Membranes fluidity

Options

A. decreases with rise in temperature
B. decreases with lowering of temperature
C. increases with lowering of temperature
D. no effect observed w.r.t. temperature

Answer

### Step-by-Step Solution:

1. **Understanding Membrane Composition**:
- Biological membranes are primarily composed of a lipid bilayer, which includes phospholipids with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. Proteins are also embedded within this lipid bilayer.

2. **Defining Membrane Fluidity**:
- Membrane fluidity refers to the viscosity of the lipid bilayer of a cell membrane, which affects the movement of proteins and lipids within the layer. A more fluid membrane allows for greater mobility of its components.

3. **Effect of Temperature on Membrane Fluidity**:
- As temperature increases, the kinetic energy of the lipid molecules also increases, leading to more movement and a more fluid membrane. Conversely, as temperature decreases, the kinetic energy decreases, leading to less movement and a more rigid membrane.

4. **Role of Lipid Saturation**:
- The presence of unsaturated lipids (lipids with double bonds in their fatty acid chains) increases membrane fluidity. This is because the kinks in the unsaturated fatty acids prevent the lipids from packing closely together, allowing for more movement.

5. **Conclusion on Fluidity and Temperature**:
- Therefore, the correct statement about membrane fluidity is that it **decreases with the lowering of temperature**. This means that as the temperature drops, the membrane becomes less fluid and more rigid.

### Final Answer:
- The correct option is: **Decreases with the lowering of temperature.**