Question

Explain why the maintenance of a high concentration of \(\mathrm{CO}_{2}\) in the bundle-sheath cells of \(\mathrm{C}_{4}\) plants is an example of active transport. How much ATP is required per molecule of \(\mathrm{CO}_{2}\) to maintain a high \(\mathrm{CO}_{2}\) concentration?

Short answer

C4 plants use 2 ATP per CO2 molecule for active transport in concentrating CO2 in bundle-sheath cells.

Step-by-step solution

Understanding Active Transport in C4 Plants

In C4 plants, carbon dioxide is initially fixed into a four-carbon compound in mesophyll cells. This compound is then transported to bundle-sheath cells where the CO2 is released for use in the Calvin cycle. Active transport refers to the movement of substances against a concentration gradient and requires energy. In this context, it involves the transportation of four-carbon compounds (such as malate or aspartate) from mesophyll cells to bundle-sheath cells.

Role of ATP in C4 Photosynthesis

In the C4 photosynthesis process, ATP is used for the regeneration of phosphoenolpyruvate (PEP) from pyruvate in mesophyll cells after the release of CO2 in bundle-sheath cells. This step is crucial as PEP is used to capture more CO2, forming oxaloacetate again, which can continue through the cycle.

Calculating ATP Usage per CO2 Molecule

For each CO2 molecule transferred to the bundle-sheath cells, the process of regenerating PEP from pyruvate costs 2 ATP molecules. This use of ATP is essential to continue capturing and concentrating CO2 in the bundle-sheath cells, thus enabling the plant to efficiently carry out photosynthesis even under conditions where CO2 is limited.

Key concepts

ATP usage in C4 photosynthesis

In C4 photosynthesis, ATP plays a critical role in creating an efficient system for capturing and concentrating carbon dioxide. The process begins in the mesophyll cells, where CO2 is initially fixed into a four-carbon compound, such as oxaloacetate, by the enzyme phosphoenolpyruvate carboxylase (PEPC). This compound is later converted into another four-carbon compound, like malate or aspartate, which is shuttled to the bundle-sheath cells.

Once in the bundle-sheath cells, the four-carbon compounds release CO2 for use in the Calvin cycle. This release results in the formation of pyruvate, which is then transported back to the mesophyll cells. Here, ATP is crucial as it provides the energy needed to regenerate phosphoenolpyruvate (PEP) from pyruvate. This regeneration is essential because PEP is needed to fix another molecule of CO2, allowing the cycle to continue.

For each molecule of CO2 that is transferred and concentrated in the bundle-sheath cells, the process consumes 2 ATP molecules. This significant investment in energy highlights the adaptability of C4 plants to efficiently carry out photosynthesis in environments where CO2 may be limited.

Role of bundle-sheath cells

Bundle-sheath cells are specialized cells that play an essential role in the overall process of C4 photosynthesis. These cells are tightly packed around the leaf veins, creating a unique environment where CO2 concentration can be optimized. This specialization helps C4 plants maintain high levels of photosynthesis efficiency, especially in hot, dry climates.

In C4 plants, the initial fixation of CO2 occurs in the mesophyll cells, producing four-carbon compounds that are then transported to the bundle-sheath cells. These cells act as a chamber, where CO2 is released from the four-carbon compounds and taken up by the Calvin cycle to synthesize sugars. The Calvin cycle is the same process used in C3 plants but takes advantage of the increased CO2 concentration.

• The tight packing of bundle-sheath cells prevents the leakage of CO2, keeping it concentrated and making photosynthesis more efficient.
• Bundle-sheath cells help bypass the photorespiration issue, which is a problem in C3 plants where oxygen interferes with the Calvin cycle.

This arrangement allows C4 plants to photosynthesize effectively under conditions where other plants might struggle.

Carbon dioxide fixation in C4 plants

Carbon dioxide fixation in C4 plants involves a unique mechanism that allows these plants to thrive in environments with low atmospheric CO2. Unlike C3 plants, which fix CO2 directly through the Calvin cycle, C4 plants first fix CO2 into a four-carbon compound in the mesophyll cells. The enzyme responsible for this initial fixation is phosphoenolpyruvate carboxylase (PEPC), which has a higher affinity for CO2 than the typical Calvin cycle enzyme rubisco.

The initial fixation step occurs in the mesophyll cells, where CO2 is combined with phosphoenolpyruvate (PEP) to form oxaloacetate. This oxaloacetate is then converted to malate or aspartate and transported to the bundle-sheath cells.

In these compartmentalized cells, CO2 is released from malate or aspartate and enters the Calvin cycle to produce sugars, using the energy carriers ATP and NADPH. This strategy effectively decouples the initial CO2 fixation from the Calvin cycle's sugar production by limiting oxygen's inhibitory effects on rubisco.

• The separation of these processes into different cell types helps avoid photorespiration, thereby enhancing water and nitrogen use efficiency in C4 plants.
• This separation is critical for enabling C4 plants to maintain high rates of photosynthesis even in stressful environmental conditions.

By utilizing this method of carbon fixation, C4 plants manage to maintain productivity under conditions that would typically not be favorable for standard C3 photosynthesis.