Question

Select the correct statement regarding the first stable product formed in Hatch and Slack pathway in \(C_{4}\) plants. (a) Oxaloacetate is formed by carboxylation of phosphoenol pyruvate (PEP) in the bundle sheath cells, (b) Oxaloacetate is formed by carboxylation of phosphoenol pyruvate (PEP) in the mesophyll cells. (c) Phosphoglyceric acid is formed in the mesophyll cells. (d) Phosphoglyceric acid is formed in the bundle sheath cells.

Short answer

(b) Oxaloacetate is formed by carboxylation of phosphoenol pyruvate (PEP) in the mesophyll cells.

Step-by-step solution

Understanding C4 Photosynthetic Pathway

The first step in the C4 pathway involves the carboxylation of phosphoenol pyruvate (PEP), which is a 3-carbon molecule, to form a 4-carbon compound, oxaloacetate (OAA). The enzyme responsible for this reaction is PEP carboxylase.

Identifying the Location of Initial Carboxylation

In C4 plants, the initial carboxylation reaction does not occur in the bundle sheath cells but in the mesophyll cells. This is a distinctive feature of C4 photosynthesis that differentiates it from the C3 pathway, where the initial carboxylation occurs in the bundle sheath cells.

Determining the Correct Statement

Based on the understanding of the C4 photosynthesis pathway, we identify that oxaloacetate is formed by the carboxylation of PEP in the mesophyll cells, not in the bundle sheath cells. Thus, the correct statement is that oxaloacetate is the first stable product and it is formed in the mesophyll cells.

Key concepts

Hatch and Slack pathway

The Hatch and Slack pathway, often referred to as the C4 photosynthetic pathway, represents a series of biochemical reactions in certain plants that adapt to hot, dry climates. Unlike the C3 pathway, which is more common among plants and where the first stable product is a 3-carbon molecule, the C4 pathway results in the first stable product being a 4-carbon compound. This modification has evolved to increase efficiency in fixing CO2 and to minimize photorespiration, a wasteful process that can occur in C3 plants under high temperature and light conditions.

By partitioning the initial and final stages of photosynthesis between two types of cells, mesophyll and bundle sheath cells, the C4 pathway effectively reduces oxygenation reactions and concentrates CO2 around the enzyme Rubisco, which is vital for carbon fixation process.

Photosynthetic pathways

Photosynthetic pathways are the routes by which plants convert light energy into chemical energy, with the primary aim of producing glucose for their growth and metabolism. There are primarily three known photosynthetic pathways: C3, C4, and CAM. The C3 pathway is the most common and is named after the 3-carbon molecule, 3-phosphoglycerate, which is the first stable product during carbon fixation. The C4 pathway, which includes the Hatch and Slack pathway, is named after the 4-carbon molecule oxaloacetate. The CAM pathway, similar to C4, also fixes carbon into a 4-carbon molecule but differs in the timing of the process; the CAM pathway closes stomata during the day to reduce water loss and opens them at night to take in CO2.

Each pathway has evolved to support plants in differing environments. C4 and CAM are more common in hot and arid climates, while C3 is ubiquitous in temperate climates.

Bundle sheath cells

Bundle sheath cells are specialized cells located surrounding the vascular bundles in C4 plants. These cells play an essential role in the C4 photosynthetic pathway. They work in concert with mesophyll cells to concentrate CO2 around Rubisco, the enzyme involved in the Calvin cycle. In the C4 pathway, after the initial fixation of CO2 in mesophyll cells, the 4-carbon compounds are shuttled to the bundle sheath cells. Here, they are decarboxylated to release CO2, thereby providing an elevated concentration of CO2 for Rubisco, which promotes efficient photosynthesis and suppresses the occurrence of the photorespiratory pathway.

Mesophyll cells

Mesophyll cells are a type of plant cell located in the leaf's interior, where most of the plant's photosynthesis occurs. In C4 plants, the mesophyll cells contain high concentrations of PEP carboxylase and are the sites of initial CO2 fixation. This creates the first product, oxaloacetate, a 4-carbon compound. This spatial separation of steps within different cell types is one of the major adaptations that differentiate C4 photosynthesis from the C3 pathway and allows C4 plants to thrive in high light intensity and high temperature environments where C3 plants might be at a disadvantage due to higher rates of photorespiration.

PEP carboxylase

PEP carboxylase, or phosphoenolpyruvate carboxylase, is an enzyme critical to the C4 photosynthetic pathway. It catalyzes the transfer of a carbon dioxide molecule to phosphoenolpyruvate (PEP), producing the 4-carbon compound oxaloacetate. The presence of PEP carboxylase is key to the efficiency of the C4 pathway since it has a higher affinity for CO2 than Rubisco and does not catalyze reactions with oxygen—a significant advantage that reduces photorespiration. This characteristic contributes to the ability of C4 plants, such as maize, sugarcane, and sorghum, to maintain high productivity even under stressful environmental conditions.