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hotosynthesis
Photosyntheis lecture from Furnam University
Another C3 and C4 photosynthesis lecture
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)
C3 are inefficient at CO2 fixation because RUBISCO has a greater affinity for oxygen than CO2
Mesophyll cells are packed with RUBISCO
Stomata open during day (CO2, oxygen, and water can all flow out)
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.
The equation:
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)
Feature of many grasses (i.e. big blue stem back campus), corn, and many arid/semi arid shrubs
Calvin cycle in bundle sheath cells where there is no oxygen to be bound by RUBISCO
Very high concentration of CO2 in bundle sheath cells
PEP carboxylase has a high affinity for CO2 so plants must open their stomata less to get CO2 and hence lose less water (especially important in arid regions)
Low levels of photorespiration and higher net photosynthesis than C3 because of low photorespiration
Costly adaptation because it requires lots of ATP (energy)—however, benefits outweigh energy costs.
Stomata are open during the day
Fixation and the calvin cyle are physically separate