Problem 120 - Objective Biology for NEET

Question

Primary \(\mathrm{CO}_{2}\) acceptor in \(\mathrm{C}_{4}\) plant occurs in (a) Inner layer cells of bundle sheath (b) Outer layer cells of bundle sheath (c) Mesophyll cells (d) None of these

Step-by-Step Solution

Answer

The primary CO2 acceptor in C4 plants can be found in (c) Mesophyll cells.

Step 1: Identify the C4 Plants

C4 plants are a type of plants that utilize the C4 carbon fixation pathway as part of their photosynthesis process. Some examples of C4 plants are corn, sugar cane, and sorghum.

Step 2: Understand the C4 Pathway

In C4 plants, the CO2 fixation happens in two stages. The first phase, CO2 fixation, occurs in the mesophyll cells. In the initial reaction of this pathway, CO2 is incorporated into a 3-carbon compound (phosphoenolpyruvate) to produce a 4-carbon compound (oxaloacetic acid). The enzyme that catalyzes this reaction is PEP carboxylase. This is the reason why the mesophyll cells are referred to as the primary CO2 acceptor in C4 plants.

Step 3: Find the correct option

With this information, we can conclude that the primary CO2 acceptor in C4 plants occurs in the mesophyll cells. Therefore, the correct answer is (c) Mesophyll cells.

Key Concepts

Photosynthesis

Photosynthesis is a vital process that allows plants, algae, and some bacteria to convert light energy into chemical energy. It takes place mainly in the chloroplasts of plant cells. During this process, plants use sunlight, carbon dioxide (\(\mathrm{CO}_2\)), and water to produce glucose and oxygen. In general, photosynthesis can be divided into two main phases:

• The light-dependent reactions: These occur in the thylakoid membranes of chloroplasts, where sunlight is captured by chlorophyll and used to split water molecules. This releases oxygen and produces ATP and NADPH, which are energy-rich molecules.
• The Calvin Cycle (light-independent reactions): In this phase, ATP and NADPH are used to convert \(\mathrm{CO}_2\) into glucose through a series of chemical reactions.

C4 plants have developed a specialized mechanism to optimize photosynthesis in hot and dry environments. This involves a modification called the C4 pathway, allowing them to efficiently fix carbon dioxide, even when stomata are closed to minimize water loss. With this method, C4 plants can photosynthesize more effectively in challenging conditions.

Mesophyll Cells

Mesophyll cells are specialized cells located within the leaves of plants, sandwiched between the upper and lower epidermis. These cells are the site of photosynthesis and are packed with chloroplasts. In C4 plants, mesophyll cells play a critical role in the C4 carbon fixation pathway. In these cells, the initial fixation of carbon dioxide takes place. It involves the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which adds \(\mathrm{CO}_2\) to a 3-carbon compound called phosphoenolpyruvate (PEP), resulting in a 4-carbon compound known as oxaloacetic acid. This process happens before the \(\mathrm{CO}_2\) is transferred to bundle sheath cells, where it is used in the Calvin Cycle.The strategic separation of the initial CO2 fixation in mesophyll cells and the subsequent Calvin Cycle in bundle sheath cells reduces photorespiration. Photorespiration is a wasteful process that occurs when the enzyme RuBisCO adds oxygen instead of \(\mathrm{CO}_2\) to the Calvin Cycle, particularly under conditions of high temperature and light.

PEP Carboxylase

PEP carboxylase (phosphoenolpyruvate carboxylase) is a crucial enzyme involved in the carbon fixation pathway of C4 plants. It is found in the cytosol of mesophyll cells and plays a significant role in helping these plants overcome the challenges of hot and dry environments. This enzyme catalyzes the reaction of CO2 with phosphoenolpyruvate (PEP), a 3-carbon compound, forming oxaloacetic acid, a 4-carbon compound. Unlike RuBisCO, PEP carboxylase has a high affinity for \(\mathrm{CO}_2\) and does not bind to oxygen. This characteristic allows C4 plants to efficiently fix carbon even when the internal oxygen concentration is high.PEP carboxylase's efficiency in capturing \(\mathrm{CO}_2\) without being affected by oxygen ensures that C4 plants maintain high rates of photosynthesis and minimize photorespiration, which is less efficient. This adaptation is particularly beneficial in climates where high temperatures and dryness might otherwise impair photosynthetic activity.