Overview on Composition of Plasma Membrane and its fluidity

The contents of a cell are enclosed in a semi-permeable membrane, called the plasma membrane. Plasma membrane are also referred to as cytoplasmic membrane, cell membrane and plasma lemma. It is ultra-thin, elastic, living, dynamic and selective transport barrier.¹ It acts as a dynamic barrier which regulates exchange of materials between the cell interior and its environment. Actions such as cell signaling, cell communication, adhesion and movement, depend upon these unique properties of plasma membrane. Thus, it can be safely concluded that, the remarkable properties of plasma membrane are building block of all life processes.

1.1 Study of Plasma membrane:

Plasma membrane is too thin to be observed by the light microscope. So, the structure and composition of plasma membrane of various cells have been studied by their isolation from living system and also by their artificial synthesis. The pure and isolated membranes are then studied by biochemical and biophysical methods.²

Human erythrocytes or red blood cells have been extensively used to study the structure and properties of plasma membrane. E. Gorter and F. Grendel (1925) explained several reasons for this choice: these cells are easy to obtain, are extremely simple, contain no intracellular organelles or membranes (so the only membrane found can be concluded as outer plasma membrane). These cells are also relatively tough and does not fragment readily. (Lucy, 1975)²

1.2 Fluid Mosaic model of Plasma membrane:

Fluid mosaic model of plasma membrane is the most accepted theory explaining the structure and composition of plasma membrane. According to this model, the plasma membrane consists of a phospholipid bilayer, surfaces of which contains protein molecules randomly in a mosaic pattern. The peripheral proteins and some parts of the integral proteins that stick on the outer surface contain chains of sugar or polysaccharides.

Fig: Structure of Plasma membrane according to fluid mosaic model (Source: Biology2e, Openstax)

1.3 Chemical composition of plasma membrane:

Plasma membrane and other membrane of different organelles (Eg. Nucleus, Mitochondria, Chloroplast, ER, etc.) have been found to contain proteins, lipids and carbohydrates. However, the ratios of these macromolecules differs along the membranes.

A. Carbohydrate:

Carbohydrates are present in the plasma membrane but absent in inner membranes of organelles. They are short, unbranched or branched chain of sugars (oligosaccharides). They are attached either to exterior ectoproteins (forming glycoprotein) or to the polar ends of phospholipids at the external surface of plasma membrane (forming glycolipid). Carbohydrates are absent at the cytoplasmic or inner surface of the plasma membrane. All oligosaccharides present in the plasma membrane are formed by the various combinations of six principal sugars: D-galactose, D-mannose, L-fucose, N-acetyl neuraminic acid (also called sialic acid), N-acetyl-D-glucosamine and N-acetyl-D-galactosamine.²

Glycoproteins: Almost all the proteins present on the outer surface of the plasma membrane contain carbohydrate components. The function of the carbohydrates in the membrane is yet to be understood completely. It has been popularly assumed that the presence of carbohydrate in the outer surface makes the membrane negatively charged. Positively charged proteins are bound to the plasma membrane through electrostatic interaction. Recent studies has also shown that glycoproteins have the capacity to bind with hormones like insulin.¹ Furthermore, cell-to-cell adhesion may be possible because of the presence of carbohydrates. Glycoproteins may also act as a site for some blood group antigens.
Glycolipids: They are usually found in the membranes of neurons in the form of gangliosides. Glycosidic bond is present between the glycerol molecule and lipid.

B. Proteins:

On the basis of degree of association with the membrane and separation technique, proteins of plasma membrane are of two types: integral proteins and peripheral proteins. Integral protein are found to associate firmly with the membrane. Peripheral proteins are weakly associated at the membrane by electrostatic interaction.

On the basis of functional importance, plasma membrane can be classified into 3 main types: structural proteins, enzymes and transport proteins. Structural proteins are lipophilic in nature and form the main bulk of the plasma membrane. Enzymes are biocatalysts and help to increase the rate of various biochemical reactions. 30 different types of enzymes are found in plasma membrane. Transport proteins transport specific substances across the plasma membrane and other cellular membranes.²

According to fluid mosaic model, proteins occur in the form of globular molecules and they are dotted about the plasma membrane in a mosaic pattern. Some proteins are attached at the polar surface of the lipid (extrinsic protein) while some others span the membrane entirely to stick out on both sides (called transmembrane protein). The most contrasting fact presented by fluid mosaic model is that unlike general understanding, the proteins are present not to give membrane its strength, but to serve as enzymes catalyzing chemical reactions within the membrane and as pump moving things across it.

Evidence in support of mosaic arrangement of proteins:

Freeze-fracture electron microscopy of plasma membrane performed by Branton (1968) revealed the presence of bumps and depression (7-8nm in diameter) which are randomly distributed. These were later shown to be transmembrane integral proteins.²

C. Lipid:

Lipids make up about 40 per cent of the chemical components of plasma membrane. Lipid compounds such as cholesterol and fatty acid esters (mainly glycerides and phospholipids) are present in the plasma membrane. The relative proportions of these lipids vary in different membranes. Phospholipids of about five different types have been identified. These are phosphatidic acid, lecithin, phosphatidyl ethanolamine,

Phosphatidyl serine and phosphatidyl inositol.¹

Cholesterol is especially abundant in the plasma membrane of mammalian cells and absent from the prokaryotic cells.

1.4 Fluidity of plasma membrane:

Membrane fluidity refers to the viscosity of the lipid bilayer in a cell membrane, which affects how easily molecules can move within the membrane. This property is crucial for various cellular processes, including the movement of proteins and lipids, cell signaling, and membrane fusion.

Mobility of membrane proteins due to fluid property of lipid bilayer was demonstrated by a classical experiment of D. Frye and Edidin (1970) through cell fusion technique.²

A. Role of lipids in maintaining membrane fluidity:

i. Movement of lipid molecule:

Lipid molecules can migrate from one lipid monolayer to another of lipid bimolecular layer. This movement is rare and is called flip-flop movement or transbilayer movement. It generally occurs only once a month for any individual molecule. However, in smooth Endoplasmic reticulum, the flip-flop movement is very rapid, catalyzed by the presence of membrane bound enzymes called phospholipid translocators. Lipid molecules readily exchange places with their neighbors within a monolayer.² This results in rapid lateral diffusion of the lipid macromolecule. These lipid movements favors membrane fluidity.

ii. Role of unsaturated fats in increasing membrane fluidity:

Double bond in unsaturated hydrocarbon chains tend to increase the fluidity of a phospholipid bilayer by making it more difficult for the chains to pack together. In order to maintain fluidity of the membrane, cells of organisms living at low temperatures have higher properties of unsaturated fatty acids in their membranes.

iii. Role of cholesterol in maintaining membrane fluidity:

Cholesterol are found in large amount in Eukaryotic plasma membrane. They are oriented in such a way that their hydroxyl groups remain close to polar head groups of the phospholipids and their rigid steroid rings partly immobilize those regions of hydrocarbon closest to the polar head, leaving the rest of the chain flexible.

Cholesterol inhibits phase transition (i.e. change of liquid state to rigid or crystalline at certain temperature) by preventing hydrocarbon chains from coming together and crystallizing. Cholesterol also decrease the permeability of lipid bilayers to small water-soluble molecules and enhance both flexibility and mechanical stability of membrane.²

REFERENCE

S.C. Rastogi. Cell and Molecular Biology. Third edition. New Age International Publishers
Dr. P.S. Verma, Dr. C.K. Agrawal. Cell Biology, Genetics, Molecular Biology, Evolution and Ecology. fourteenth. (Bharatnagar S, Pradhan S, eds.). S.CHAND & COMPANY PVT.LTD.; 2016.