Question

During Hatch and Slack pathway, PEP combines the presence of enzyme PEPcase, to form OAA. of initial fixation of \(\mathrm{CO}_{2}\) occurs in (a) mesophyll cells (b) bundle sheath (c) both (a) and (b) (d) none of these \(\mathrm{E}\) (b)

Short answer

The initial fixation of CO2 during the Hatch and Slack pathway occurs in (a) mesophyll cells.

Step-by-step solution

Understanding the Hatch and Slack Pathway

The Hatch and Slack pathway, also known as C4 photosynthesis, involves two types of cells: the mesophyll cells and the bundle sheath cells. In this process, Phosphoenolpyruvate (PEP) reacts with Carbon dioxide (CO2) in the presence of the enzyme PEP carboxylase (PEPcase) to form oxaloacetate (OAA).

Identifying the Site of Initial CO2 Fixation

The initial fixation of CO2 occurs in the mesophyll cells where PEP is present and combines with CO2 in the presence of the enzyme PEPcase to form OAA.

Choosing the Correct Option

Based on the understanding of where initial CO2 fixation takes place in the Hatch and Slack pathway, the correct option is (a) mesophyll cells.

Key concepts

Hatch and Slack Pathway

The Hatch and Slack pathway, commonly known as C4 photosynthesis, is a specialized photosynthetic process used by certain plants to minimize photorespiration and maximize the efficiency of carbon fixation, particularly in hot and dry environments. Unlike the Calvin cycle used in typical C3 photosynthesis, the C4 pathway involves an additional set of steps that effectively concentrate carbon dioxide in the bundle sheath cells, thereby reducing the oxygenase activity of Rubisco and the loss of fixed carbon as CO2.

Essentially, the strategy is to convert CO2 into a four-carbon compound as the first step in carbon fixation. The process starts in the mesophyll cells where PEP carboxylase adds carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate (OAA). This OAA is then typically converted to a transportable form such as malate or aspartate, which is shuttled to the bundle sheath cells where carbon dioxide is released for use in the Calvin cycle.

PEP Carboxylase (PEPcase)

Phosphoenolpyruvate carboxylase, or PEPcase, is a crucial enzyme in the C4 photosynthetic pathway. It has a powerful affinity for carbon dioxide and is responsible for the initial carbon dioxide fixation. Unlike Rubisco, PEPcase does not interact with oxygen, which makes it much more efficient in environments where CO2 levels are low, as in hot and arid conditions.

PEPcase catalyzes the reaction of PEP with CO2 to produce oxaloacetate (OAA). This reaction is the first step in creating a C4 organic acid that serves as a carrier for CO2 to be transported to the bundle sheath cells. This process ensures a high concentration of carbon dioxide near Rubisco, averting the oxygenation activity and photorespiration that can dissipate energy and carbon.

Initial CO2 Fixation

Initial CO2 fixation in C4 plants is distinct from that in C3 plants. The whole purpose of this initial step is to trap the CO2 in a form that can be transported to another cell type for further processing. This stage uses PEPcase to fix bicarbonate (rather than CO2 directly) to PEP, forming a stable four-carbon compound, oxaloacetate, which can then be converted into other C4 compounds.

The specificity of PEPcase for bicarbonate over oxygen ensures that the initial products of photosynthesis are not lost through photorespiration as they could be when the less specific enzyme Rubisco fixes CO2 in C3 plants. The initial CO2 fixation is a fast, high-affinity reaction, ensuring that carbon is quickly captured and stabilized under a wide range of environmental conditions.

Mesophyll Cells

Mesophyll cells are the primary sites of photosynthesis in plant leaves. In C4 photosynthesis specifically, these cells are the location for the initial CO2 fixation. They contain a high concentration of PEPcase which quickly converts CO2 into a four-carbon molecule, maximizing carbon fixation efficiency.

The role of mesophyll cells is pivotal as they not only facilitate the initial fixation but also are involved in the spatial separation of the C4 pathway's steps. They are anatomically adapted to perform their function effectively, often having large numbers of chloroplasts and enhanced cytoplasmic streaming to quickly transport the fixed carbon to bundle sheath cells for the subsequent steps of photosynthesis.