Question

In case of \(\mathrm{C}_{4}\) pathway, \(\mathrm{CO}_{2}\) combines with (a) PGA (b) PEP (c) RuDP (d) RMP

Short answer

The correct answer is (b) PEP.

Step-by-step solution

Understanding the C4 Pathway

The C4 pathway, also known as the Hatch-Slack pathway, is a carbon fixation process in some plants. In this pathway, carbon dioxide is first fixed into a 4-carbon compound before entering the Calvin cycle. The name 'C4' comes from this initial 4-carbon molecule that is produced.

Identifying the Correct Compound

In the first step of the C4 pathway, carbon dioxide combines with a 3-carbon molecule called phosphoenolpyruvate (PEP). The result of this combination is the 4-carbon compound oxaloacetate. Hence, the molecule that CO2 combines with in the C4 pathway is PEP.

Key concepts

Carbon Fixation

Carbon fixation is a crucial process in photosynthesis, allowing plants to convert carbon dioxide from the atmosphere into organic compounds. This process provides the foundation for the energy supply in most ecosystems. The primary goal of carbon fixation is to create a stable form of carbon that can be used to synthesize sugars and other organic molecules.

In the context of the C4 pathway, carbon fixation takes a unique route compared to the more common C3 pathway. In C4 plants, carbon fixation occurs in two stages, optimizing the process for environments where CO2 may be limited or conditions are arid. This specialized adaptation helps certain plants, like corn and sugarcane, to thrive in hot and dry environments.

What sets the C4 pathway apart is the initial fixation of carbon into a 4-carbon compound, hence the name. This initial fixation occurs in the mesophyll cells, leading to the creation of a molecule named oxaloacetate. This strategic separation of steps in both time and space helps to avoid photorespiration, a wasteful process that can occur in plant cells.

Phosphoenolpyruvate (PEP)

Phosphoenolpyruvate (PEP) is a critical 3-carbon molecule in the C4 carbon fixation pathway. It acts as a starting anchor for capturing carbon dioxide before it is eventually used to form carbohydrates. PEP's role is essential because it combines with carbon dioxide to form oxaloacetate, a 4-carbon molecule.

This reaction is facilitated by the enzyme PEP carboxylase, which adds carbon dioxide to PEP. Unlike Rubisco (an enzyme used in the C3 pathway), PEP carboxylase has a strong affinity for CO2, making it more efficient at lower CO2 concentrations. This makes PEP vital for the functioning of plants in environments where carbon dioxide is not as readily available.

Once oxaloacetate is formed, it undergoes further transformations into malate or aspartate and is transported to the bundle sheath cells. Here, the CO2 is released for use in the Calvin cycle where it is incorporated into sugars. This process underscores the value of phosphoenolpyruvate as a crucial intermediary in efficient carbon utilization.

Oxaloacetate

Oxaloacetate is a 4-carbon acid produced during the initial stages of the C4 pathway. When carbon dioxide is fixed onto PEP, the product is oxaloacetate. This molecule stands at the crossroads of many metabolic pathways within the cell.

In the C4 pathway, oxaloacetate serves as a vehicle to transport carbon dioxide from the mesophyll cells to the bundle sheath cells. This movement is vital for segregating the initial carbon fixation from the subsequent stages of photosynthesis that occur in the Calvin cycle. The conversion of oxaloacetate into malate or aspartate allows the shuttling of CO2 while maintaining the separation necessary to minimize photorespiration.

Once in the bundle sheath cells, oxaloacetate-derived molecules release CO2, ensuring a high concentration around Rubisco for photosynthesis. This efficient transportation and concentration method enhances overall carbon fixation efficiency, especially in environments where CO2 levels are low. Thus, oxaloacetate is crucial in optimizing the photosynthetic process in C4 plants.