Plasma Membrane Structure

1.0Definition of Plasma Membrane

The plasma membrane is a biological membrane that separates and protects the interior of all cells from the outside environment. It regulates the passage of materials into and out of the cell and is crucial for maintaining homeostasis..Structurally the cell` membrane of prokaryotes is similar to the eukaryotes.

2.0Functions of the Plasma Membrane:

The plasma membrane serves several essential functions:

• Selective Permeability: It controls the movement of substances in and out of the cell.
• Protection: It acts as a barrier to protect the cell from harmful substances.
• Cell Communication: It contains receptors for external signals like hormones.
• Structural Support: The membrane helps maintain the shape of the cell.
• Cell Recognition: It helps identify cells for immune responses.
• Endocytosis and Exocytosis: The processes of engulfing and secreting materials.

3.0Composition of the Plasma Membrane:

The plasma membrane is composed of:

• Phospholipids: Form the bilayer.
• Proteins: Integral and peripheral proteins, involved in transport, signaling, and structural support.
• Cholesterol: Provides rigidity and stability.
• Carbohydrates: Form glycoproteins and glycolipids for recognition and communication.
• The percentage of components  of plasma membrane vary from cell to cell.In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids.

4.0What is the Structure of the Plasma Membrane:

(A). Sandwich model of Plasma Membrane:- Proposed by Davson and Danielli.

 The sandwich model suggests that phospholipid bilayers form structures with hydrophilic heads facing outward and hydrophobic tails inward.The hydrophilic ends of fatty acid covered by protein layer thus arrangement of protein and lipid layer remain in P-L-L-P form This model has been replaced by the fluid mosaic model.

(B). Fluid mosaic model:-

Proposed by Singer & Nicolson (1972)

The fluid mosaic model describes the plasma membrane as consisting of a phospholipid bilayer, embedded proteins,peripheral protein, cholesterol, and carbohydrates. This model emphasizes the quasifluid nature of the membrane and structure of plasma membrane.

1. Lipids:-

Phospholipid is the main component of cell membrane because it

forms a continuous structural frame of the cell membrane. The phospholipid layer provides fluidity to the plasma membrane because phospholipids are rich in unsaturated fatty acid.

Cholesterol is also present in plasma membranes. Cholesterol is more rigid than phospholipid. So it helps in stability of membrane structure.

• The Quasifluid nature of lipids enable lateral movement of protein within the overall bilayer.

This ability to move within the membrane is measured as its fluidity.

2. Proteins:-

Two types of protein are present in plasma membrane. (On the basis of ease of extraction)

1. Extrinsic or peripheral protein :-

These are superficially arranged on the surface of the lipid layer and can be separated easily. These proteins have enzymatic activity.

2. Intrinsic or integral protein :-

These proteins are tightly binds with phospholipid. Thus, they can not be easily removed from the membrane.

i. Partially buried – Incompletely buried through the thickness of the membrane.

ii. Totally buried – Totally buried through the complete thickness of membrane.

(Carrier Proteins and Tunnel Proteins.)

(c) Carbohydrates (sugar):-

Oligosaccharides (sugar), the glycolipids & glycoproteins on the outer surface of plasma membranes are involved in cell to cell recognition.

Best example of cell recognition is “Fertilization (where sperm & egg recognize each other)” and “Blood group Antigens”.