Fluid mosaic model for the structure of plasma membrane explains

Each phospholipid molecule has a head that is attracted to water (hydrophilic: hydro = water; philic = loving) and a tail that repels water (hydrophobic: hydro = water; phobic = fearing). Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside; the hydrophobic tails form the inside of the bilayer.

Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), the plasma membrane forms a circle around each cell so that the water-loving heads are in contact with the fluid, and the water-fearing tails are protected on the inside.

The fluid-mosaic model of plasma membranes.

The molecules that are embedded in the plasma membrane also serve a purpose. For example, the cholesterol that is stuck in there makes the membrane more stable and prevents it from solidifying when your body temperature is low. (It keeps you from literally freezing when you’re “freezing.”) Carbohydrate chains attach to the outer surface of the plasma membrane on each cell. These carbohydrates are specific to every person, and they supply characteristics such as your blood type.

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.

Table of Contents

• Components of Plasma Membrane
• Factors Affecting Fluidity of Plasma Membrane
• Restriction to Fluidity of Plasma Membrane
• Key Points on Fluid Mosaic Model

Also Read: Cell membrane and plasma membrane

Components of Plasma Membrane

Phospholipids

Phospholipids are amphipathic molecules with a hydrophilic head and a hydrophobic tail. These are attached to a glycerol molecule by a covalent bond.

Cholesterol

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.

Proteins

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:

Temperature

Phospholipids are found close together when it is cold. When it’s hot, they move apart.

Cholesterol

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

The fluidity of plasma membrane is restricted due to:

Lipid Rafts

These are the lipid domains found on the external leaflet of the plasma membrane. Cholesterol, glycosphingolipids, glycosylphosphatidylinositol are the building blocks of lipid rafts.

Protein Complexes

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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