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Alternative Mechanisms of Carbon Fixation

Alternative Mechanisms of Carbon Fixation. Stomata. Allow for the entry of CO 2 and exit of water vapor (transpiration). On sunny, hot, dry days, guard cells close to preserve water, but this poses a problem for photosynthesis.

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Alternative Mechanisms of Carbon Fixation

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  1. Alternative Mechanisms of Carbon Fixation

  2. Stomata • Allow for the entry of CO2 and exit of water vapor (transpiration). • On sunny, hot, dry days, guard cells close to preserve water, but this poses a problem for photosynthesis. • As O2 accumulates, it competes with CO2 in binding with rubisco.

  3. Photorespiration • When oxygen more plentiful than CO2, RuBP is oxidized.

  4. Photorespiration • Recap: in C3: • 3 CO2 + 3 RuBP (5C)  6 PGA (3C) • In Photorespiration: • 3 O2 + 3 RuBP (5C)  3 PGA (3C) + 3 glycolate (2C) • Some carbons of glycolate returned to the Calvin cycle as Glyceraldehyde 3-phosphate (G3P) • Some carbons converted to CO2.

  5. Results of Photorespiration: • Decreases the production of carbs due to removal of PGA from the Calvin cycle. • ¼ - ½ of carbon fixed in C3 plants are lost by photorespiration • Optimal temperature for photorespiration (30-47 degrees Celsius) is much higher than photosynthesis (15 – 25 degrees Celsius) • Relate back to Enzymes: O2 more likely to be the substrate of the enzyme Rubisco when temperatures are _______________. • Hot, dry, bright days facilitate _____________. • Global warming?

  6. The Non-Evolution of Rubisco • Rubiscoworked fine in early Earth, when oxygen levels weren’t very high and CO2 was plentiful. • Photosynthesis increased amount of O2 in atmosphere. • As oxygen levels increased, rubisco did not adapt to rid itself of its ability to oxidate. • Solution: evolution of alternate mechanisms of carbon fixation where CO2 is concentrated at the site of Rubisco suppress rate of photorespiration. • C4 photosynthesis • Crussalacean Acid Metabolism (CAM)

  7. C4 Plants (I) • Several thousand species of plants undergo C4 • Phosphenolpyruvate carboxylase (PEP carboxylase) first catalyzes addition of CO2 molecule to PEP (3C)  oxaloacetate (OAA) (4C) • C3 vs. C4?

  8. C4 Plants (II) • Leaf anatomy and function of C4 plants: two types of photosynthetic cells: • Mesophyl cells located around bundle-sheath cells. • IN THE CYTOPLASM, NOT THE CHLOROPLAST: • PEP carboxylase: CO2 + PEP (3C)  OAA (4C) • OAA  malate • Bundle-sheath cells surrounding a vein • Malate diffuses from mesophyll cells into bundle-sheath cells through cell-cell connections called plasmodesmata. • CO2 removed from malate (decarboxylation)  pyruvate (3C) • This CO2 enters the Calvin Cycle: catalyzed by rubisco • Pyruvate  mesophyll cell  converted to PEP

  9. C4 Plants (III) • Why all the hassel? • Reduces the amount of photorespiration by continually pumping CO2 towards rubisco. • CO2 outcompetes O2 • Sugar production maximized • Costs plant 2 ATP molecules per CO2 produced. • C3 plants: 18 molecules of ATP used/glucose • C4 plants: 30 molecules of ATP used/glucose • When is it worth it?

  10. CAM Plants • Occur in succulents: water-storing plants • Cacti • Pineapples • Open stomata at night and close them during the day. • Closing stomata during the day helps ___________________ but prevents _____ from entering the leaves. • When stomata open, CO2 taken in, incorporated into C4 organic acids (using PEP carboxylase). • Organic acids stored in vacuoles until morning  CO2 molecules enter the C3 Calvin cycle  ______________.

  11. CAM vs. C4 • C4: first part of carbon fixation and Calvin cycle occur in separate compartments of the leaf (spacial separation) • CAM: two steps occur in the same compartments, but at different times of the day (temporal separation)

  12. CAM and C4 • Both represent evolutionary solutions to the problem of maintaining photosynthesis when stomata are closed. • Initially produce organic acids that eventually transfer CO2 to the Calvin cycle.

  13. Summary

  14. Seatwork/Homework • Pg. 172 # 1 - 6

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