 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.  As O 2 accumulates, it competes with CO 2 in binding with rubisco.

 When oxygen more plentiful than CO2, RuBP is oxidized.

 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.

 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?

 Rubisco worked 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)

 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?

 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

 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?

 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  ______________.

 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)

 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.

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