Question

1. Discuss how and why a plasma membrane lipid bilayer is fluid and what types of physical properties of the lipids contribute to the fluidity of the membrane. This should include a discussion of the different types of fatty acids and other lipids and environmental interactions that can change fluidity.

Answer 1

The fluid mosaic model of the plasma membrane lipid bilayer describes the plasma membrane as a fluid combination of phospholipids, cholesterol and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward facing surface of the membrane. This composition of phospholipids, cholesterol, proteins and carbohydrates is said to be the reason for fluidic character of the plasma membrane.

Why plasma membrane needs to be fluidic is to carry out the below functions.

1) It allows membrane proteins rapidly in the plane of bilayer.

2) It permits membrane lipids and proteins to diffuse from sites where they are inserted into bilayer after their synthesis.

3) It enables membranes to fuse with one another and mix their molecules.

The different types of fatty acids found in plasma membrane are oleic acid, vaccenic acid, palmitoleic acid, palmitic acid and myristic acid, the two major types of lipids are phospholipids and sterols in plasma membrane.

Some of the environmental interactions which affects the fluidity are changes in ambient temperature and osmolarity.

Temperature:

As temperature increases, so does phospholipid bilayer fluidity. At lower temperatures, phospholipids in the bilayer do not have as much kinetic energy and they cluster together more closely, increasing intermolecular interactions and decreasing membrane fluidity. At high temperatures the opposite process occurs, phospholipids have enough kinetic energy to overcome the intermolecular forces holding the membrane together, which increases membrane fluidity.