Cell membrane: Structure and Functions

October 4, 2021

It is a protective outer covering sheath that envelopes the cell body.
It is known by other name called plasma lemma and plasma membrane.
It separates extracellular fluid (ECF) which is outside the cell and intracellular fluid that is inside the cell.
It is a semi permeable membrane as it allows exchange of certain substances inside and outside the cell i.e., between ECF and ICF.
It is composed of 55% protein 40% lipids and 5% carbohydrates.

Structure of Cell Membrane

Cell membrane is a unit membrane or a three layered membrane on the basis of its structure.
The three layer of cell membrane is revealed by electron microscope where one layer is central electron-lucent layer and two electron-dense layers.
The two electron dense layers are present one on either side of central layer.
The central layer is formed by lipid substances so it is a lipid layer and the other two layers are formed by proteins so they are protein layers.
It also contains some carbohydrates molecule

Structure model of cell membrane

There are various model of cell membrane. Some of the models are explained here below.

A) Danielli-Davson model

It is one of the basic model of membrane structure proposed at first by James F Danielli and Hugh Davson in 1935 which was accepted for many years by various scientists.
This model was basically a ‘sandwich of lipids’ covered by proteins on both sides.

Image Source: Microbenotes

B) Unit membrane model

This model was proposed by JD Robertson in 1957 on the basis of electron microscopic studies.
This model replaced the model given by Danielli and Davson.

C) Fluid mosaic model

This model was proposed by SJ Singer and GL Nicholson in 1972 as ‘Fluid mosaic model’ where the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement of different colored pieces of stone, tile, glass or other such materials).
This is the most popular model which is accepted by all scientists till now.
According to this model, the proteins are found to float in the lipid layer instead of forming the layers of the sandwich-type model.

Lipid layers of the Cell Membrane

Lipid layer which lies centrally is a bi-layered structure formed by thin film of lipids.
Due to the fluid nature of lipid layer, the portions of the membrane move from one point to another point along the surface of the cell.
The materials content of the lipid layer also move to all areas of the cell membrane.
Phospholipids and cholesterol are two major lipids found in cell membrane.

i) Phospholipids

Phospholipids are those lipid substances containing phosphorus and fatty acids.
Some major phospholipids present in lipid layer of cell membrane are aminophospholipids, sphingomyelins, phosphatidylcholine, phsophatidyletholamine, phosphatidylglycerol, phosphatidylserine and phosphatidylinositol etc.
The molecules of phospholipids are arranged in two layers each molecule of which resemble headed pin in shape.
Head portion is the outer part of phospholipid molecule whereas tail portion is the inner one.
Head portion is soluble in water and is hydrophilic i.e. has strong affinity for water so it is the polar end.
Tail portion is insoluble in water and is hydrophobic i.e. gets repelled by water and hence is the non-polar end.
The arrangement of the two layers of phospholipids is such that the hydrophobic tail portions meet in the centre of the membrane whereas head portion of outer layer face the ECF and those of the inner layer face ICF.

ii) Cholesterol

Between the phospholipid molecules, the molecules of cholesterol are arranged.
Due to the softness and oily structures of phospholipids, they are packed in the membrane with the help of cholesterol.
So, the structural integrity of lipid layer of the cell membrane is possible by cholesterol.

Functions of Lipid Layer in Cell membrane

This layer allows only the fat soluble substances like oxygen, carbon dioxide and alcohol to pass through it due to its semi permeability.
The water soluble substances are unable to pass through this layer.

Protein layers of the cell membrane

These are electron dense layers which covers the two surfaces of the central lipid layer.
The protection is provided to the central lipid layer by this protein layers.
Glycoproteins are the mostly present protein substances in these layers.
There are two categories of protein molecules. They are:

Image source: SparkNotes Image Source: Socratic

Integral proteins

These proteins pass through entire thickness of cell membrane from one side to the other side.
They are in tight contact (bound) with the cell membrane.
For examples: cell adhesion proteins, cell junction proteins, some carrier (transport) proteins, channel proteins, some hormone receptors, antigens, some enzymes etc.

Peripheral proteins

These proteins are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane.
They are loosely bound with the integral proteins or lipid layer of the cell membrane.
So these protein molecules can easily dissociate from the cell membrane.
For examples: proteins of cytoskeleton, some carrier proteins, some enzymes, etc.

Functions of proteins in cell membrane

Structural integrity of the cell membrane is provided by integral proteins.
Channel proteins are responsible for the diffusion of water soluble substances like glucose and electrolytes.
Carrier protein provides function of the transportation of substances across the cell membrane by means of active or passive transport.
Acting like a pump, some carrier proteins transports ions actively across the cell membrane.
The work of receptor sites for hormones and neurotransmitters is done by receptor proteins.
Some chemical reactions within the cell membrane are controlled by the enzymes formed by some of the protein molecules.
Antibody formation occurs due to the activity of some proteins in the form of antigen.
Attachment of the cells to their neighbors or to basal lamina is done by cell adhesion molecules or the integral proteins.

Carbohydrates of the cell membrane

Some of the carbohydrate molecules in the cell membrane are attached to the protein and some to the lipids thus forming glycoprotein (Proteoglycans) and glycolipids respectively.
The molecules of carbohydrate form a thin and loose covering over the entire surface of the cell membrane which is known as glyco-calyx.

Functions of Carbohydrates in Cell membrane

They do not permit the movement of negatively charged substances in and out of the cell as they are negatively charged themselves.
There is tight fixation of cells with one another by the help of glyco-calyx from the neighboring cells.
Some molecules of carbohydrates act as receptors for some hormones also.

Functions of Cell Membrane

It protects the cytoplasm and the organelles present in the cytoplasm.
Acting as a semi-permeable membrane, it allows only some substance to pass through it and hence acts as a barrier for other substances.
Absorption of nutrients into the cell occurs though cell membrane.
Through cell membrane, the metabolites and other wastes products are excreted.
Entry of oxygen into the cell from the blood and leaving of carbon dioxide from the cell and entering into the blood occurs due to cell membrane.
The shape and size of the cell is maintained by cell membrane.

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