Problem 106 - Objective Biology for NEET

Question

In \(\mathrm{C}_{4}\) plants, Calvin cycle occurs in (a) Stroma of bundle sheath chloroplast (b) Mesophyll chloroplast (c) Grana of bundle sheath chloroplast. (d) Does not occur as \(\mathrm{CO}_{2}\) is fixed mainly by PEP and no \(\mathrm{CO}_{2}\) is left for the Calvin cycle.

Step-by-Step Solution

Answer

The correct choice is (a) Stroma of bundle sheath chloroplast.

Step 1: Understanding C4 photosynthesis

In C4 plants, the process of photosynthesis is slightly different than that in C3 plants. In these plants, the first product of carbon fixation is a 4-carbon compound, hence the name C4 photosynthesis. The two types of photosynthetic cells in C4 plants are mesophyll and bundle sheath cells.

Step 2: Identifying the site of the Calvin cycle

The Calvin cycle, also known as the C3 photosynthetic pathway, is the part of photosynthesis where carbon dioxide is fixed by the enzyme Rubisco to form a 3-carbon compound. This process takes place in some specific part of the chloroplast.

Step 3: Matching the options with the concept

From the given options, we need to choose the one that correctly identifies the site of the Calvin cycle in C4 plants. From the understanding in step 1 and step 2, it can be concluded that Calvin cycle occurs in the stroma of bundle sheath chloroplast.

Key Concepts

Calvin Cycle

The Calvin cycle is a crucial part of photosynthesis, specifically in the light-independent reactions. It's where plants convert carbon dioxide ( CO₂ ) and other compounds into glucose. This process occurs in the stroma of the chloroplasts. In simpler terms, the stroma is the "soup" where all the enzymatic activities take place, away from the direct light. The cycle involves a series of biochemical reactions that utilize energy in the form of ATP and NADPH, which are produced during the light-dependent reactions of photosynthesis. Here are the main stages of the Calvin Cycle:

• Carbon Fixation: CO₂ is attached to a 5-carbon sugar named ribulose bisphosphate (RuBP) by the enzyme RuBisCO.
• Reduction Phase: ATP and NADPH are used to convert the 3-phosphoglycerate molecules into glyceraldehyde-3-phosphate.
• Regeneration Phase: Some glyceraldehyde-3-phosphate molecules go on to form glucose, while others are recycled to regenerate RuBP.

In C4 plants, the Calvin Cycle still takes place, but it mainly occurs in the bundle sheath cells, keeping most of the oxygen out and avoiding photorespiration.

Carbon Fixation

Carbon fixation is the process of converting inorganic carbon (in the form of CO₂ ) into organic compounds within plants. In C4 plants, carbon fixation happens differently than it does in C3 plants. The process starts in the mesophyll cells, where carbon dioxide is initially fixed into a four-carbon compound (oxaloacetate) by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) instead of RuBisCO. This is the primary step that distinguishes C4 photosynthesis.

• PEP carboxylase is important because it's more efficient than RuBisCO in capturing CO₂ and doesn't work with oxygen. This avoids the process known as photorespiration, which is wasteful.
• The 4-carbon compounds are then transported to the bundle sheath cells.
• Once in the bundle sheath cells, carbon is released as CO₂ again, which enters the Calvin Cycle.

This separation of steps is what enables C4 plants to photosynthesize efficiently in hot, sunny environments, where CO₂ concentration might otherwise be low.

Bundle Sheath Cells

Bundle sheath cells are a special feature of C4 plants that play a crucial role in their photosynthetic process. These cells form a tightly packed layer around the vascular bundles of plant leaves and stems.
They are integral to the C4 photosynthetic pathway, providing a unique environment for critical phases of photosynthesis to occur. Here’s how they function:

• Carbon Concentration: Bundle sheath cells help concentrate CO₂ for the Calvin Cycle, diminishing oxygen's presence and reducing photorespiration.
• Calvin Cycle Delivery: They receive the 4-carbon compounds from mesophyll cells. The compounds are then decarboxylated to release CO₂ .
• Energy Efficiency: Within these cells, the conditions protect the Calvin Cycle from potential disruptions due to oxygen, facilitating a more energy-efficient photosynthesis.

Efficient photosynthesis in bundle sheath cells makes C4 plants notably adaptive to stressful environments, like high temperatures and intense sunlight, which many plants would find challenging for photosynthesis.