Guard cell regulate

Options

A. Respiration
B. Transpiration
C. Photosynthesis
D. Photoerspiration

Answer

### Step-by-Step Solution:

1. **Understanding Guard Cells**:
- Guard cells are specialized cells that surround the stomata (small openings on the surface of leaves). Their main function is to regulate the opening and closing of these stomatal pores.

2. **Identifying the Functions of Guard Cells**:
- The primary role of guard cells is to control the rate of transpiration, which is the process of water vapor loss from the plant to the atmosphere. This is crucial for maintaining water balance and gas exchange in plants.

3. **Differentiating Between Options**:
- **Respiration**: This is the process by which plants convert glucose and oxygen into energy, but it is not directly regulated by guard cells.
- **Photosynthesis**: This is the process of converting light energy into chemical energy (food), which occurs in chloroplasts and is not directly regulated by guard cells.
- **Photorespiration**: This is a process that occurs in some plants where oxygen is used instead of carbon dioxide, leading to a waste of energy and resources. It is not the primary function of guard cells.
- **Transpiration**: This is the loss of water vapor from plant surfaces, primarily through the stomata, and is directly regulated by the opening and closing of guard cells.

4. **Conclusion**:
- Based on the functions of guard cells and the definitions of the processes, the correct answer is that guard cells regulate **transpiration**.

### Final Answer:
Guard cells regulate **transpiration**.

---