Rubisco, the enzyme of the Calvin cycle, has a flaw: it can grab O₂ instead of CO₂. When that happens, the plant loses fixed carbon and energy in a process called photorespiration. You saw in Water Balance in Plants that stomata close when the plant is short of water. Then CO₂ inside the leaf drops, O₂ from the light reactions builds up, and rubisco makes the mistake more often, especially in heat.
Key idea
C4 plants (corn, sugarcane) separate CO₂ capture and the Calvin cycle in space: mesophyll cells fix CO₂ into a 4-carbon acid, which carries it to bundle-sheath cells where rubisco works in a CO₂-rich spot. CAM plants (cactuses, pineapple) separate them in time: they open stomata and store CO₂ at night, and run the Calvin cycle by day with stomata closed.
| C3 (most plants: wheat, rice, soybean) | C4 (corn, sugarcane) | CAM (cactuses, pineapple) | |
|---|---|---|---|
| First CO₂ capture | Rubisco, onto RuBP | PEP carboxylase, into a 4-carbon acid | PEP carboxylase, into a 4-carbon acid |
| Where or when the Calvin cycle runs | Mesophyll cells, by day | Bundle-sheath cells, by day | Mesophyll cells, by day |
| Stomata open | By day | By day (can open less) | At night |
| Best suited to | Cool, moist places | Hot, sunny places | Hot, very dry places |
Why it works
The first enzyme in C4 and CAM plants, PEP carboxylase, binds CO₂ even when CO₂ is scarce and does not grab O₂. It works like a pump: it collects CO₂ and stores it in an acid. Later the acid releases CO₂ right where rubisco is, so rubisco is surrounded by CO₂ and rarely makes its mistake. The pump costs extra ATP, which is why C4 and CAM pay off in heat and drought but not in cool, wet places.
Worked examples
Worked example: Spotting a CAM plant from data
A succulent's leaves are measured at dawn and at dusk. At dawn the leaf sap is very acidic; at dusk it is much less acidic. Its stomata are open at night. What kind of plant is it?
- Open stomata at night means CO₂ is taken in at night.
- The CO₂ is stored as organic acid, so acid builds up overnight: high at dawn.
- By day the acid releases CO₂ for the Calvin cycle, so it falls by dusk.
Answer: A CAM plant.
Worked example: Hot and dry
On a hot, dry afternoon, a wheat plant (C3) and a corn plant (C4) both close their stomata partway. Which loses more carbon to photorespiration?
- In wheat, rubisco takes CO₂ straight from the air spaces of the leaf, which is running low on CO₂.
- In corn, PEP carboxylase keeps CO₂ concentrated around rubisco in the bundle-sheath cells.
Answer: The wheat plant.
Common mistake
Thinking CAM plants run the Calvin cycle at night. At night they only take in CO₂ and store it as acid. The Calvin cycle still needs ATP and NADPH from the light reactions, so it runs by day, using the stored CO₂ while the stomata stay closed.