In C4 plants, what is the sequence of CO2 fixation and release?

• A. CO2 fixed in the bundle-sheath cells by RuBisCO, then transported to mesophyll cells.
• B. CO2 fixed in the mesophyll cells by RuBisCO to form 3-PGA, then transported to mitochondria.
• C. CO2 fixed in mesophyll cells by PEP carboxylase to form oxaloacetate, transported to bundle-sheath cells, where CO2 is released for fixation by RuBisCO.
• D. CO2 is fixed only in the thylakoid, no transport involved.

Answer: (C)

In C4 plants the CO2 fixation happens in two steps with a spatial separation between the cells that perform each step. First, CO2 is fixed in the mesophyll cells by the enzyme PEP carboxylase to form a four-carbon compound (oxaloacetate, usually converted to malate). This C4 acid is then transported to the bundle-sheath cells, where it is decarboxylated to release CO2. The locally concentrated CO2 is then fixed again by RuBisCO in the Calvin cycle. The four-carbon carrier is recycled back to the mesophyll as pyruvate to regenerate the starting molecule (PEP) and continue the cycle. This arrangement concentrates CO2 around RuBisCO and minimizes photorespiration.