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Plant bio
C4 photosynthesis
| Question | Answer |
|---|---|
| Carbon fixation | - Rubisco: (Ribulose bisphosphate carboxylase oxygenase) a protein allowing for carbon fixation - photosynthetic efficiency limited by photorespiration |
| Rubisco | CO2 and O2 are competitive substrates Enzyme either fixes, or is responsible ultimately for release of CO2 In C3 plants, up to 30% of CO2 captured by carboxylase activity is lost by oxygenase activity Remove the oxygen, no competition, no loss |
| Photorespiration equation | C5 sugar + O2 → C3 + C2 2C2 → C3 + CO2 |
| Respiration Photosynthesis | C6H12O6 + 6O2 → 6CO2 + 6 H2O |
| CO2 fixation in classical C4 plants (a CO2 pump concentrating CO2) 1) | 1) Initial fixation in the mesophyll by the enzyme PEP Carboxylase the substrate is bicarbonate produces a (C4) compound |
| CO2 fixation in classical C4 plants a CO2 pump concentrating CO2 2) | 2) The C4 compound is passed to the mesophyll and decarboxylated (releases CO2) |
| CO2 fixation in classical C4 plants a CO2 pump concentrating CO2 3) | 3) which is fixed by Rubisco in the C3 Calvin process - the remaining C3 compound enters the mesophyll cell for recycling |
| C3 plants | - wheat, rice, potatoes - widely distributed (fields) - bundle sheath not green |
| C4 plants | - sugar cane, maize - warm to hot, open sites - bundle sheath with chloroplasts |
| Increasing ambient CO2 concentration | - C3 plants fix more in a growing concentration - C4 plants round off and fix none |
| Optimum temperatures for CO2 assimilation | - C3 assimilates CO2 at a lower temp - C4 assimilates CO2 at a higher temp |
| CO2 pump energetic costs | - PEP carboxylase has no oxygenase activity - Rubisco has oxygenase activity, but this is limited by low O2 |
| Water use efficiency | - gCO2 fixed/Kg H2O lost is lower for C3 than C4 - although they have similar max. stomatal conductances, the C3 plant is losing CO2 via photorespiration |
| Stomatal physiology of CAM plants | - stomatal closed in light, open in dark |
| Crassulacean acid metabolism | - a CO2 concentrating mechanism, an adaptation to drought conditions with C4 and C3 cycles separated in time |
| Internal gases | - organic acid accumulation by night, mobilization by day, releasing CO2 fixed by Calvin-cycle - reflected in massive increases in internal gas-phase CO2 |
| Advantages of fixing CO2 by night | Lower temps Diffusive loss of H2O vapour lessened Air temperature affects vapour pressure of atmosphere |
| Advantages of fixing CO2 by night | Water vapour content of hot air is greater than cold air Cool the air and the vapour pressure increases Therefore vapour pressure deficit (leaf-atmosphere) is reduced |
| 4 phases of gas exchange and metabolism in well-watered CAM plants | 1) Dark CO2 fixation & malate synthesis by PEPcarb, vacuolar accumulation |
| 4 phases of gas exchange and metabolism in well-watered CAM plants | 2) initial light period, stomata close- but for a moment both PEPc and Rubisco are active together |
| 4 phases of gas exchange and metabolism in well-watered CAM plants | 3) Light, Malate decarboxylation, v high CO2 concttn behind closed stomata, high Rubisco activity |
| 4 phases of gas exchange and metabolism in well-watered CAM plants | 4) initial dark period, as decarboxylation ceases CO2 falls and stomata open |
Created by:
rose.coo
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