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Alternative Mechanisms of Carbon Fixation

Alternative Mechanisms of Carbon Fixation

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Alternative Mechanisms of Carbon Fixation

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  1. Alternative Mechanisms of Carbon Fixation Section 3.4 Page 168

  2. Normal plants... • Gas exchange through stomata: • Let CO2 in • Let O2 out • Stomata: • Open during day • Closed at night

  3. Plants in hot, dry weather... • Conserve water by closing stomata • Problems: • Buildup of O2 in cells Photorespiration • How to get CO2 into cells?

  4. Photorespiration • Rubisco catalyzes the first reaction (carbon fixation) in the Calvin Cycle. • Rubisco’s reactions: • Carboxylation (addition of CO2 to RuBP) • Oxidation (addition of O2 to RuBP), aka photorespiration

  5. O2 competes with CO2 for rubisco’s active site. When oxygen is more plentiful than carbon dioxide, rubisco will catalyze its binding more often.

  6. PHOTORESPIRATION = BAD It removes carbon from the Calvin cycle

  7. O2 + RuBP (5-C) PGA (3-C) + glycolate (2-C) partial conversion returns to Calvin cycle CO2 (released through stomata)

  8. No carbon input, but carbon is released as CO2 • Under normal conditions, 20% of fixed carbon is lost by a C3 plant through photorespiration • No ATP production • Reduces photosynthetic efficiency, and slows plant growth

  9. Photorespiration, like cellular respiration, involves: • O2 uptake • Release CO2

  10. Reducing the rate of photorespiration Plants evolved two strategies: • C4 photosynthesis (in C4 plants) • Crassulacean acid metabolism (CAM)

  11. C4 PLANTS • E.g., sugar cane, corn • Hot, dry climates • Unique leaf structure

  12. “NORMAL” (C3) LEAF

  13. C4 PLANT LEAF • Two types of photosynthetic cells: • Bundle-sheath cells surrounding a vein • Mesophyll cells located around the bundle sheath

  14. Strategy: • CO2 is continually pumped from mesophyll to the bundle-sheath cells to enter Calvin cycle • Keeps [CO2] high so it can outcompete O2 – minimize photorespiration • Extra ATP is used as compared with C3 • Benefit outweighs cost

  15. CAM plants • Succulents – cacti, pineapples • Hot, dry, desert climates • These plants open their stomata at night, and close them during the day (opposite of other plants) • Helps conserve water, but prevents CO2 from entering leaves during day

  16. ...but why wait until day??

  17. Strategy: • Keep stomata closed during day – minimize water loss • Take in CO2 at night and store it for use during day

  18. C4 vs. CAM photosynthesis Same steps: • CO2 temporarily fixed into a 4-carbon organic acid • C4: OAA • CAM: malate • The organic acid releases CO2 to the Calvin cycle

  19. Spatial vs. Temporal separation of steps

  20. Homework Pg. 172 #1-6