Why do plants often struggleto achieve high concentrations of CO2 to sustain photosynthesis?
2. EvolutionaryAspects of Rubisco
Photorespiration – A bad deal?
-> 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
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
○ Uptake of CO2
○ Temporaryfixation into organic