Question

Which of the following cells of \(\mathrm{C}_{4}\) plants are prominently loaded with starch? (a) Epidermal cells (b) Mesophyll cells (c) Bundle sheath cells (d) All of these

Short answer

The bundle sheath cells of C4 plants are prominently loaded with starch.

Step-by-step solution

Understand the role of different cells

In C4 plants, photosynthesis takes place in two types of cells: mesophyll cells and bundle sheath cells. The epidermal cells are the outer protective layer of cells in plants, and they are not involved in the process.

Recall starch storage in C4 plants

The key detail to recall here is that, in C4 plants, the mesophyll cells initially fix the carbon dioxide into a four-carbon compound (hence the name 'C4'). This compound is then transported to the bundle sheath cells where they are broken down into carbon dioxide and a three-carbon compound. The carbon dioxide is then used in the Calvin cycle to produce sugars, while the three-carbon compound is sent back to the mesophyll cells to be recycled. The sugars produced in the Calvin Cycle can be stored as starch in the bundle sheath cells.

Select the correct answer

Based on the information from Step 2, we can conclude that the bundle sheath cells in C4 plants are prominently loaded with starch, not the mesophyll or epidermal cells.

Key concepts

Photosynthesis

Photosynthesis is the process through which plants convert light energy into chemical energy stored as sugars. This complex process occurs in two main stages: the light-dependent reactions and the Calvin cycle.
During the light-dependent reactions, chlorophyll within the leaf cells absorbs sunlight, and this energy is used to convert water and carbon dioxide into oxygen and energy-rich compounds like ATP and NADPH. These molecules are then utilized in the Calvin cycle to synthesize sugars from carbon dioxide.
C4 plants, such as corn and sugarcane, have adapted a unique process where photosynthesis occurs in two different types of cells to efficiently capture carbon dioxide and minimize photorespiration.

• In mesophyll cells, carbon dioxide is captured and initially fixed into a four-carbon compound.
• This compound is then transported to bundle sheath cells, where carbon dioxide is released and re-assimilated in the Calvin cycle to produce glucose.

Understanding this division and specialization of photosynthesis highlights the efficiency of C4 plants in hot and sunny environments.

Bundle Sheath Cells

Bundle sheath cells play a crucial role in C4 plant photosynthesis. They are a layer of cells in plant leaves and stems that wrap around the vascular bundles.
In C4 plants, these cells are important because they house the Calvin cycle, where carbon dioxide is ultimately fixed into sugars. The earlier fixed four-carbon compound is transported into these cells where it decomposes, releasing carbon dioxide directly to the Calvin cycle.
This spatial separation minimizes photorespiration, increasing photosynthetic efficiency under high light and temperature conditions.

• Bundle sheath cells protect the Calvin cycle from the high oxygen concentrations in the mesophyll.
• The carbohydrates generated in the Calvin cycle can be temporarily stored here as starch, which is quite vital for balancing energy during periods when photosynthesis is less efficient.

Thus, the accumulation of starch seen in these cells makes them a key focus in understanding C4 plant adaptations.

Starch Storage

Starch storage in C4 plants is an important survival mechanism. In the context of photosynthesis, starch serves as a way to store the sugars produced so they can be used later when needed.
C4 plants store most of their starch in the bundle sheath cells, which helps them manage energy and carbon flow efficiently.

• When surplus energy is generated, it's converted into starch, serving as an energy reserve.
• During times of low sunlight or increased energy demand, starch is broken down to release glucose for continued metabolic activities.

This starch storage system ensures that energy is available consistently, supporting growth and development even under fluctuating environmental conditions.

Mesophyll Cells

Mesophyll cells are an essential component of the leaf structure in C4 plants. These cells are located just beneath the epidermal layer and are rich in chloroplasts, the organelles where photosynthesis initially begins.
In C4 plants, mesophyll cells have the specialized task of capturing carbon dioxide and fixing it into a four-carbon compound using the enzyme PEP carboxylase.
This compound is then transported to the bundle sheath cells, bypassing the space where high oxygen concentration might impede photosynthesis through a process known as photorespiration.

• They serve as the initial site of carbon dioxide uptake and concentration.
• Mesophyll cells help maintain a high concentration of carbon dioxide for the Calvin cycle in bundle sheath cells.

The unique collaboration between mesophyll and bundle sheath cells is what makes C4 photosynthesis so effective, especially in hot and arid environments.