Question

In \(\mathrm{C}_{4}\) plants, \(\mathrm{CO}_{2}\) combines with PEP in the presence of (a) PEP carboxylase (b) RuBP carboxylase (c) RuBP oxygenase (d) Hydrogenase

Short answer

In C4 plants, CO2 combines with PEP in the presence of PEP carboxylase.

Step-by-step solution

Identifying Enzymes and Their Functions

Each of the options provided in the questions signifies an enzyme with a specific function. PEP carboxylase, RuBP carboxylase, RuBP oxygenase and Hydrogenase all participate in various biochemical and metabolic reactions within plants.

Understanding C4 Photosynthesis

Much unlike C3 plants, C4 plants perform a slightly modified form of photosynthesis. This is done to overcome photorespiration, which often affects productivity in high temperature and light conditions. In C4 plants, the carbon fixation process happens in two stages. The first reaction combines CO2 with PEP.

Determining the Correct Enzyme

The first reaction of carbon fixation in C4 plants is the combination of CO2 and PEP. This reaction is facilitated by the enzyme PEP carboxylase. Hence, the correct answer to this question would be (a) PEP carboxylase.

Key concepts

PEP carboxylase

PEP carboxylase is a crucial enzyme in the photosynthetic pathway of C4 plants. It plays a fundamental role in the first step of carbon fixation. This enzyme catalyzes the reaction between phosphoenolpyruvate (PEP) and carbon dioxide ( CO_2 ), forming oxaloacetate.

• Unlike RuBisCO, which is found in C3 plants, PEP carboxylase operates more efficiently in hot, arid environments.
• It has a higher affinity for CO_2 and does not react with oxygen. This allows C4 plants to minimize water loss through their stomata.

Due to its efficiency, PEP carboxylase helps C4 plants avoid photorespiration, which can be a significant disadvantage for C3 plants under similar conditions.

Carbon fixation

Carbon fixation is a pivotal part of the photosynthesis process. It involves the incorporation of carbon dioxide into organic compounds. In C4 photosynthesis, this process is uniquely divided into two stages.

• First, CO_2 combines with PEP in the mesophyll cells to form a four-carbon compound, usually oxaloacetate. This is catalyzed by PEP carboxylase.
• Oxaloacetate is then quickly converted into malate or aspartate, which are transported to the bundle-sheath cells.
• In the bundle-sheath cells, the four-carbon compound is decarboxylated, releasing CO_2 for the Calvin cycle, which is why these plants are named "C4".

This two-stage carbon fixation strategy is efficient and reduces the chances of photorespiration, allowing C4 plants to thrive in conditions that would otherwise cause problems for C3 plants.

Photorespiration

Photorespiration is a process that occurs primarily in C3 plants when RuBisCO mistakenly binds oxygen instead of CO_2 . This usually happens under high light and temperature conditions, leading to reduced photosynthetic efficiency.

• It is an energy-consuming process that leads to the loss of fixed carbon through the release of CO_2 rather than its fixation.
• In C3 plants, this can significantly reduce productivity as they have to spend additional energy to manage photorespiration.
• C4 plants, however, generally do not face this issue because PEP carboxylase only binds CO_2 , allowing them to avoid the negative impacts of photorespiration.

The adaptation of C4 plants to minimize photorespiration is one of the key evolutionary responses to thrive in hot, sunny environments.