Which statement best describes C4 carbon fixation?

Which statement best describes C4 carbon fixation?

• A. It fixed carbon directly in the Calvin cycle.
• B. It fixes carbon at night to reduce water loss.
• C. It uses a three-carbon compound to minimize photorespiration.
• D. It uses a four-carbon compound to minimize photorespiration.

Answer: (D)

The key idea is concentrating CO2 before the Calvin cycle by using a four-carbon molecule. In C4 plants, CO2 is first fixed in mesophyll cells by the enzyme phosphoenolpyruvate carboxylase into a four-carbon compound (oxaloacetate, quickly turned into malate). This four-carbon shuttle carries the carbon to bundle-sheath cells, where CO2 is released and then fixed by the Calvin cycle. By delivering CO2 in a high-CO2 environment, RuBisCO is less likely to oxygenate it, which minimizes photorespiration—an advantage in hot, dry conditions when stomata are often closed to save water. Other statements don’t fit because fixing carbon directly in the Calvin cycle describes the C3 pathway, not C4; fixing at night is a feature of CAM plants; and using a three-carbon compound would describe a different, non-C4 scenario. The use of a four-carbon compound to concentrate CO2 is the defining aspect of C4 carbon fixation.

The key idea is concentrating CO2 before the Calvin cycle by using a four-carbon molecule. In C4 plants, CO2 is first fixed in mesophyll cells by the enzyme phosphoenolpyruvate carboxylase into a four-carbon compound (oxaloacetate, quickly turned into malate). This four-carbon shuttle carries the carbon to bundle-sheath cells, where CO2 is released and then fixed by the Calvin cycle. By delivering CO2 in a high-CO2 environment, RuBisCO is less likely to oxygenate it, which minimizes photorespiration—an advantage in hot, dry conditions when stomata are often closed to save water.

Other statements don’t fit because fixing carbon directly in the Calvin cycle describes the C3 pathway, not C4; fixing at night is a feature of CAM plants; and using a three-carbon compound would describe a different, non-C4 scenario. The use of a four-carbon compound to concentrate CO2 is the defining aspect of C4 carbon fixation.