2. AP Biology 2006-2007 Photosynthesis: Variations on the Theme

3. AP Biology Remember what plants need…  Photosynthesis  light reactions  light H2OH2O  Calvin cycle  CO 2 What structures have plants evolved to supply these needs?  sun  ground  air

4. AP Biology Leaf Structure H2OH2O CO 2 O2O2 H2OH2O phloem (sugar) xylem (water) stomate guard cell palisades layer spongy layer cuticle epidermis O2O2 CO 2 Transpiration vascular bundle

5. AP Biology A second inside a leaf…  Gas exchange & water flow  CO 2 in   O 2 out   H 2 O out  photosynthesis gas exchange water loss xylem (water) phloem (sugars) O2O2 CO 2 O2O2 H2OH2O for Calvin cycle waste from light reactions for light reactions

6. AP Biology Controlling water loss from leaves  Hot or dry days  stomates close to conserve water  guard cells  gain H 2 O = stomates open  lose H 2 O = stomates close  adaptation to living on land, but… creates PROBLEMS!

7. AP Biology Closed stomates  Closed stomates lead to…  O 2 builds up  from light reactions  CO 2 is depleted  in Calvin cycle  causes problems in Calvin Cycle The best laid schemes of mice and men… and plants!

8. AP Biology Inefficiency of Rubisco: CO 2 vs O 2  Rubisco in Calvin cycle  carbon fixation enzyme  normally bonds C to RuBP  reduction of RuBP  building sugars  when O 2 concentration is high  Rubisco bonds O to RuBP  O 2 is alternative substrate  oxidation of RuBP  breakdown sugars photosynthesis photorespiration

9. AP Biology 6C unstable intermediate 1C CO 2 Calvin cycle review 5C RuBP 3C PGA ADP ATP 3C NADP NADPH ADP ATP G3P to make glucose Rubisco 3C G3P

10. AP Biology 1C CO 2 Calvin cycle review 5C RuBP 3C PGA Rubisco C3 plants 3C G3P to make glucose G3P

11. AP Biology Calvin cycle with O 2 5C RuBP Rubisco 3C 2C to mitochondria ––––––– lost as CO 2 without making ATP photorespiration O2O2 A good enzyme, gone BAD!

12. AP Biology 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 C 6 H 12 O 6 (food) produced  if photorespiration could be reduced, plant would become 50% more efficient  strong selection pressure to evolve alternative systems

13. AP Biology Reducing photorespiration  Separate carbon fixation from Calvin cycle  C4 plants  physically separate carbon fixation from actual Calvin cycle  different enzyme to capture CO 2  PEP carboxylase stores carbon in 4C compounds  different leaf structure  CAM plants  separate carbon fixation from actual Calvin cycle by time of day  fix carbon (capture CO 2 ) during night  store carbon in organic acids  perform Calvin cycle during day

14. AP Biology C4 plants  A better way to capture CO 2  1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase  store as 4C compound  adaptation to hot, dry climates  have to close stomates a lot  different leaf anatomy  sugar cane, corn, other grasses…

15. AP Biology sugar canecorn C4 Plants

16. AP Biology PEP carboxylase O2O2 light reactions  PEP carboxylase enzyme  higher affinity for CO 2 than O 2 (better than Rubisco)  fixes CO 2 in 4C compounds  regenerates CO 2 in inner cells for Rubisco phosphoenolpyruvate (3C) + CO 2  oxaloacetate (4C)

17. AP Biology Comparative anatomy  Separate reactions in different cells  light reactions  carbon fixation  Calvin cycle C3C4 Location, location, location!

18. AP Biology C4 photosynthesis CO 2 O2O2 O2O2  Outer cells  light reaction & carbon fixation  pumps CO 2 to inner cells  keeps O 2 away from inner cells  away from Rubisco  Inner cells  Calvin cycle  glucose to veins Physically separated C fixation from Calvin cycle

19. AP Biology CAM ( Crassulacean Acid Metabolism ) plants  Different 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 “storage” compounds  organic acids: malic acid, isocitric acid  in day, close stomates & release CO 2 from “storage” compounds to Calvin cycle  increases concentration of CO 2 in cells  succulents, some cacti, pineapple It’s all in the timing!

20. AP Biology CAM plants

21. AP Biology C4 vs CAM Summary C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally at 2 different times solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge

22. AP Biology Why the C3 problem?  Possibly evolutionary baggage  Rubisco evolved in high CO 2 atmosphere  there wasn’t strong selection against active site of Rubisco accepting both CO 2 & O 2  Today it makes a difference  21% O 2 vs. 0.03% CO 2  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 photorespiration We’ve all got baggage!

23. AP Biology 2006-2007 Any Questions??