Cell (Plasma) Membrane- Structure, Composition, Functions


The cell (plasma) membrane is the thin layer of lipids and proteins that separates the inside of a cell from its surrounding environment. It is also called the cell surface membrane or plasmalemma. The cell membrane is essential for the life and function of every cell, as it regulates what enters and exits the cell, communicates with other cells and molecules, and participates in various biochemical reactions.

The cell membrane is composed of a phospholipid bilayer, which consists of two layers of phospholipids arranged with their hydrophilic (water-loving) heads facing the aqueous environments inside and outside the cell, and their hydrophobic (water-fearing) tails facing each other in the middle. The phospholipid bilayer forms a fluid and flexible barrier that allows the cell to maintain its shape and integrity.

Embedded within the phospholipid bilayer are various proteins that perform different functions for the cell. Some proteins act as channels or transporters that facilitate the movement of specific substances across the membrane. Some proteins act as receptors that bind to hormones, neurotransmitters, or other signaling molecules and trigger a response in the cell. Some proteins act as enzymes that catalyze chemical reactions on or near the membrane. Some proteins act as structural components that link the membrane to the cytoskeleton or the extracellular matrix.

In addition to lipids and proteins, the cell membrane also contains carbohydrates, which are attached to either lipids or proteins. These carbohydrates form a layer called the glycocalyx, which covers the outer surface of the membrane and serves as a recognition and protection system for the cell. The glycocalyx helps the cell to identify self from non-self, to adhere to other cells or surfaces, and to resist mechanical damage or infection.

The cell membrane is not a static or uniform structure, but rather a dynamic and diverse mosaic of molecules that constantly move and change. The fluidity and diversity of the membrane allow it to adapt to different conditions and stimuli, and to perform various functions for the cell. The cell membrane is therefore often described by the fluid mosaic model, which was proposed by S.J. Singer and G.L. Nicolson in 1972.