The fluid mosaic model was proposed by S.J. Singer and Garth L. Nicolson. This model explains the structure of the plasma membrane of animal cells as a mosaic of components such as phospholipids, proteins, cholesterol, and carbohydrates. These components give a fluid character to the membranes.
Each phospholipid has a hydrophilic head pointing outside and a hydrophobic tail forming the inside of the bilayer.
Cholesterol and proteins are embedded in the bilayer that gives the membrane a mosaic look. Each component has a specific function to perform.
Also Read: Cell membrane and plasma membrane
Components of Plasma Membrane
|Phospholipid||The main fabric of plasma membrane|
|Cholesterol||Between phospholipids and phospholipid bilayers|
|Integral proteins||Embedded within phospholipid layers|
|Peripheral proteins||Inner or outer surface of the phospholipid bilayer|
|Carbohydrates||Attached to proteins on outside membrane layers|
Phospholipids are amphipathic molecules with a hydrophilic head and a hydrophobic tail. These are attached to a glycerol molecule by a covalent bond.
It helps the plasma membrane to retain the fluidity. It is present between the phospholipids and prevents the compaction of hydrophilic tails at low temperatures and their expansion at high temperatures.
The plasma membrane has three types of proteins:
- Integral Proteins: These proteins form channels to allow the movement of large molecules and ions across the hydrophobic layer of the membrane.
- Peripheral Proteins: These are found embedded in a single leaflet of the membrane. They carry signals from one segment of the membrane and relay it to the another.
- Glycoproteins: They stabilize the membrane and are responsible for intercellular communication.
Factors Affecting Fluidity of Plasma Membrane
The fluidity of the cell membrane is influenced by three factors:
Phospholipids are found close together when it is cold. When it’s hot, they move apart.
The cholesterol molecules are randomly distributed along the phospholipid bilayer and hold it preventing it from separating too far, or compact too tightly.
Saturated and Unsaturated Fatty Acids
Fatty acids make up the phospholipid tails. Saturated fatty acid chains have a single bond between the carbon atoms whereas, unsaturated fatty acid chains have double bonds between the carbon atoms.
Double bonds make it harder for the chain to pack tightly by creating kinks. These kinks increase the fluidity of the membrane.
Also Read: Lipids
Restriction to Fluidity of Plasma Membrane
These are the lipid domains found on the external leaflet of the plasma membrane. Cholesterol, glycosphingolipids, glycosylphosphatidylinositol are the building blocks of lipid rafts.
Proteins and glycoproteins are diffused within the plasma membrane. These help in the transport of ions and metabolites, cell signalling, adhesion, and migration.
Key Points on Fluid Mosaic Model
- The plasma membrane comprises amphiphilic, phospholipid molecules.
- The second important component of the plasma membrane is integral proteins that are integrated completely into the membrane.
- Carbohydrates are found on the external surface of the membrane where they are bound to proteins or lipids.
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