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CO 2 -concentrating mechanisms improve photosynthetic efficiency:

CO 2 -concentrating mechanisms improve photosynthetic efficiency: two major CO 2 -concentrating mechanisms in angiosperms: C 4 photosynthetic pathway CAM (Crassulacean Acid Metabolism) pathways operate in addition to the Calvin cycle. C 4 photosynthetic pathway - occurs in C 4 plants

gail-zamora
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CO 2 -concentrating mechanisms improve photosynthetic efficiency:

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  1. CO2-concentrating mechanisms improve photosynthetic efficiency: two major CO2-concentrating mechanisms in angiosperms: C4 photosynthetic pathway CAM (Crassulacean Acid Metabolism) pathways operate in addition to the Calvin cycle

  2. C4 photosynthetic pathway - occurs in C4 plants - plants have unique anatomical features - Kranz anatomy

  3. C4 leaf – note large bundle sheath cells, closely spaced veins

  4. C3 leaf - note less distinct bundle sheath fewer small leaf veins, more widely spaced

  5. in C4 plants: “division of labor” bundle sheath cells presence of RuBP carboxylase mesophyll cells absence of RuBP carboxylase, presence of PEP carboxylase phosphoenolpyruvate + HCO3- oxaloacetate + Pi (PEP - a 3C molecule) (1C) (OAA - a 4 C molecule)

  6. PEP carboxylase has strong affinity for HCO3- ; no competing affinity for O2 additional reactions allow trapped C to be used in the Calvin cycle alternate carboxylation reaction plus additional reactions = C4 photosynthetic pathway

  7. Reactions following carboxylation: OAA is converted into malate (also 4C) OAA + NADPH malate + NADP+ malate transportedto bundle sheath cells malate undergoes decarboxylation malate + NADP+ pyruvate + CO2 + NADPH

  8. products of decarboxylation 1) CO2, 2) NADPH, and 3) pyruvate 1) CO2 will enter the Calvin cycle - improved chances of carboxylation 2) NADPH will be used in the Calvin cycle 3) pyruvate transported back to mesophyll cells

  9. in mesophyll cells, pyruvate used to regenerate PEP: pyruvate + ATP + PiPEP + AMP + PPi and (ATP + AMP 2 ADP) (** 1 pyruvate regenerates 1 PEP, using two ATP) catalyzed by pyruvate, phosphate dikinase

  10. C4 photosynthesis more "costly" than Calvin cycle alone - helps to avoid photorespiration - allows plants to keep their stomates partially closed

  11. Crassulacean Acid Metabolism - also uses PEP carboxylase - PEP carboxylase and Rubisco both found in mesophyll cells; activity is temporally separated open their stomates (take in CO2) at night - can't use the Calvin cycle at this time light reactions occur during day - but stomates are closed

  12. in the dark: 1) starch used to produce PEP 2) OAA produced by PEP carboxylase; then reduced to malate 3) malate stored overnight in vacuoles in the light: 1) malate is transported out of vacuoles 2) malate undergoes decarboxylation 3) CO2 is used in the Calvin cycle; pyruvate used to regenerate starch

  13. in C4 plants, PEP carboxylase is active only during the day in CAM plants, PEP carboxylase is active only during the night

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