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

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.

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.