Each cell is surrounded by a membrane called Plasma membrane. From unicellular microorganism to complex multicellular organisms, Cell Membrane is the basic cover for each cell. Though the Prokaryotic and Eukaryotic both type of cells have quite similar plasma membrane but there are some different in composition of the membrane.
Discovery of Cell Membrane
The detail structure of the plasma membrane was studied only after the advent of the electron microscope in 1950. Many scientist give various model and hypothesis to describe the plasma membrane. The most widely accepted theory of plasma membrane is “Fluid Mosaic Model“ given by Singer and Nicolson in 1972.
Composition of Cell Membrane
The plasma membrane is mainly composed of lipids and proteins. It also contain some carbohydrate and cholesterol to perform specific function.
Lipids
The major lipids of the plasma membrane are Phospholipids that are arranged in a bilayer. The major phospholipids are Phosphatidyl choline and Phosphatidyl ethanolamine which are amphipathic in nature.
Proteins
Along with phospholipids, the plasma membrane contain many protein.
- Some protein are bound tightly with the surrounding lipids within the membrane, they are not easily dislodged from the membrane. These type of proteins are called Integral protein.
- Some other proteins lie at the outer or inner surface of the membrane, these are called Peripheral protein.
- The membrane proteins that allow the passage of ions across the plasma membrane. These are known as Ion channels.
- The membrane proteins transport substances along their electrochemical gradient without expenditure of energy are termed as Carrier protein.
- Some special proteins that transport substances across the membrane against the electrochemical gradient with expenditure of energy. These protein structures are called Pump.
Carbohydrates
The cell membrane contains carbohydrate like simple sugar, oligossacharides to compound sugar, glycoprotein which are used as signaling molecules and receptors.
Cholesterol
The plasma membrane of the animal cell contain cholesterol for maintaining stability. The cholesterol holds the phospholipids together, restricting movement across the movement. Cholesterol is not present in bacterial plasma membrane.
Property of Cell Membrane
Polarity :
As the phospholipids are amphipathic in nature, their ‘phosphate’ portion forms the polar, hydrophilic end directed towards the surface of the membrane. In contrast, the lipid portion containing two fatty acid chains from the nonpolar, hydrophobic end.
Permeability :
Being contain the hydrophobic-hydrophilic moiety, various receptors and voltage or ligand gated channel, the plasma membrane is selectively permeable in nature. It influx or efflux various substance as per cellular requirement and the condition of external environment.
Fluidity :
The quasi-fluid nature of lipids enables lateral movement of lipid within the overall bilayer.
- The lipids are not only rotate but also move laterally throughout the membrane. Transfer of lipid from one side of bilayer to another side is called Flip-flop movement.
- The protein can only move laterally. They can not show flip-flop movement.
Functions of Cell Membrane
The main function of the plasma membrane is to cover the protoplam. Other special functions are —
- regulate transport of substances into and out of the cell i.e, Cellular transport in various means like Uniport, Coport (Symport and Antiport).
- connect adjacent cell through membrane junctions like Tight junctions, Adhering junctions and Gap junctions.
- receiving signals from chemical messengers like hormones, neurotransmitter etc. through receptors.
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