C3, C4, and CAM Photosynthesis
Three evolutionary answers to the same problem: photorespiration in hot, dry climates.
The problem with rubisco
Rubisco is not selective: it binds whichever gas reaches its active site. On hot, dry days plants close their stomata to conserve water, CO₂ inside the leaf drops, O₂ from the light reactions builds up, and rubisco increasingly fixes O₂.
That reaction - photorespiration - consumes ATP and releases CO₂ without producing sugar, cutting photosynthetic efficiency by as much as 25% in C3 plants.
C4: spatial separation
In C4 plants (corn, sugarcane, crabgrass), PEP carboxylase in the mesophyll fixes CO₂ into a 4-carbon acid. PEP carboxylase has no affinity for O₂, so it works even when CO₂ is scarce.
The 4-carbon acid moves into bundle-sheath cells and releases CO₂ there, creating a locally high CO₂ concentration around rubisco. The Calvin cycle then runs normally with little photorespiration.
CAM: temporal separation
CAM plants (cacti, pineapple, jade) open their stomata only at night, when evaporative loss is low, and store fixed carbon as organic acids in the vacuole. During the day the stomata close and those acids release CO₂ internally for the Calvin cycle.
The trade-off is throughput: CAM plants grow slowly because nighttime storage limits how much carbon they can fix, but they survive where C3 plants cannot.
Key terms
3
- Photorespiration
- Rubisco fixing O₂ instead of CO₂, consuming ATP and releasing CO₂ with no sugar produced.
- C4 pathway
- CO₂ is first fixed into a 4-carbon acid in mesophyll cells, then released to the Calvin cycle in bundle-sheath cells.
- CAM
- Crassulacean acid metabolism; stomata open at night to fix CO₂ into organic acids used the next day.
Sign-off
Finish this lesson
A lesson only counts once you've read all the way through and completed every activity on the page.
- Read the full lesson (scroll to the end)
- Complete all activities (0/1 done)