C3, C4, and CAM

Notes from Friday, September 13, 2002


Also read Ricklef’s!


For more information visit

            Botany Online:Photosynthesis

            Photosyntheis lecture from Furnam University

            Another C3 and C4 photosynthesis lecture


C3 plantsall 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 plantscarbon 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


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.


  • Because the stomata are closed during the day, able to capture large levels of CO2
  • Adaptation found in succulents, cacti, and euphorbs for example that are found in desert (very arid) environments.
  • Very high water use efficiencies because stomata don’t open during the hottest times of day when transpiration is the greatest.
  • Also energy expensive

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Last updated September 2002.
Copyright ©2002 Earlham College.
Send corrections or comments to vandeel@earlham.edu