C4 Pathway and CAM

The C4 pathway separates carbon fixation and the Calvin cycle in space, occurring in different cell types. PEP carboxylase fixes CO2 in mesophyll cells, then malate transports it to bundle sheath cells where oxygen concentration is low. This allows plants to close stomata and conserve water while maintaining efficient photosynthesis and avoiding photorespiration.

C4 plants separate carbon fixation and the Calvin cycle in space using different cell types, while CAM plants separate these processes in time. CAM plants open stomata at night to fix CO2 into malate, storing it in vacuoles until daylight when the Calvin cycle proceeds. This temporal separation prevents water loss during hot days.

CAM plants open stomata at night to prevent water loss during hot daytime conditions. They fix CO2 into malate and store it in vacuoles overnight. During the day, stomata remain closed while stored malate releases CO2 for the Calvin cycle, allowing photosynthesis to proceed without excessive water evaporation.

PEP carboxylase is the enzyme that initiates carbon fixation in C4 plants by combining CO2 with the 3-carbon compound phosphoenolpyruvate to form the 4-carbon compound oxaloacetate. This reaction occurs in mesophyll cells and is the first step that concentrates CO2 before it enters the Calvin cycle in bundle sheath cells.

Photorespiration occurs when oxygen concentration is high and the rubisco enzyme binds O2 instead of CO2, halting photosynthesis and consuming energy. C4 and CAM plants overcome this by maintaining low oxygen concentrations in specific locations or times, allowing rubisco to bind CO2 efficiently and preventing energy loss.

Malate is stored in vacuoles of mesophyll cells in CAM plants. This storage allows plants to accumulate fixed carbon during the cool night when stomata are open, then release it during the day for the Calvin cycle without reopening stomata and losing water to evaporation.

After malate is broken down in bundle sheath cells, pyruvate is regenerated and transported back to mesophyll cells where it is converted back into phosphoenolpyruvate. This regenerated PEP is then available for the next round of CO2 fixation, creating a continuous cycle that concentrates carbon for efficient photosynthesis.