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Lecture 8

BIO204 Lecture 8 (CH10)

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University of Toronto Mississauga
Sanja Hinic- Frlog

8 (CH10) October-14-13 10:18 AM Why do plants often struggleto achieve high concentrations of CO2 to sustain photosynthesis? 2. EvolutionaryAspects of Rubisco Photorespiration – A bad deal? • Protection?! -> Adaptation to high light stress -> Avoidance of photoinactivationand damage of photosyntheticmembranes • Photorespirationhelps keeping a balance ○ Keeps [CO2] in leaves close to CO2-compensationpoint, even when stomata are closed ○ Sustains turnover of photosyntheticapparatus when stomataare closed How do plants from hot and dry environments deal with photorespiration? A simple solution: More is always better - Keep intracellular CO2 concentrationhigh!  C4 and CAM C4 Photosynthesis – Initial carbon fixation in C4 plants differs from C3 plants C4 Photosynthesis – Initial carbon fixation in C4 plants differs from C3 plants C4Plants: Carbon Fixation Occurs Independently of the Calvin cycle C4photosynthesis is a solution to photorespiration C4 pathway acceleratesphotosynthesisthrough concentrating CO2  Carboxylationreaction is increasingly inefficient when temperatureincreases: ○ Increase of the Oxygenase activity (O2 to Rib1,5bP) ○ Decreaseof the Carboxylationactivity (CO2 to Rib1,5bP)  Trick in C4 with 3 components: 1. Separate place of carbon fixation and rubisco carboxylation 2. Use PEP-carboxylasefor carbon fixation (not affected by high O2, higher affinity for CO2) 3. Increase CO2 concentrations directly where the Calvin-cycle occurs within photosyntheticactive cells Examples of C4 plants  C4 pathway occurs mostly in plants from hot, dry habitats  Limits the damaging effects of photorespiration CAM Plants – details of their exciting night life • Crassulacean acid metabolism (CAM) • Carbon fixation and Calvin cycle separated in time • Plants from hot, dry habitats • Closure of stomataduring day, open at night • Nighttime ○ Uptake of CO2 ○ Temporaryfixation into organic
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