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Photosynthesis: Variations on the Theme

Photosynthesis: Variations on the Theme. Remember what plants need…. Photosynthesis light reactions light H 2 O Calvin cycle CO 2.  sun.  ground.  air. What structures have plants evolved to supply these needs?. xylem (water). phloem (sugar). cuticle. epidermis. palisades

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Photosynthesis: Variations on the Theme

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  1. Photosynthesis:Variations on the Theme

  2. Remember what plants need… • Photosynthesis • light reactions • light • H2O • Calvin cycle • CO2  sun  ground air What structures have plants evolved to supply these needs?

  3. xylem (water) phloem (sugar) cuticle epidermis palisades layer spongy layer stomate guardcell vascular bundle Leaf Structure O2 H2O CO2 O2 H2O CO2 Transpiration

  4. O2 CO2 O2 CO2 H2O A second inside a leaf… • Gas exchange & water flow • CO2 in  • O2 out  • H2O out  for Calvin cycle waste from light reactions for light reactions photosynthesis xylem (water) phloem (sugars) gas exchangewater loss

  5. 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!

  6. Closed stomates • Closed stomates lead to… • O2 builds up  from light reactions • CO2 is depleted  in Calvin cycle • causes problems in Calvin Cycle The best laidschemes ofmice and men…and plants!

  7. Inefficiency of Rubisco: CO2 vs O2 • Rubisco in Calvin cycle • carbon fixation enzyme • normally bonds CtoRuBP • reduction ofRuBP • building sugars • when O2 concentration is high • Rubisco bonds OtoRuBP • O2 is alternative substrate • oxidation of RuBP • breakdown sugars photosynthesis photorespiration

  8. 1C 3C CO2 RuBP 5C 6C unstable intermediate ATP ADP 3C ATP PGA ADP 3C NADPH NADP Calvin cycle review Rubisco G3P to make glucose G3P

  9. 1C 3C CO2 RuBP 5C G3P to make glucose 3C PGA G3P Calvin cycle review Rubisco C3 plants

  10. O2 RuBP 5C 3C 2C to mitochondria ––––––– lost as CO2 without making ATP Calvin cycle with O2 Rubisco A good enzyme,gone BAD! photorespiration

  11. Impact of Photorespiration • Oxidation of RuBP • short circuit of Calvin cycle • loss of carbons to CO2 • 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 evolve alternative systems

  12. Reducing photorespiration • Separate carbon fixation from Calvin cycle • C4 plants • physically separate carbon fixation from actual Calvin cycle • different enzyme to capture CO2 • 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 CO2) during night • store carbon in organic acids • perform Calvin cycle during day

  13. C4 plants • A better way to capture CO2 • 1st step before Calvin cycle, fix carbon with enzymePEP carboxylase • store as 4C compound • adaptation to hot, dry climates • have to close stomates a lot • different leaf anatomy • sugar cane, corn, other grasses…

  14. C4 Plants corn sugar cane

  15. O2 PEP carboxylase light reactions • PEP carboxylase enzyme • higher affinity for CO2 than O2 (better than Rubisco) • fixes CO2 in 4C compounds • regenerates CO2in inner cells for Rubisco phosphoenolpyruvate (3C) + CO2 oxaloacetate (4C)

  16. Location,location, location! Comparative anatomy • Separate reactions in different cells • light reactions • carbon fixation • Calvin cycle C3 C4

  17. CO2 O2 O2 CO2 C4 photosynthesis Physically separated C fixation from Calvin cycle • Outer cells • light reaction & carbon fixation • pumps CO2 to inner cells • keeps O2 away from inner cells • away from Rubisco • Inner cells • Calvin cycle • glucose to veins

  18. 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 CO2 from “storage” compounds to Calvin cycle • increases concentration of CO2 in cells • succulents, some cacti, pineapple It’s all inthe timing!

  19. CAM plants

  20. C4 vs CAM Summary solves CO2 / O2 gas exchangevs. H2O losschallenge C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally at2 different times

  21. We’ve all gotbaggage! Why the C3 problem? • 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 photorespiration

  22. Any Questions??

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