Carbon Dioxide Acceptor in C4 Plants: Phosphoenol Pyruvic Acid (PEP)

In C4 plants, carbon dioxide (CO2) is initially fixed into a four-carbon compound called oxaloacetate (OAA) in the mesophyll cells of the leaves. The enzyme responsible for this initial fixation is phosphoenolpyruvate carboxylase (PEP carboxylase).

1. C4 Photosynthesis
C4 plants have evolved an efficient mechanism called C4 photosynthesis to overcome the limitations of the enzyme Rubisco, which has a low affinity for CO2 and can also catalyze reactions with oxygen, leading to photorespiration. C4 plants have specialized leaf anatomy, with two types of cells: mesophyll cells and bundle sheath cells.

2. Mesophyll Cells
In C4 plants, CO2 is first fixed into a three-carbon compound called phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase in the mesophyll cells. This process occurs in the presence of ATP, and the reaction forms a four-carbon compound called oxaloacetate (OAA). This initial fixation of CO2 occurs in the mesophyll cells and is independent of light.

3. PEP Carboxylase
PEP carboxylase is an enzyme found in the mesophyll cells of C4 plants. It has a higher affinity for CO2 than Rubisco and does not catalyze reactions with oxygen. PEP carboxylase is not inhibited by oxygen and has a higher optimal temperature than Rubisco, allowing C4 plants to efficiently fix CO2 even under high temperatures.

4. Role of PEP in C4 Plants
The oxaloacetate (OAA) formed in the mesophyll cells is then converted into malate or aspartate, which can be transported to the bundle sheath cells, where the Calvin cycle takes place. In the bundle sheath cells, malate or aspartate is decarboxylated, releasing CO2, which is then fixed by Rubisco in the Calvin cycle.

5. Advantages of C4 Photosynthesis
C4 plants have several advantages over C3 plants (plants that use the Calvin cycle for photosynthesis):
- Increased efficiency in carbon fixation, especially under high temperature and light conditions
- Reduced photorespiration, leading to higher rates of photosynthesis and growth
- Ability to grow in environments with limited water availability and high temperatures, such as tropical areas

Overall, the carbon dioxide acceptor in C4 plants is phosphoenolpyruvate (PEP), which is fixed into oxaloacetate (OAA) by the enzyme PEP carboxylase in the mesophyll cells. This initial fixation of CO2 allows C4 plants to overcome the limitations of Rubisco and efficiently perform photosynthesis even under challenging environmental conditions.

The C4​ plants are called so because the first stable product of this process are the four carbon atom containing organic acids like oxalo-acetic acid and malic acid. The initial acceptor of carbon dioxide is phosphoenol pyruvic acid (PEP) and the initial carboxylating enzyme is PEP carboxylase. The initial carboxylation reaction occurs in the mesophyll cells. In the C4​ organic acids are decarboxylated in the bundle sheath cells to release carbon dioxide. In the bundle sheath cells the carbon dioxide is fixed with the help of RuBisCO enzyme. Hence the ans is option A