Question

In the mesophyll cells of C₄ plants, CO₂ is bonded to ________ to form oxaloacetate.

a. phosphoenolpyruvate
b. malate
c. phosphoglycolate
d. glycerol
e. pyruvate

Answer 1

In the mesophyll cells of C₄ plants, CO₂ is bonded to phosphoenolpyruvate to form oxaloacetate.

Answer 2

In C₄ plants, CO₂ is initially fixed into phosphoenolpyruvate to form oxaloacetate. The correct answer is a. phosphoenolpyruvate.

Step-by-step explanation:

In C₄ plants, carbon fixation occurs in mesophyll cells, where CO₂ is first incorporated into phosphoenolpyruvate (PEP) by the enzyme phosphoenolpyruvate carboxylase (PEPC). This reaction forms oxaloacetate, a four-carbon compound. The enzyme PEPC has a high affinity for CO₂ and does not undergo the oxygenation reaction that occurs in the Calvin cycle of C₃ plants, reducing photorespiration. The resulting oxaloacetate is then converted to malate, which is transported to bundle-sheath cells, where the Calvin cycle takes place.

In the first step of the C₄ pathway, the carboxylation of PEP to oxaloacetate is a crucial process. The equation for this reaction is:

`\[ \text{PEP} + \text{CO}_2 \rightarrow \text{Oxaloacetate} \]`

This initial fixation of CO₂ into a four-carbon compound allows for more efficient carbon fixation and reduces the losses associated with photorespiration. The C₄ pathway is an adaptation to improve the efficiency of photosynthesis, especially in conditions of high temperature and aridity.

In summary, phosphoenolpyruvate serves as the initial acceptor of CO₂ in the mesophyll cells of C₄ plants, forming oxaloacetate, which is then further processed in the C₄ pathway to enhance photosynthetic efficiency.

The correct answer is a. phosphoenolpyruvate.