Which of the following changes in the cell sap of the guard cells is responsible for keeping stomata open during day time?

Options

A. Decrease in osmotic pressure but increase in turgor pressure
B. Increase in osmetic pressure but decrease in turgor pressure
C. Increase in both osmotic and turgor pressures
D. Decrease in both osmotic and turgor pressures

Answer

To determine which changes in the cell sap of the guard cells are responsible for keeping stomata open during the daytime, we can analyze the options provided:

### Step-by-Step Solution:

1. **Understanding Guard Cells**:
- Guard cells are specialized cells that surround the stomata (pores) on the leaf surface. Their primary function is to regulate the opening and closing of stomata, which controls gas exchange and water loss.

2. **Role of Potassium Ions**:
- During the daytime, when photosynthesis occurs, potassium ions (K+) are actively transported into the guard cells. This influx of potassium ions is crucial for the changes in osmotic pressure.

3. **Formation of Potassium Malate**:
- The increase in potassium ions leads to the formation of potassium malate in the vacuoles of guard cells. This compound contributes to the osmotic potential of the cell sap.

4. **Osmotic Pressure Increase**:
- As potassium malate accumulates, the osmotic concentration inside the guard cells increases. This causes water to move into the guard cells via osmosis.

5. **Turgor Pressure Increase**:
- The influx of water into the guard cells increases their turgor pressure. Turgor pressure is the pressure of the cell contents against the cell wall, which is essential for maintaining cell structure and function.

6. **Stomata Opening**:
- The increase in turgor pressure causes the guard cells to swell and bend, resulting in the opening of the stomata. This allows for gas exchange (CO2 in, O2 out) necessary for photosynthesis.

7. **Evaluating the Options**:
- **Option 1**: Decrease in osmotic pressure but increase in turgor pressure - Incorrect, as stomata cannot open with decreased osmotic pressure.
- **Option 2**: Increase in osmotic pressure but decrease in turgor pressure - Incorrect, as stomata require increased turgor pressure to open.
- **Option 3**: Increase in both osmotic and turgor pressures - Correct, as both are necessary for stomata to open.
- **Option 4**: Decrease in both osmotic and turgor pressures - Incorrect, as this would lead to stomata closing.

8. **Conclusion**:
- The correct answer is **increase in both osmotic and turgor pressures**, which is responsible for keeping stomata open during the daytime.