase DAY sheath cell (carbon fixation) Day Perform “light” reactions during the day (a) Spatial (b) Temporal - mostly cyclic e flow to produce ATP (low O2) separation CALVIN 2 Organic acids CALVIN separation of steps. In C CYCLE CYCLE + 4 release CO2 to of steps. In CAM decarboxylate malate to yield CO2 and NADPH + H plants, carbon fixation Calvin cycle plants, carbon fixation and the Calvin cycle and the Calvin cycle perform C3 reactions (Calvin Cycle) to produce occur in different Sugar Sugar Figure 10.20 occur in the same cells sugars and starch 17 types of cells. at different18 times. 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.
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