What is the C4 pathway?

• A. A photosynthetic pathway that helps plants avoid photorespiration by fixing CO2 into a four-carbon compound.
• B. The respiratory pathway in mitochondria that fixes CO2 into carbonic acid.
• C. A process that converts CO2 directly to glucose in one step.
• D. A pathway that converts CO2 to CO2 via decarboxylation.

Answer: (A)

The C4 pathway is a photosynthetic adaptation that concentrates CO2 to reduce wasteful photorespiration. In this route, CO2 is first fixed in mesophyll cells by the enzyme PEP carboxylase to form a four-carbon molecule (oxaloacetate), which is quickly converted to malate and transported to neighboring bundle-sheath cells. There, malate is decarboxylated to release CO2, which is then used by Rubisco in the Calvin cycle. By delivering high CO2 levels directly to Rubisco, this pathway minimizes the oxygenation reaction that leads to photorespiration, especially when stomata are closed under hot, dry conditions. This comes with an energy cost to move and regenerate carbon skeletons, but it’s advantageous in high light and heat environments and is characteristic of C4 plants like maize and sugarcane.