|C3, C4, and CAM|
Notes from Friday, September 13, 2002
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C3 plants—all of carbon fixation and photosynthesis happens in mesophyll cells just on the surface of the leaf. C3 plants include most temperate plants (except many grasses)—more than 95% of all earth’s plants.
The equation for the Calvin Cycle:
CO2 (Carbon dioxide in from stomata) + RuBP (Ribulose bisphosphate already in plant) + the enzyme RUBISCO (Ribulose bisphosphate carboxylase) “fixes” carbon from the atmosphere à 2PGA (phospholygerate)
PGA enters Calvin cycle in Mesophyll cells à more RuBP (to fix more CO2) + sugar (CH2O)
Photorespiration undoes CO2 assimilation
2PGA à CO2 + RuBP
increases when there is lots of O2, low levels of CO2, and increased temperature
C4 plants—carbon fixation and photosynthesis split between the mesophyll cells and bundle sheath cells.
In mesophyll (carbon fixation):
CO2 + PEP (phosphoenol pyruvate) + PEP carboxylase fixes carbon à OAA (oxaloacetic acid)
OAA diffuses to bundle sheath cells
In bundle sheath (Calvin Cycle):
OAA à malic acid and aspartic acid is decarboxylated à CO2 + pyruvate
Then the Calvin Cycle CO2 + RuBP + the enzyme RUBISCO à 2PGA à RuBP + CH2O
Pyruvate with ATP is moved back to mesophyll and turned into PEP (to fix more CO2)
CAM Crassulacean acid metabolism –photosynthesis takes place in the mesophyll cells, but carbon fixation (and opening of stomata) takes place at night and the Calvin cycle happens during the day.
The equations are the same as for C4 plants. Substitute “night” for mesophyll and “day” for bundle sheath.
Last updated September 2002.
Copyright ©2002 Earlham College.
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