Plasma Membrane: Structure and Functions

All types of cells are bounded by a thin membrane which is known as the plasma membrane. It is also known as the cell membrane, cytoplasmic membrane or plasmalemma. It is a living ultra-thin, elastic porous selectively permeable membrane that separates the cell content from the external environment.

The term cell membrane was first coined by C. Nageli and C. Cramer in 1855 while the term plasmalemma has been given by J.Q. Plowe in 1931. According to some Scientists, cell membrane originated from the endoplasmic reticulum. Some hold that the cell membrane is formed by the modification of the outer surface of the cytoplasm. Plasma membrane lies between the cell wall and cytoplasm in the bacteria and plant cells and it is the outer limiting membrane of most animal cells.

Structure of Plasma Membrane

The plasma membrane or cell membrane is porous, thin and invisible. Sometimes the membrane may be distinguished because it is either folded to form brush border or sac-like structure which is called pinocytic vacuoles. When it is examined under an electron microscope, brush border looks finger-like processes, called microvilli. In between two adjacent cells, the plasma membranes become thicker in certain regions. From these areas, many fine filaments are seen, known as tonofilaments radiate towards the interior of the cell. Such thickened areas of the plasma membrane are known as desmosomes.

According to Dannielli and Davson (1935), the plasma membrane is about 75-80 Å in thick. They also observed that the membrane is made up of trilaminar (triple-layered) structure. J.D. Roberson (1959) described this basic trilaminar structure of all cell membrane as a unit membrane model. At high magnification with an electron microscope, the cell membrane consists of double layers of lipid molecules of 35 Å thick. They are sandwiched within the two densely stained protein layers. The thickness of each protein layer is 20 Å. The lipid layer is mostly phospholipids of which the head end contains the water-soluble and positively charged phosphate group called polar ends. The tail end contains the water-insoluble and negatively charged lipid group, called non-polar ends remain side by side. 

The polar ends remain attached to protein layers probably by the hydrogen bonds, ion bonds or electroscopic forces. In some eukaryotic cells (animal cells) beside the plasma membrane, there is another covering is present which is known as glycocalyx or cell coat. It is made up of protein and carbohydrate.

Also read: Cell Wall: Structure and Functions

The Fluid Mosaic Model of Plasma Membrane

The lipid-globular protein mosaic model suggests, as the name implies, that instead of a continuous layer of protein on the surface of the membrane there is discontinuous mosaic globular protein. They remain partially embedded in and partially protruding from the phospholipid bilayer. There are also some discontinuous peripheral globular proteins arranged just outside and along the surface of the phospholipid bilayer.

This model was observed by English Scientists S. J. Singer and Garth Nicolson in 1972. This model is also known as Singer – Nicolson’s fluid mosaic model.  According to this model, the plasma membrane looks like a mosaic which contains some components like phospholipids, cholesterol, proteins, and carbohydrates, etc. which gives the membrane a fluid character.  Generally, the percentages of  proteins, carbohydrates, and lipids in the plasma membrane vary with cell type. In myelin, the proportion of proteins and lipid are 18% and 76% respectively while the inner membrane of mitochondrial contains 76% protein and 24% lipid.

According to this theory, the main component of the cell membrane is a bimolecular lipid layer which actually consists of two rows of amphiphilic phospholipids molecules. Each phospholipid molecule contains three-carbon glycerol backbone with two fatty acid molecules which are attached to carbons 1 and 2 and a phosphate-containing group that is attached to the third carbon.

Each phospholipid molecule has a water-soluble polar head and two fat-soluble non-polar tails. The polar head of phospholipids is hydrophilic (hydro=water, philic=loving) that is attracted to water and the non-polar tail is hydrophobic (hydro=water, phobic(=fearing) that always try to avoid water.  Within the phospholipid bilayer, different types of protein and cholesterol molecules are embedded which gives the plasma membrane look of a mosaic. In this way, phospholipids build up a magnificent lipid bilayer cell membrane that isolates fluid inside the cell from the fluid outside of the cell.

The plasma membrane is illustrated to be fluid of its hydrophobic basic parts; for example, lipids and membrane proteins that move along the side or sideways all through the membrane. That implies the membrane isn't solid however more like a fluid. That is the reason the plasma membrane is depicted utilizing the fluid mosaic model.

Chemically, the second major component of plasma membranes is proteins. Some protein molecules exist outside the lipid layer; called peripheral protein molecule and some are partially or entirely pass across the lipid layer, called integral protein molecules. Integral protein molecules create an ion channel through the cell membrane for passing water-soluble molecules. A single integral protein usually consists of 20–25 amino acids.

The third major component of plasma membranes is oligosaccharide molecules (carbohydrates). These oligosaccharide molecules attached to some protein and lipid molecules of the outer side of the cell membrane to form glycoprotein and glycolipid respectively. Generally, these carbohydrate chains contain 2–60 monosaccharide units which can be either branched or straight.

Chemical Composition of Plasma Membrane

Chemically cell membrane is made up of the following components:

Functions of the Plasma Membrane

Also read: Cell Structure and Functions