The Fluidity of a Phospholipid Membrane

Phospholipids are the main components of cell membranes and play a crucial role in maintaining the integrity and fluidity of the membrane. The fluidity of a phospholipid membrane refers to the ability of its individual phospholipids to move within the membrane.

The fluidity of a phospholipid membrane is influenced by several factors, including the length and saturation of the fatty acid chains that make up the phospholipids.

Length and Degree of Unsaturation

The length of the fatty acid chains refers to the number of carbon atoms in the chain, while the degree of unsaturation refers to the presence of double bonds in the chain.

Chain Length

- Shorter chain length: Phospholipids with shorter fatty acid chains have a lower melting point and are more fluid. This is because shorter chains have less surface area and fewer van der Waals forces between adjacent phospholipids, allowing for more movement within the membrane.
- Longer chain length: Phospholipids with longer fatty acid chains have a higher melting point and are less fluid. Longer chains have more surface area and stronger van der Waals forces, restricting the movement of phospholipids within the membrane.

Degree of Unsaturation

- Decreased degree of unsaturation: Phospholipids with saturated fatty acid chains have a higher melting point and are less fluid. This is because saturated fatty acids have straight chains, allowing for close packing and stronger van der Waals forces.
- Increased degree of unsaturation: Phospholipids with unsaturated fatty acid chains have a lower melting point and are more fluid. Unsaturated fatty acids have kinks or bends at the sites of double bonds, preventing close packing and reducing the strength of van der Waals forces.

Explanation of the Correct Answer (Option B)

According to the correct answer, the fluidity of a phospholipid membrane increases when the chain length decreases and the degree of unsaturation increases.

This means that shorter fatty acid chains and unsaturated fatty acids contribute to a more fluid membrane. Shorter chains and unsaturated fatty acids result in less surface area, fewer strong van der Waals forces, and more movement within the membrane.

On the other hand, longer chain lengths and saturated fatty acids lead to a less fluid membrane. Longer chains and saturated fatty acids have more surface area, stronger van der Waals forces, and restricted movement within the membrane.

Therefore, the correct answer (Option B) accurately explains that the fluidity of a phospholipid membrane increases when the fatty acid chain length decreases and the degree of unsaturation increases.