Cell Membrane structure and function

The outer living boundary of the cell is called as the cell membrane or ‘Plasma membrane’. The term cell membrane was coined by C.J. Nageli and C. Crammer in 1855. Apart from the cell membrane, each
and every organelle in the cell is also covered by membranes. The cell membrane not only limits the cell cytosol, but it has a variety of functions like membrane transport, signal transduction and neuro transmission.

Chemical Composition.: Four major constituents are present in the cell membrane. They are (i) lipids (28 – 79%) (ii) proteins (20 – 70%). (iii) oligosaccharides (only 1 – 5%) and (iv) water (20%).

Lipids: Depending upon the tissue from which the cell membrane isisolated, the composition also differs. Nearly 80% of the myelin sheath is made up of lipids, while in liver, it constitutes only 28%.The main lipid components of the membranes are phospholipids, cholesterol and glycolipids. The major phospholipids present are phophatidyl choline (lecithin), phophatidyl ethanolamine, phophatidyl serine and phophatidyl inositol.
Membrane lipids are amphipathic in nature and they have a head portion, which is hydrophilic and a tail portion which is hydrophobic. As the membranes are exposed to the hydrophilic environments, the lipids
arrange themselves to form a bilayer in which the hydrophobic core is buried inside the membrane.
Proteins:All the major functions of the plasma membrane are executed by the proteins present in the membrane. Proteins account for about 20 – 70% of the membrane depending on the type of the cell. They can be classified into two types. Integral membrane proteins and peripheral membrane proteins
Integral Proteins:Some of the membrane proteins are tightly embedded in the membrane and they cannot be isolated unless, the membrane is disintegrated. They are called as Integral or Intrinsic membrane proteins.
They are again classified into two. (a). Transmembrane proteins, which traverse (pass through) or span the membrane. These proteins will have domains on either side of the membrane. Many cell surface receptors
belong to this class. (b). Lipid anchored proteins that are present either on the cytosolic side or on the extracytosolic side. They insert themselves in the membrane by a lipid (acyl chain) attached to the N terminal end.
Transmembrane proteins are of two types. Single pass transmembrane proteins that traverse the membrane only once. Multipass transmembrane proteins that traverse the membrane more than once.
Peripheral Proteins:Those proteins that are present on the surface of the membrane are called as peripheral proteins. They can be easily isolated from the membrane. eg. spectrin present in the RBC membrane.
Models proposed for the plasma membrane
Monolayer Model:Overton was the pioneer to postulate that plasma membrane is a thin layer of lipid. He proposed this because he found that lipid soluble substances are easily transported across the membrane.
 Lipid Bilayer Model
The amount of lipids present in the erythrocyte membrane was nearly twice that of its total surface area. This made Gorten and Grendelto propose that lipids in the membrane exist as bilayers.
 Unit Membrane Model
This model was proposed by Davson and Danielli and was shapedby Robertson. Experiments showed that the surface tension of the biologicalmembranes are lower than that of the pure lipid bilayers, suggesting the
presence of proteins in them. Based upon this, Davson and Danielliproposed that proteins are smeared over the lipid bilayer.When electron microscope was invented, plasma membraneappeared as three layers. With this observation, Robertson formulated aunit membrane model, which states that the proteins are present on either side of the lipid bilayer. According to this model, the membrane will be like a lipid layer sandwiched between two protein layers.
 Fluid Mosaic model
This is the universally accepted model for plasma membrane. On the basis of several experiments, S.J. Singer and G.L. Nicolson in 1972 proposed this model.
The essential features of the Fluid mosaic model are
1. Lipids and proteins are present in a mosaic arrangement within the bilayer.
2. Phospholipids act as a fluid matrix, in which some proteins are integral and others are associated with the surface of the membrane.
3. Lipids and proteins are mobile in the membrane.
4. They can move laterally, rotate but not from one monolayer to the other.
5. The membrane is asymmetric in nature, the outer and inner leaflets of the bilayer differ in composition.