DARK REACTIONS energy utilization. The Calvin Cycle. Lecture 17 Oct 10, Photosynthesis II. Calvin Cycle. Lecture Outline

Transcription

1 Lecture 17 Oct 10, Photosynthesis II. Calvin Cycle Lecture Outline 1. The Calvin Cycle fixes carbon makes reduced carbon compounds 2. Reactions of the Calvin Cycle anabolic pathway input of H, input of 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 2 DARK REACTIONS energy utilization The Calvin Cycle 3

2 The purpose of the Carbon-fixation (Calvin Cycle) Reactions H C 6 H 12 O 6 NADP ADP P i carbohydrate Note: synthesis of carbohydrate from is favorable only because coupled to very favorable reactions to NADP and to ADP P i energy released is greater than it costs to make carbohydrate 4 The Calvin cycle has three phases Carbon fixation Reduction (energy input, reducing equiv input) Regeneration of the acceptor (energy input priming step ) 5 Overview of Carbon-Fixation Reactions 2 3 Carbon Acid with 2 phosphates on it ADP P i 2024 ADP P i NADP 3 Carbon Aldehyde Carbon - Glucose with NO phosphate on it O R-C- = R-C-H Higher Energy Compound 6 Reduction O =

3 Carbon Fixation Carried out by the enzyme rubisco (ribulose 1,5 bisphosphate carboxylase oxygenase) Do need to know this enzyme7 Key regulatory enzyme Priming Step Carbon Fixation Acid - CO Rubisco Input of energy Reduction Aldehyde - H Regenerate What started with 8 Regulation of Rubisco 1 st Enzyme in Calvin Cycle SubstrateProduct availability Allosterically regulated by and Very Narrow ph optimum ph 8 ph7 ph 8 or above, inactive at 7 Enzyme must be in reduced form 9

4 Integration of Light-Dependent and Light-Independent Reactions They generally occur AT THE SAME TIME Light reaction H 2 O Calvin cycle Light NADP ADP P 1 RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I G3P Figure Chloroplast O 2 Starch (storage) Amino acids Fatty acids 10 Sucrose (export) PhotoRespiration- the OXYGEN PROBLEM Oxygen is a competing substrate for the 1st enzyme in the C 3 cycle (Rubisco) with 2 phosphates on it 3 Carbon Acid 3 Carbon Acid Net increase in material make glucose with extra O 2 with 2 phosphates on it 2 Carbon Acid 3 Carbon Acid Energy Wasted With NO synthesis of glucose 11 Some Plants Deal with this problem by a DIVISION OF LABOR BETWEEN CELLS C 4 Plants Mesophyll s perform usual noncyclic Light-Dependent Reactions make oxygen, and Do NOT perform the C 3 (Calvin cycle) reactions Bundle Sheath s perform UN UNusual cyclic Light-Dependent Reactions a lot of but very little and very little O 2 PERFORM the usual C 3 (Calvin Cycle) reactions 12

5 Photosynthetic s of C 4 plant leaf C 4 leaf anatomy and the C 4 pathway Vein (vascular tissue) C 4 leaf anatomy Mesophyll Produce and Stomata Mesophyll Mesophyll Bundle- sheath Bundle- Sheath PEP carboxylase insensitive to O 2 Malate brings across reducing equivalents PEP carboxylase Oxaloacetate (4 C) PEP (3 C) ADP used Malate (4 C) Malate (4 C) Oxaloacetate (4 C) regenerated Cyclic e- flow Little O 2 Pyruate (3 C) CALVIN CYCLE CO 2 Figure Vascular tissue 13 Mesophyll Cell C4 Metabolism CH 2 H-C-O-H malate carries Reducing equivalents NADP H CH 2 P i Oxaloacetate Needs and NAPDPH H Non-cyclic electron flow PEP carboxylase CH 3 O-P i PEP CH 3 O-H pyruvate 14 ADP Bundle Sheath - Cell C4 Metabolism CH H 2 H-C-O-H NADP carries malate Reducing equivalents Needs Only Cyclic electron flow Very low O 2 CH 2 Oxaloacetate Calvin Cycle glucose CH 3 O-H pyruvate 15

6 Mesophyll s provide a means for bundle sheath s to acquire H reducing power Mesophyll s provide carbon dioxide to bundle sheath s at higher concentration than in air Bundle Sheath s not making oxygen, so very little competitor with C 3 reactions Costs more energy to do business this way but has the advantage when is limiting (when stomates are closed - like on hot days) Who cares as long as the sun is shining? 16 is not limiting CAM Plants Cacti, pineapple Open their stomata only at night, too hot during day survive very adverse (dry) conditions NIGHT Perform PEP carboxylase reaction at night ( assimilation) accumulate malate to high concentration in central vacuole use sugar oxidationcatabolism to power (NADH and ) carbon fixation DAY Perform light reactions during the day mostly cyclic e - flow to produce (low O 2 ) decarboxylate malate to yield and H perform C3 reactions (Calvin Cycle) to produce 17 sugars and starch Generally Slow growing Sunlight Powers cane Pineapple C 4 CAM Mesophyll Cell Both Organic acid 1 CO Organic acid 2 incorporated phases Bundlesheath into four-carbon organic acids (carbon fixation) (a) Spatial separation CALVIN 2 Organic acids CALVIN of steps. In C CYCLE CYCLE 4 release to plants, carbon fixation Calvin cycle and the Calvin cycle occur in different Figure types of s. Night Oxidation Powers One Phase Day (b) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same s at different 18 times.

7 Summary 1. Photosynthetic light reactions produce and reducing potential H 2. Dark reactions use 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 fixation) 3. Rubisco enzyme regulated tightly by allosteric modulators ph, and reducing status of stroma 4. O 2 interferes with carbon fixation by Rubisco enzyme 5. Metabolic tricks to avoid photorespiration - C4 metabolism - CAM metabolism 19