Plasma Membrane: Subject Matter and Functions (With Diagram)

In this article we will discuss about the Plasma Membrane:- 1. Subject Matter of the Plasma Membrane 2. Chemical Composition of Plasma Membrane 3. Specialized Structure 4. Functions.

Subject Matter of the Plasma Membrane:

The protoplasm of the cell is bounded by an exceedingly thin elastic, semi-permeable living limiting membrane called the plasmalemma or plasma membrane or cytoplasmic membrane or cell membrane. A plasma membrane forms the outer limiting boundary of most animal cells.

In plant cell and bacterial cell the plasma membrane lies between the cell wall and cytoplasm. Many botanists prefer the term plasmalemma to plasma membrane. The earlier biologists doubted its appearance. Existence of plasma membrane was proved when phenol-red dye was injected into the cytoplasm.

In the experiment it was found that the dye coloured the whole of cytoplasm and did not pass out of the cell. Further when some colourless cells were placed in the dye, only the cell wall was coloured and not the cytoplasm.

These observations suggested the existence of a boundary layer between the outer non-living cell wall and internal cytoplasm. W. Pfeffer (1890) established that the plasma-membrane could not be differentiated from the inner plasma with the help of light microscope. It can, however, be easily studied under electron microscope due to better resolution and it appears as a unit membrane of 75 to 100 Å thickness.

Now it is an accepted view that all the unit membranes have the same structure. The unit membrane forms not only the plasma membrane, but also the limiting layer of most of the cell organelles. Such a view was initially put forth by Robertson in 1960.

In broad sense, the term cell membrane refers to the limiting membrane of the cell organelles like plastid, mitochondria, lysosomes and the other membranous systems like endoplasmic reticulum, Golgi-complex and nuclear envelopes.

The structure of plasma membrane has not been marked out in every type of cell because of the problem of technique for isolation. The electron microscope has been quite helpful in study of cell membranes.

The detailed structure of plasma membrane could be studied only after the improvement of micro-technique for isolation of pure and intact plasma membrane from the living cell and their subsequent electron microscopy.

The studies made by E. Gorter and F. Grendel (1925) on mammalian red blood cells and the myelin shealth of the nerve fibre have provided bulk of information regarding the structure and functions of the plasma membrane.

The recent observations on the plasma membranes of many other cell types are similar to the findings on the membranes of these two cells and hence applicable to all others cell types. The red blood cells are treated to remove all protoplasmic contents except plasma membrane.

The residue of such treatment is called a ‘red cell ghost’. Mitchell (1959) isolated and analysed the plasma membrane of a bacterium by removing the cell wall through enzyme treatment and then by bursting the cell in hypotonic solution.

Ponder and others have raised objections on the grounds that there is no direct evidence for the ghost to be a true likeness of plasma membrane of living cells. Some workers have suggested that the ghost is simply an artifact of technique involved in the isolation of plasma membrane. Nevertheless, the studies on red blood cells, nerve cells and some other cells have provided basic information about the plasma membrane.

Chemical Composition of Plasma Membrane:

The chemical composition of plasma membranes differs in different cells. The membranes are composed mainly of proteins, phospholipids and a small percentage (1-5%) of oligosaccharides.

In general, plasma membranes contain about 60% protein and 40% lipids by dry weight but there is wide variation in the protein lipid ratio of different cell membranes. Oligosaccharides may be found attached to either lipids (glycolipids) or proteins (glycoproteins).

Carbohydrates usually form 1-10% of the total dry weight. In the membranes of some cell organelles the presence of nucleic acids, DNA and RNA has also been reported but they are presumed to be contaminations.

i. Proteins of the Plasma Membranes:

Plasma membranes contain three different classes of protein: structural proteins, enzymes and carrier proteins.

The structural proteins form the mechanical structure of the membrane. They have little catalytic activity and are extremely hydrophilic in nature. The plasma membrane consists largely of structural proteins, the average molecular weight being 3x 10⁴.

The structural proteins of plasma membrane have been classified into two main groups:

1. The peripheral or extrinsic proteins.

2. Intrinsic or integral proteins.

1. The peripheral proteins are weakly associated with the membrane surface and may be separated by mild treatments. They are soluble in aqueous solutions and are usually free from phospholipids. The most common examples of peripheral proteins are spectrin found in erythrocytes, ATPase and Cytochrome c found in mitochondria and acetylcholinesterase, etc.

2. Integral or Intrinsic proteins are associated strongly with the membrane and represent more than 70% of the structural proteins. They are usually associated with the lipid extending all the way through it and require drastic procedures for isolation such as treatment with hydrophobic bond breaking agents like detergents, organic solvents, chaotropic agents, etc.

They are usually insoluble in aqueous solution. The most common examples of integral proteins are rhodopsin found in rod cells of retina and cytochrome oxidase found in mitochondrial membrane.

ii. Enzymes of Plasma Membranes:

Enzymes form the major part or component of many membranes and are catalytic proteins. The endoplasmic reliculum, mitochondria, and other membranes contain many enzymes. The structure of enzymes varies from membrane to membrane.

About 30 enzymes have been reported from the plasma membrane, the most common being 5-nucleotidase, Mg⁺⁺ AT Pase, activated Ma⁺ K⁺ activated Mg⁺⁺ ATP Pase, alkaline phosphatase, adenylcyclase, acid phospho monoesterase and RNAase.

Some of the enzymes have specific localizations, e.g., alkaline phosphatase are more abundant in the bile capillaries while disaccharides are present in the microvilli of intestine.

Carrier Proteins:

Carrier proteins are permeases which transport substances across the membrane against concentration gradient. The carrier proteins are more or less similar to structural proteins in molecular weight.

iii. Lipid Fraction of Plasma Membrane:

The lipid fraction constitutes about 20-79% of the plasma membrane. The main lipid constituents of cell membrane are phospholipids, cholesterol and galactolipids in variable proportions in different cells.

Phospholipid components are of two kinds:

(i)Neutral phospholipids:

Neutral phospholipids, such as phosphatidicholine, phosphatidylethanolamine and sphingomyline which tend to pack tightly in two layers.

(ii) Acidic phospholipids:

Acidic phospholipids, such as phosphatidyl inocetol, phosphatidyl serine, cardiolipid, phosphatidyl glycerol, sulpholipids which are negatively charged and associated with proteins by way of lipid protein interactions.

The lipids of plasma membrane are polar lipids which contain hydrophilic heads (glycerol) and two hydrophobic tails (fatty acids). The hydrophobic and hydrophilic regions may be bridged by glycerol moiety, a sphinganine derivative or homologue of a sterol.

Plasma membranes of different cells consist of a variety of fatty acids in their lipid layer, e.g., Ted blood cells of mammal contain fatty acids such as palmitic, stearic, oleic, linoleic and arachidonic acids.

iv. Carbohydrate Fraction of Plasma Membranes:

According to Bell (1962) the polysaccharides are present in the outer protein layer of the plasma membrane which provides some stability to the lipoprotein complex. The common oligosaccharides which remain bounded in plasma membrane of red blood cells and liver cells of mammals are hexose, hexosamine, Fucose, sialic acid.

Specialized Structure of Plasma Membrane:

In order to perform specialized functions, plasma membranes are variously modified.

The most specialized structures of plasma membrane are as follows:

1. Specialization due to Evagination of Plasma Membrane (Microvilli):

The plasma membranes of some cells which perform active absorption give out many minute finger-like processes. These are called microvilli. The microvilli are 0.6-0.8 mµ long and 0.1 mµ in diameter. The cytoplasmic core of microvilli contains fine microfilaments along with a-actinin, myocin molecule, and microtubules.

The outer surface of microvilli is covered by coat of filamentous material (Fuzzy coat) of glycoprotein (Figs. 2.8 and 2.9). A single cell may develop as many as 3,000 microvilli.

The narrow spaces between the microvilli form a kind of sieve through which substances may pass during absorption. These microvilli increase the absorptive surface of the cells, such as intestinal epithelium, hepatic cells, mesothelial cells of convoluted tubules of kidney, epithelial cells of gall bladder, uterous and yolk sac, etc.

2. Invaginations of Plasma Membrane:

Certain cells which perform active transportation, such as cells of kidney, may show infoldings or invaginations of plasma membrane. The bases of these folds may develop septa and thus narrow compartments are formed.

These invaginations are in contact with many mitochondria which possibly provide energy to the plasma membrane for the active transportation of solutes.

3. Specialization of Plasma Membrane due to Contact (Junctional Complex):

Though the adjacent cells remain separated by a uniform intercellular gap of 100-200 Å, their plasma membranes may come too close or may go too far. Sometimes the contact between the cells may result in the modification of plasma membrane. In the specialized regions the membrane proteins of adjacent cells interact in a highly specific way to provide a intercellular junctions.

These are called Junctional complexes. Some junctions are devices to make the adjacent cells adhere together, some are barriers to prevent fluids from passing between the cells and some junctions serve primarily to establish channels of communications from one cell to other.

So the cell junctions are assigned various names that describe their shapes and relative closeness with the adjacent membranes, such as:

(i) Zonule:

A stripe that passes completely around the cell.

(ii) Macule:

It is a contact spot in the plasma membrane.

(iii) Occludens:

It is a zone where the outer surfaces of the two membranes are very close or even fused at several points containing sealing strands which prevent the free passage of substances across the membranes.

(iv) Adherens:

It is a wide separation band between the cell membranes.

Functions of the Plasma Membrane:

The plasma membrane is of great physiological importance because of its semi-permeable nature. It controls the cellular semi-permeability, resorption, secretion and excretion.

It regulates the cytoplasmic contents of a cell and acts as a selective barrier between the living cell contents and its external environment because of the fact that it permits only those substances to pass through which are needed by the cytoplasm and does not allow those substances to enter which are not required.

Plasma membrane is permeable to water. The diffusion of ions through plasma membrane depends upon the concentration and electrical gradients.

Plasma membrane is not only a passive diffusion barrier but also carries catalytically active regions. The machinery which utilizes energy for carrying out osmotic work is also located in or near plasma membrane.

The important means by which materials can be transported into the cell are “phagocytosis” and “pinocytosis”. The process of ingestion of solid substances by the cell is referred to as phagocytosis and the ingestion of fluid material in bulk is known as pinocytosis (Lewis, 1931). These two processes involve invagination of plasma membrane to form vacuoles containing solids or liquids.

The membrane bound vesicles or vacuoles are termed phagosomes and pinosomes respectively (Fig. 2.11). The phagosomes and pinosomes along with external materials pass into the cell where they fuse with the secretory granules. In the food vacuoles the digestion of food substance takes place and the digested food is diffused to the groundplasm.

A reverse process to phagocytosis or pinocytosis may occur for throwing the waste materials out through plasma membrane.

The plasma membrane is the integral organelle of the cell without which protoplasm cannot survive. Like other organelles, the plasma membrane repairs itself if it is injured.