C4 Pathway
Plants in hot climates (e.g. corn, sugar cane) exhibit a modification of C3 that involves
- extra step in CO2 assimilation
- physical separation of initial assimilation step and Calvin cycle
CO2 + PEP ? OAA
- PEP = phosphoenal pyruvate (3 C)
- OAA = oxaloacetic acid (4 C)
Process
- assimilation of CO2 occurs in mesophyll cells, but photosynthesis occurs in bundle sheath cells
- OAA diffuses into bundle sheath cells where it breaks down to CO2 and pyruvate
- Pyr moves back to bundle sheath cells - converted to PEP
- CO2 released in bundle sheath enters Calvin cycle
Benefits
- enzyme PEP carboxylase, has high affinity for CO2, therefore stomata can be closed
- [CO2] determined by [OAA], therefore [CO2] higher than atmos - Calvin cycle operates more efficiently