Photosynthesis-Carbon
Fixation-4 
C3 versus C4 Photosynthesis |
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| Comparison of CO2 Fixation by two Atriplex Species. One A. patula is a C3 plant, while the other A. rosea is a C4 plant. The C4 species performs better under all three conditions! |
Path of CO2 in C4 Photosynthesis |
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 CO2 crosses the Cell Wall and Plasmalemma of a Mesophyll Cell. It is fixed by
PEP Carboxylase to form a 4-Carbon Acid like Malate. This occurs in the
Cytoplasm.The 4-Carbon Acid is transported via Plasmodesmata to a Bundle Sheath Cell. It loses one CO2 (Decarboxylation) and enters a Bundle Sheath Chloroplast where CO2 is fixed by RUBISCO using the C3 Calvin Cycle.PEP is transported into the Mesophyll Cell where it can accept another CO2. |
PEP Carboxylase has a much higher affinity for CO2 than RUBISCO.PEP Carboxylase does NOT have Oxygenase activity. The CO2 concentration in the Bundle Sheath Chloroplasts greatly favors Carboxylation by RUBISCO & virtually eliminates Photorespiration. |
C4 Carbon Fixation requires more energy than C3 photosynthesis. However, the increased efficiency of CO2 fixation far outweighs the increased energy requirements.
C4 plants photosynthesize better than C3 plants under dry, hot conditions.
The greater affinity of PEP Carboxylase for its substrate means that the enzyme is saturated at low ambient CO2 levels.
Consequently, stomata may be closed for longer periods of time with C4 plants. This obviously helps to conserve water.
C4 plants are more abundant in hot arid climates, as might be expected.
