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48 ch10c4cam2008

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48 ch10c4cam2008

  1. 1. Photosynthesis: Variations on the ThemeAP Biology 2007-2008
  2. 2. Remember what plants need…  Photosynthesis  light reactions O  light ← sun C O  H2O ← ground  Calvin cycle  CO2 ← air What structures have plants evolved toAPsupply these needs? Biology
  3. 3. vascular bundle Leaf Structure xylem (water) cuticle phloem (sugar)epidermispalisades layer O2 H O spongy 2 CO2 layer stomate guardTranspiration cell O2 H 2O CO2Gas exchangeAP Biology
  4. 4. Controlling water loss from leaves  Hot or dry days  stomates close to conserve water  guard cells  gain H2O = stomates open  lose H2O = stomates close  adaptation to living on land, but… creates PROBLEMS!AP Biology
  5. 5. When stomates close…  Closed stomates lead to…  O2 build up → from light reactions  CO2 is depleted → in Calvin cycle  causes problems in Calvin Cycle The best laid schemes of O2 xylem mice and men… (water) and plants! phloem (sugars)CO2 O2 CO2 AP Biology H2O
  6. 6. Inefficiency of RuBisCo: CO2 vs O2  RuBisCo in Calvin cycle  carbon fixation enzyme  normally bonds C to RuBP  CO2 is the optimal substrate  reduction of RuBP photosynthesis  building sugars  when O2 concentration is high  RuBisCo bonds O to RuBP  O2 is a competitive substrate  oxidation of RuBP photorespirationAP Biology  breakdown sugars
  7. 7. Calvin cycle when CO2 is abundant 1C CO2 RuBP ATP 5C RuBisCo unstable ADP 6C intermediate G3Pto make 5Cglucose PGA C3 plants 3C G3P 3C NADPH ATP NADP ADPAP Biology 3C
  8. 8. to Calvin cycle when O2 is high mitochondria ––––––– lost as CO2 O2 without RuBP making ATP 5C RuBisCo Hey Dude, 2Care you high on oxygen! 3C It’s so sad to see a good enzyme, go BAD! photorespirationAP Biology
  9. 9. Impact of Photorespiration  Oxidation of RuBP  short circuit of Calvin cycle  loss of carbons to CO 2  can lose 50% of carbons fixed by Calvin cycle  reduces production of photosynthesis  no ATP (energy) produced  no C6H12O6 (food) produced  if photorespiration could be reduced, plant would become 50% more efficient  strong selection pressure to evolveAP Biology alternative carbon fixation systems
  10. 10. Reducing photorespiration  Separate carbon fixation from Calvin cycle  C4 plants  PHYSICALLY separate carbon fixation from Calvin cycle  different cells to fix carbon vs. where Calvin cycle occurs  store carbon in 4C compounds  different enzyme to capture CO2 (fix carbon)  PEP carboxylase  different leaf structure  CAM plants  separate carbon fixation from Calvin cycle by TIME OF DAY  fix carbon during night  store carbon in 4C compounds  perform Calvin cycle during dayAP Biology
  11. 11. C4 plants  A better way to capture CO2  1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase corn  store as 4C compound  adaptation to hot, dry climates  have to close stomates a lot  different leaf anatomy  sugar cane, corn, other grasses…AP Biology sugar cane
  12. 12. PEP (3C) + CO2 → oxaloacetate O2(4C) C4 leaf anatomy light reactions CO2 PEP carboxylase C3 anatomy bundle stomate sheath CO2 cell RuBisCo PEP carboxylase enzyme  higher attraction for CO2 than O2  better than RuBisCo  fixes CO2 in 4C compounds  regenerates CO2 in inner cells for RuBisCo AP Biology C4 anatomy
  13. 13. Comparative anatomy Location, location,location! C3 C4 PHYSICALLY separate C fixation from Calvin cycleAP Biology
  14. 14. CAM (Crassulacean Acid Metabolism) plants  Adaptation to hot, dry climates  separate carbon fixation from Calvin cycle by TIME  close stomates during day  open stomates during night  at night: open stomates & fix carbon in 4C “storage” compounds It’s all in the timing!  in day: release CO from 4C acids 2 to Calvin cycle  increases concentration of CO2 in cells  succulents, some cacti, pineappleAP Biology
  15. 15. CAM plants cacti succulentsAP Biology pineapple
  16. 16. C4 vs CAM Summary solves CO2 / O2 gas exchange vs. H2O loss challengeC4 plants CAM plantsseparate 2 steps separate 2 stepsof C fixation of C fixationanatomically in 2 temporally =different cells 2 different times night vs. day AP Biology
  17. 17. We’ve all got Why the C3 problem? baggage!  Possibly evolutionary baggage  Rubisco evolved in high CO2 atmosphere  there wasn’t strong selection against active site of Rubisco accepting both CO2 & O2  Today it makes a difference  21% O2 vs. 0.03% CO2  photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day  strong selection pressure to evolve better way to fix carbon & minimize photorespirationAP Biology
  18. 18. It’s not so easy as it looks… Any Questions??AP Biology 2007-2008
  19. 19. Ghosts of Lectures Past (storage)AP Biology 2007-2008
  20. 20. A second look inside a leaf…  Gas exchange & water flow  CO2 in → for Calvin cycle  O out → 2 waste from light reactions  H O out → for light reactions 2 photosynthesis xylem (water) O2 CO 2 phloem (sugars) gas exchange water loss H2O O2AP Biology CO2
  21. 21. C4 photosynthesis PHYSICALLY separated C fixation from Calvin cycle  Outer cells  light reaction & carbon fixation  pumps CO to inner 2 cells  keeps O away from 2 inner cells  away from RuBisCoCO2  Inner cells O2 O2 CO2  Calvin cycle  glucose to veins AP Biology

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