How do C4 plants initially fix carbon?

• A. CO2 is fixed into oxaloacetate, then converted to malate.
• B. CO2 is fixed directly into ribulose-1,5-bisphosphate.
• C. CO2 is stored as bicarbonate in vacuoles.
• D. CO2 is reduced to glyceraldehyde-3-phosphate.

Answer: (A)

C4 carbon fixation starts with a specific enzyme in the mesophyll cells: CO2 is fixed not directly into the usual Calvin cycle receptor, but into a four-carbon molecule. The CO2 combines with phosphoenolpyruvate to form oxaloacetate, which is then quickly reduced to malate. This malate is transported to the adjacent bundle-sheath cells, where it is decarboxylated to release CO2. That CO2 is then available at a high concentration for the Calvin cycle, allowing Rubisco to fix it efficiently and reducing wasteful oxygenation (photorespiration) under hot, dry conditions.

This differs from direct fixation into ribulose-1,5-bisphosphate, which is the typical entry point in C3 photosynthesis and occurs without the initial PEP carboxylase step. Storing CO2 as bicarbonate in vacuoles isn’t how carbon is fixed into organic molecules in plants. Reducing CO2 to glyceraldehyde-3-phosphate happens later in the Calvin cycle after CO2 is fixed, not at the initial fixation step in C4 metabolism.