Question

Photorespiration is avoided by \(\mathrm{C}_{4}\) plants because a. these plants separate the formation of a four-carbon molecule from the rest of the Calvin cycle in different cells. b. these plants carry out only anaerobic respiration. c. the enzyme PEP carboxylase functions to maintain high \(\mathrm{CO}_{2}\) concentrations in the bundle-sheath cells. d. all of the above. e. a and c only.

Short answer

The correct answer is e. a and c only.

Step-by-step solution

Understand the process in C4 plants

C4 plants have developed a special mechanism to avoid photorespiration. They initially fix carbon dioxide into a four carbon molecule in the mesophyll cells, separate from the rest of the Calvin cycle which occurs in the bundle-sheath cells. This process is facilitated by an enzyme called PEP carboxylase.

Analyze the statements

Statement a accurately describes the mechanism of carbon fixation in C4 plants, and statement c correctly mentions the role of PEP carboxylase. However, statement b is incorrect, because C4 plants, just like other plants, do carry out aerobic respiration.

Conclude based on analysis

The statements a and c accurately describe how photorespiration is avoided by C4 plants, but statement b does not. Therefore, the correct choice is e - a and c only.

Key concepts

C4 plants

C4 plants have evolved a unique mechanism to minimize the process of photorespiration, improving their efficiency in photosynthesis. Photorespiration is a wasteful pathway that occurs when the enzyme rubisco reacts with oxygen instead of carbon dioxide. C4 plants, such as corn and sugarcane, avoid photorespiration by spatially separating the steps of photosynthesis. They initially fix carbon dioxide into a four-carbon compound in the mesophyll cells. This compound is then transferred to bundle-sheath cells, where the Calvin cycle occurs.

By separating these steps into different cell types, C4 plants maintain a high concentration of carbon dioxide in the bundle-sheath cells. This separation reduces the chance of rubisco interacting with oxygen, thus minimizing photorespiration. As a result, C4 plants are more efficient in hot and dry environments, where the concentration of oxygen is typically higher.

PEP Carboxylase

PEP carboxylase is an essential enzyme in the C4 photosynthetic pathway. It plays a critical role in the initial fixation of carbon dioxide. Unlike rubisco, PEP carboxylase has no affinity for oxygen, making it efficient for minimizing photorespiration.

The enzyme facilitates the reaction that combines phosphoenolpyruvate (PEP) with carbon dioxide to form oxaloacetate, a four-carbon compound. This reaction occurs in the mesophyll cells. By effectively capturing carbon dioxide, PEP carboxylase ensures that the Calvin cycle in the bundle-sheath cells has a concentrated supply of carbon dioxide.

• PEP carboxylase works in low carbon dioxide conditions.
• Keeps oxygen levels from interfering with carbon fixation.
• Enables C4 plants to thrive in adverse environments.

Calvin Cycle

The Calvin cycle, also known as the light-independent reactions, is a crucial component of photosynthesis. It takes place in the chloroplasts of plant cells, converting carbon dioxide into glucose. In the case of C4 plants, this cycle occurs in the bundle-sheath cells, which are anatomically distinct from the mesophyll cells where initial carbon fixation happens.

During the Calvin cycle, carbon dioxide is assimilated into organic molecules through a series of reactions. This process relies heavily on the enzyme rubisco, which catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP). The separation of C4 pathways from the Calvin cycle in specific cell types helps prevent oxygen from interfering with rubisco's function, significantly reducing photorespiration. As a result, the Calvin cycle in C4 plants operates with higher efficiency than in C3 plants.

Carbon Fixation

Carbon fixation is a fundamental process in photosynthesis where atmospheric carbon dioxide is converted into organic compounds. This process marks the beginning of carbon assimilation in plants. In C4 plants, carbon fixation is split into two stages:

1. **Initial Carbon Fixation in Mesophyll Cells:** The enzyme PEP carboxylase captures carbon dioxide and forms a four-carbon compound, oxaloacetate. This step helps in concentrating carbon dioxide and avoids photorespiration.

2. **Conversion and Use in Bundle-Sheath Cells:** The four-carbon compound is transported to bundle-sheath cells, where carbon dioxide is released and enters the Calvin cycle. This two-step process is highly efficient in maintaining a high carbon dioxide concentration around rubisco.

• Prevents the wasteful process of photorespiration.
• Improves photosynthetic efficiency under high light intensity and temperature.
• Enables agricultural productivity in warmer climates.

By understanding carbon fixation's role in photosynthesis, especially in C4 plants, we gain insight into how plants adapt to diverse environmental conditions and enhance photosynthetic output.