Plasma Membrane

The cell has a different internal milieu from that of its external environment. This difference is maintained throughout the life of the cell by the thin surface membrane, the plasmalemma or plasma membrane. It controls the entrance and exit of molecules and ions.

The limiting membrane of the cell organelles like Golgi complex, endoplasmic reticulum, nuclear membrane etc., is known as cell membrane. The cell membrane and plasma membrane are collectively known as biological membranes.

Cell membranes may be defined as the membranes that serve as boundary of cytoplasm or organelles, exerts selective influence on the molecules and ions entering or leaving the cell or organelles, help in controlling intracellular melieu and influence the cell or organelle behaviour.

In some eukaryotic cells the different membrane systems make up as much as 80 percent of the total dry cell mass. Membranes serve not only as barriers separating aqueous compartments with different solute composition but also as the structural base to which certain enzymes and transport systems are firmly bound. They are very thin (about 8 nm) and flexible.

Composition of membrane:

The cell membrane mainly consists of proteins and lipid or lipoproteins. In certain cases polysaccharides, sialic acid, RNA and DNA have also been found. The ratio of protein to lipid varies from 1:0.8 to 1:4 and it depends, on the type of membrane, type of cell and type of organism.

All these chemical components are held together in a thin film – like pliable sheet of about 75A⁰ thickness, in three concentric layers. The middle layer is about 35A⁰ and outer layer on either side are 20 A⁰ thick.

Lipids:

The amount of lipids found in the membrane varies from 30 percent (mitochondria) to 70 percent of the total dry matter of cell membrane.

The most common lipids found in the membrane are phospholipids, glycolipids and sterols. All of them are amphipathic i.e., they have both hydrophilic and hydrophobic portions within a molecule.

The major proportion of membrane phospholipids is represented by phosphatidylcholine, phosphatidylethanolamine and sphingomylein. Glycolipids contain a sugar (glucose or galactose) in addition to fatty acids and sphingosine. The examples of sterols are cholesterols present in animal tissues, phytosterols present in plant cells and ergosterol present in ergot and  yeast.

Membrane Proteins:

Proteins represent the main component of most biological membranes. They play an important role, not only in the structure of the membrane, but also as carriers or channels, serving for transport, they may also be involved in regulatory or ligand-recognition properties. Numerous enzymes, antigens, and various kinds of receptor molecules are present in plasma membranes.

The different types of proteins present in membranes are:

i. Structural proteins: that form the backbone of the membrane and are strongly lipophilic in nature.

ii. Functional proteins: like enzymes that catalyse several physiological reactions (e.g., glucose 6 phosphatase found in membranes of endoplasmic reticulum and cytochrome oxidase present in mitochondria)

iii. Carrier proteins: that bring about the diffusion of substances across the membrane.

The proteins present in the plasma membrane are further differentiated into integral or intrinsic or integral proteins and peripheral or extrinsic proteins.

The intrinsic proteins make up 70 percent or more of the total membrane proteins. They are very tightly bound to the lipid portion and can be removed only by drastic procedures for isolation. The intrinsic proteins are highly insoluble in water solutions and are associated with lipids. These proteins may be attached to oligosaccharides , thus forming glycoproteins.

The peripheral or extrinsic proteins are loosely attached to the membrane surface, free of lipids and can easily be removed in soluble form by mild extraction procedures.

Structure of Plasma Membrane or Molecular models of Cell Membrane

Since substances soluble in lipid solvents penetrate the plasma membrane easily, in 1902 Overton postulated that the plasma membrane is composed of a thin layer of lipid.

Lipid Bilayer Model:

In 1925, E.Gorter and F.Grendel made the first proposal that the plasma membrane might contain a lipid bilayer. They extracted the lipids from human erythrocytes and concluded that the plasma membrane contained a bimolecular layer of lipids, or simply a lipid bilayer.

They also suggested that the polar groups of each molecular layer were directed towards the outside of the bilayer.

Later, it was found that some water soluble molecules could move across the cell surface and this resulted in the suggestion that the  plasma membrane was a mosaic of lipid and non-lipid substances.

Danielli – Davson Model:

In 1935, James Danielli and Hugh Davson studied the starfish egg membrane and proposed that the plasma membrane is made up of two layers of lipids sandwiched between two continuous layers of proteins.

According to this model, the protein and lipid molecule layers are electrostatically held together the former being the charged and the latter negatively charged. The lipophilic(hydrophobic) tails of fatty acids face one another in the centre and their hydrophilic heads face outwards.

They further envisioned the polar ends of the lipid molecules as being associated with a monomolecular layer of polar globular proteins. Each protein layer is found to be 20 A⁰ thick and lipd layers 35A⁰ thick, making the membrane 75 – 100 A⁰ thick.

This model cannot be applied to all membranes, as it reveals the definite proportion of lipids and proteins, which has not been found true. Moreover, recent studies reveal that proteins are found in form of particles and not as continuous layer.

Unit Membrane Concept:

It was proposed by Robertson . This concept was based on the study of the myelin sheath of a nerve fibre.

Robertson studied the structure of membrane with the help of electron microscope and found that all biological membranes show three layers (trilaminar) i.e., two outer layers  (electron dense) separated by a lighter middle layer. These correspond to two layers of proteins and a middle layer of phospholipids.

The molecular chains of lipids remain parallel to each other thus forming a bimolecular structure. The two chains remain linked by non-polar and hydrophobic inner ends of lipid molecules. Van der Waals forces bind the two lipid layers together.

The protein layer provides elasticity and mechanical resistance to plasma membrane. The unit membrane model is similar to Danielli-Davson model except that the protein layers at the exterior and interior are different. The outer surface has mucoproteins, while the inner surface has un-conjugated protein molecules are present. This shows the asymmetry of the protein surfaces on either side of the lipid bilayer.

Robertson provided evidence that the membranes of other cell organelles like endoplasmic reticulum, mitochondria, golgi complex have the unit membrane structure.

The Robertson’s unit membrane model was widely approved but remained unsatisfactory because it could not explain the dynamic nature and functional specificity of the membrane.

Fluid Mosaic Model

The most satisfactory model of membrane structure was postulated by S.J. Singer and G.L. Nicolson called the fluid-mosaic model.

This model postulates that the phospholipids of membranes are arranged in a bilayer to form a fluid, liquid-crystalline matrix or core. In this bilayer individual lipid molecules can move laterally, endowing the bilayer with fluidity, flexibility and a characteristically high electrical resistance and relative impermeability to highly polar molecules.

The fluid mosaic model postulates that that the membrane proteins are globular and are dispersed in the lipid bilayer. Thus, the proteins would form a mosaic like structure in the fluid phospholipid bilayer. In a way, these protein molecules resemble floating icebergs in the sea of lipids.

This mosaic is not fixed or static, since the proteins are free to diffuse laterally. Some of the proteins are partially embedded in the membrane, penetrating in the lipid layer (integral or intrinsic proteins) and some are superficially attached (peripheral or extrinsic proteins.

The important concepts of fluid mosaic model can be summarized as i) that biological membranes are quasi-fluid structures in which both the lipids and integral proteins are able to perform translational movements within the bilayer, ii) that the lipid and integral proteins are disposed in a kind of mosaic arrangement.

The fluid mosaic model accounts satisfactorily for many features and a properties of biological membranes. It provides for membranes with widely different protein content, varying thickness of different types of membranes, it account for the asymmetry of natural membranes, it accounts for the electrical properties and permeability of membranes.

The Micellar Model:

Many cell biologists suggested that membrane might consists of closely packed repeating units which are similar in structure.

In 1953, Hillier and Hoffman suggested that the membranes have a non-lamellar pattern, consisting of globular subunits known as micelles.

Each micelle is about 40-70A⁰ in diameter with a lipid core and a hydrophilic shell of polar groups. The protein components of the membrane form a monolayer on either side of the plane of lipid micelles.

The space between micelles represent water filled pore (4A⁰). These pores are lined partly by the polar groups of micelles and partly by the polar groups of associated proteins.

Functions of Biological Membranes

1. Membranes are living boundaries of the cytoplasm and cellular organelles. They form and continuous and unbroken sheets that divide the living matter into self-sustaining units to perform specialised functions.

2. Acts as a selectively permeable membrane- allowing appropriate substances into the cell and removing the inappropriate substances out of cell.

3. Allow the transport of metabolic essential substances

4. membranes have on their outer surface specific protein molecules acting as receptors, which unit with complementary substances or ligands providing external stimuli to the cell.

5. Perform different biochemical activities in the presence of enzymes.