Which plant type fixes carbon through a four-carbon compound to minimize photorespiration?

Which plant type fixes carbon through a four-carbon compound to minimize photorespiration?

• A. CAM plants
• B. C3 plants
• C. C4 plants
• D. All plants

Answer: (C)

Concentrating CO2 around the enzyme that fixes carbon is how this strategy reduces wasteful photorespiration. In C4 plants, CO2 is first fixed in the mesophyll cells by PEP carboxylase into a four-carbon compound (oxaloacetate, usually converted to malate). This four-carbon molecule is then transported to bundle-sheath cells, where it is decarboxylated to release CO2 directly near the Calvin cycle. The locally high CO2 concentration minimizes RuBisCO’s oxygenase activity, so less carbon is lost to photorespiration. This spatial separation—fixation in one cell type and decarboxylation in another—lets C4 plants photosynthesize efficiently even when stomata are partially closed in hot, dry conditions.

CAM plants also use four-carbon acids, but they do so by fixing CO2 at night and storing it as acids, providing temporal separation. The question’s mechanism—four-carbon fixation to minimize photorespiration in a daytime CO2-rich environment—points to C4 plants.