Problem 112 - Objective Biology for NEET

Question

How many types of photosynthetic cells occur in \(\mathrm{C}_{4}\) plant? (a) One type (b) Two types (c) Four types (d) Eight types

Step-by-Step Solution

Answer

C4 plants have two types of photosynthetic cells - Mesophyll cells and Bundle sheath cells. Thus, the answer is (b) Two types.

Step 1: Understand photosynthesis in C4 plants

C4 plants have a unique mechanism of photosynthesis, known as the C4 pathway. This pathway efficiently captures carbon dioxide in conditions of high light intensity and low carbon dioxide concentration. It is an adaptation to hot, dry conditions and involves the separation of carbon fixation and the Calvin cycle in space, within two different types of cells.

Step 2: Identify the types of photosynthetic cells in C4 plants

In C4 plants, there are two types of photosynthetic cells present: Mesophyll cells and Bundle sheath cells. Carbon dioxide is initially fixed in the mesophyll cells to form a four-carbon compound (hence the name C4). This compound is then transferred to the bundle sheath cells, where it releases carbon dioxide that enters the traditional Calvin cycle. Each cell type has a specific role in the C4 photosynthetic pathway, making this a two-step process involving two types of cells.

Key Concepts

Mesophyll Cells

In C4 plants, mesophyll cells play a crucial role in the first steps of photosynthesis. These cells are located at the outer part of the leaf and are primarily involved in capturing carbon dioxide from the atmosphere. Their significance lies in their ability to fix carbon dioxide into a four-carbon compound, which is integral to the C4 photosynthetic pathway.

• The four-carbon compound formed is usually oxaloacetate or malate, which serves as a carrier of carbon dioxide.
• This initial fixation of carbon dioxide is facilitated by the enzyme phosphoenolpyruvate (PEP) carboxylase, which is more efficient than Rubisco under certain conditions, such as low atmospheric CO2 concentrations.

By fixing carbon dioxide quickly and efficiently, mesophyll cells allow C4 plants to thrive in environments with intense sunlight and limited water. This step is crucial in ensuring that the photosynthetic process remains effective even when atmospheric conditions are not ideal.

Bundle Sheath Cells

Following the carbon fixation in mesophyll cells, the C4 plants utilize bundle sheath cells to complete the photosynthesis process. These cells are found surrounding the veins of the leaf, forming a tightly packed layer. Their role in the C4 pathway is vital for ensuring the efficient processing of carbon into sugars.

• Bundle sheath cells receive the four-carbon compound from mesophyll cells and are the site where the Calvin cycle occurs.
• Within these cells, the compound releases carbon dioxide, which is then fixed again by the enzyme Rubisco during the Calvin cycle.

The spatial separation of the initial carbon fixation and the Calvin cycle in two different cell types reduces photorespiration, a process that can waste energy in plants. This characteristic is particularly beneficial for C4 plants in hot climates, helping them to maintain high rates of photosynthesis when compared to C3 plants.

Calvin Cycle

The Calvin Cycle, also known as the light-independent reactions or dark reactions, is central to the photosynthetic process in all plants, including C4 plants. This cycle occurs in the bundle sheath cells for C4 photosynthesis.

• The Calvin Cycle begins once carbon dioxide is released from the four-carbon compounds.
• Rubisco, an enzyme, plays a critical role by catalyzing the first major step of carbon fixation in the cycle.
• Through a series of reactions, carbon dioxide is eventually converted into glucose, a simple sugar that plants can use as energy.

This cycle does not directly require light but depends on ATP and NADPH produced during the light-dependent reactions of photosynthesis. In C4 plants, the Calvin Cycle is especially efficient because it occurs in bundle sheath cells where oxygen levels are kept low, minimizing the possibility of photorespiration. This efficiency allows C4 plants to adapt to varying environmental stresses better than C3 plants.