Why do CAM and C4 plants use PEP carboxylase for initial CO2 fixation?

• A. It has a high affinity for O2.
• B. No O2 affinity; fixes CO2 even at low concentrations.
• C. It produces NADPH.
• D. It requires Rubisco.

Answer: (B)

The main idea is that CAM and C4 plants rely on PEP carboxylase for the first CO2 capture because this enzyme can fix CO2 efficiently without being affected by oxygen, which is crucial when CO2 is scarce or stomata are closed. PEP carboxylase combines CO2 with phosphoenolpyruvate to form oxaloacetate, a four-carbon compound, and it does not have an oxygenase activity. This means it won’t waste energy by reacting with O2 (the problem Rubisco faces under low CO2 or high O2 conditions, leading to photorespiration). The CO2 captured by PEPC is then stored as a four-carbon compound and later released near the site of the Calvin cycle, where Rubisco operates, effectively increasing local CO2 concentration and reducing wasteful oxygenation. Because of this, the initial fixation step is robust even when CO2 levels fall or stomata are closed. The other statements aren’t correct: PEPC’s role is not to produce NADPH, and the initial fixation does not require Rubisco.