In mesophyll cells of C4 plants, what enzyme adds carbon dioxide to a three-carbon molecule to generate oxaloacetate?
The correct answer and explanation is:
The enzyme that adds carbon dioxide to a three-carbon molecule in the mesophyll cells of C4 plants to generate oxaloacetate is phosphoenolpyruvate carboxylase (PEP carboxylase).
Explanation:
In C4 plants, photosynthesis involves a special mechanism to efficiently capture carbon dioxide (CO2) and minimize photorespiration. This mechanism is known as the C4 pathway. The process starts in the mesophyll cells, where carbon dioxide is initially fixed.
Phosphoenolpyruvate carboxylase is the key enzyme responsible for the first step of carbon fixation in C4 plants. It catalyzes the reaction between carbon dioxide and a three-carbon compound called phosphoenolpyruvate (PEP). When CO2 is added to PEP, a four-carbon molecule called oxaloacetate is produced. This reaction is crucial because PEP carboxylase has a very high affinity for CO2 and does not bind oxygen, which helps the plant avoid the wasteful process of photorespiration that occurs in C3 plants.
Oxaloacetate formed in the mesophyll cells is then usually converted into malate or aspartate. These four-carbon compounds are transported to the bundle sheath cells, where CO2 is released and enters the Calvin cycle. By concentrating CO2 in the bundle sheath cells, the plant increases the efficiency of photosynthesis, especially under conditions of high temperature, drought, or low atmospheric CO2.
This spatial separation of carbon fixation and the Calvin cycle allows C4 plants to conserve water and maintain higher photosynthetic rates compared to C3 plants under stressful environmental conditions. Examples of C4 plants include maize, sugarcane, and sorghum.
In summary, phosphoenolpyruvate carboxylase plays an essential role in C4 photosynthesis by fixing CO2 into oxaloacetate in the mesophyll cells, setting the stage for efficient carbon assimilation.