Q.2 Structure of cell membrane and its functions.

The structure of a cell membrane is a phospholipid bilayer with protein molecules embedded in it. It is called fluid mosaic model and was put forward by the scientists Singer and Nicolson. It is also called Singer and Nicolson or Fluid mosaic model of cell membrane.

The phospholipid bilayer consists of two layers of phosphate head and lipid tail. The phosphate head is hydrophilic and lipid tail is hydrophobic. The two layers are aligned in such a way that the lipid ends face each other and phosphate end are in opposite direction. The phospholipid bilayer is fluidic allowing the cell to undergo considerable changes in the shape without disruption of structural integrity. The globular proteins float in the fluid phospholipid bilayer giving a fluid mosaic pattern.

Proteins are arranged in the cell membrane in two forms

1. Lipoproteins: They are proteins containing lipids and function as enzymes and ion channels
2. Glycoproteins: They are proteins containing carbohydrates which function as hormones and neurotransmitters.

Based on the arrangement proteins on the cell membrane are of two types

Peripheral proteins
They are attached to the cell membrane inside the cell or outside the cell. They function as enzymes and cell adhesion molecules.

Integral proteins
They are embedded in the cell membrane passing through and through it. They function as channels, carriers, receptors, antigens and pumps.

The carbohydrates are attached to the outer surface of the cell membrane and forms a thin layer known as glycocalyx. They functions as receptors and help in tight fixation of cell to neighboring cells.

Functions of cell membrane

1. The cell membrane forms and outer covering of the cell and hence protect the cell from outer environment.
2. The cell membrane being fluidic help the cell to change its size and shape without rupturing.
3. The proteins in the cell membrane act as channels which transport various ions in and out of the cell. Eg: Sodium channel, Potassium channel.
4. The proteins in the cell membrane act as carrier proteins such as GLUT which transports glucose into the cell.
5. The proteins in the cell membrane act as receptors for many hormones through which they exert their actions.
6. The proteins in the cell membrane act as antigens. Eg: Antigens on RBC cell membrane.
7. The proteins in the cell membrane act as pumps such as Na-K Pump which maintains the resting membrane potential of the cell.
8. The carbohydrates of the cell membrane being negatively charged prevents negatively charge particles from exiting the cell.
9. The carbohydrates of the cell membrane also act as receptors for various hormones.
10. The carbohydrates on the cell membrane helps in tight fixation of the cell to another cell or the basal lamina.