Branched chain lipids present in plasma membrane of Archaebacteria :

Options

A. Increases membrane fluidity and increase tolerance to extremes of heat as well as low `pH`
B. Decrease membrane fluidity and increase tolerance to extremes of heat as well as high `pH`
C. Stabilize membrane fluidity and increase tolerance to extremes of heat as well as low `pH`
D. Decrease tolerance to extremes of heat as well as low `pH`

Answer

### Step-by-Step Solution:

1. **Understanding the Structure of Archaebacterial Membranes**:
- Archaebacteria possess unique branched chain lipids in their plasma membranes, which differ from the unbranched chain lipids found in other bacteria. This structural difference is crucial for their survival in extreme environments.

2. **Composition of Archaebacterial Membranes**:
- The lipids in the membranes of archaebacteria are composed of hydrocarbon chains that are connected to glycerol via ether linkages. This is a distinctive feature that contributes to their stability.

3. **Function of Branched Chain Lipids**:
- The branched structure of these lipids plays a significant role in maintaining the integrity of the plasma membrane, especially under extreme conditions such as high temperatures. The branched chains help prevent the membrane from breaking down, which is essential for the survival of archaebacteria in harsh environments.

4. **Effects of Temperature and pH**:
- The branched chain lipids decrease membrane fluidity, which is beneficial at high temperatures. If the membrane were too fluid, it would compromise the cell's integrity. Additionally, archaebacteria can tolerate high pH levels, which is another aspect of their adaptability.

5. **Identifying the Correct Statement**:
- Based on the characteristics of the branched chain lipids, we can evaluate the provided options:
- The first option suggests increased membrane fluidity, which is incorrect as it would not support survival at high temperatures.
- The second option states decreased membrane fluidity and increased tolerance to extreme heat and high pH, which aligns with the properties of archaebacterial membranes.
- The third and fourth options are incorrect based on the same reasoning.

6. **Conclusion**:
- Therefore, the correct statement regarding the branched chain lipids present in the plasma membrane of archaebacteria is: "Decreased membrane fluidity and increased tolerance to extreme heat as well as high pH."