In this chemical composition of biological membrane post we have briefly explained about lipid, protein, enzymes, salt, and water and carbohydrates composition of cell membrane.
Plasma membrane is primarily composed of protein and lipid, although carbohydrate is often present in association with protein (as glycoprotein) or lipid (as glycolipid). However, the relative proportions of protein and lipid vary considerably in membranes from different sources.
Chemical Composition of Cell Membrane
The plasma membrane contains about 20 to 79% lipids mainly of three types like phospholipids, cholesterol and glycolipids. The phospholipids which make up between 55% and 75% of the total lipid content, consists chiefly of lecithin and cephalin. A phosphoglycerides is made up of two fatty acid chains, a glycerol backbone and a phosphorylated alcohol.
The outer layer of phospholipids consists mainly of lecithin and sphingomyeline, while the inner layer is composed mainly of phosphatidyl ethanolamine and phosphatidyl serine (both are phosphoglycerides). The glycolipids (sugar containing lipids) are mainly in the outer half of the bilayer.
Chemical Composition of Cell Membrane: A phospholipids cholesterol complex of cell membrane
Cholesterol is present in eukaryotes but not in prokaryotes. Plasma membrane of cells such as erythrocyte, liver cells and myelinated nerve cells are rich in cholesterol.
Membrane lipids are amphipathic molecules. They contain both a hydrophobic and hydrophilic moiety. Hydrophilic unit is also called the polar head groups, is represented by a circle and their hydrocarbon tails are depicted by straight or wavy lines.
Polar head groups have affinity for water, whereas their hydrocarbons tails avoid water. This can be accomplished by forming a micelle, in which polar head groups are on the surface and hydrocarbon tails are directed inside.
Chemical Composition of Cell Membrane: A phospholipids molecule
Another arrangement of lipid molecule in a membrane is a bimolecular sheet, which is also called a lipid bilayer. Phospholipids and glycolipids are key membrane constituents of bimolecular sheets. Hydrophobic interactions are the major driving force for the formation of lipid bilayer. The lipid bilayer of the membrane is interrupted only by the proteins that traverse it. This bilayer consists primarily of:
Neutral Phospholipids and Cholesterol: These include phosphatidylenoline, lecithin cerebroside, and sphingomyeline and phosphatidyl ethanolamine. They are without any electric charge at neutral pH and are closely packed in the bilayer along with cholesterol.
Acidic Phospholipids: These constitute about 5% to 20% fractions of the total phospholipids of plasma membrane. They are negatively charged and are associated with proteins by way of lipid-protein interactions.
Proteins are the main component of plasma membrane. Myelin sheath (membrane surrounding some nerve axons) is composed of about 80% lipids and 20% protein and presence of lipid makes myelin an excellent insulator.
Eukaryotes membrane which serves primarily as permeability barriers possesses about 50% proteins and 50% lipid. Plasma membrane that are actively involved in energy transfer, such as inner membrane of mitochondria, chloroplasts and membranes of aerobic prokaryotes have large amounts of proteins i.e. about 75%.
They not only provide mechanical support but also act as carriers or channels, serving for transport. In addition numerous enzymes, antigens and various kinds of receptor molecules are present in plasma membranes. Membrane proteins are classified as integral (intrinsic) or peripheral (extrinsic) according to the degree of their association with the membrane.
They are also called extrinsic proteins associated with membrane surface. These can be separated by addition of salts, soluble in aqueous solutions and usually free of lipids. They are bound to the surface by electrostatic and hydrogen bond interactions. They form outer and inner layers of the lipid bilayer of plasma membrane. Common examples are cytochrome-C found in mitochondria, acetyl cholinesterase in electroplax membrane and spectrin found in erythrocytes.
These proteins penetrate the lipid layer wholly or partially and represent more than 70% of the two protein types. Their polar ends protrude from the membrane surface while non-polar regions are embedded in the interior of the membrane. Usually they are insoluble in water solutions and can be separate them from the membrane by detergents or organic solvents.
Chemical Composition of Cell Membrane
About 30 enzymes have been found in various membranes. Those most constantly found are 5′-nucleotidase, Na+-K+ activated ATPase, alkaline phosphatase, adenylcyclase, RNAse and acid phosphomonoestrase.
Na+-K+ activated Mg+ ATPase plays an important role in the ionic exchange and may also act as carrier protein or permease across the plasma membrane. Some enzymes have a preferential localization. For example, alkaline phosphatase and ATPase are more abundant in bile capillaries, while disaccharides are present in microvilli of the intestine.
Enzymes are asymmetrically distributed, for example in the outer surface of erythrocytes there are acetylcholinestrase, nicotinamide adenine dinucleotidase and Na+-K+ ATPase. In the inner surface there is NADH-diaphorase, G3PD, adenylate cyclase, protein kinase and ATPase.
The membranes of eukaryotic cells usually contain 2% to 10% carbohydrates in the form of glycolipids and glycoproteins. Hexose, hexosamine, fucose and sialic acid are the commonest carbohydrates found in the membrane.
Plasma membranes of neuronal surface contain gangliosides (Lapertina, 1967) and are probably involved in the ion transfers. The distribution of oligosaccharides is also highly asymmetrical.
Salt and Water
They are also present in cell membranes. Water in cell membranes forms parts of membrane structure as it does in all cell constituents.