The plasma membrane, cell membrane, cytoplasmic membrane and permeable membrane collectively these are same membrane which surround the cell and the components of cell. Every living cell is externally covered by a thin transparent, electron microscopic, plastic, regenerative and semipermeable membrane called cell membrane (called plasma membrane or plasmalemma). It was called “cell” membrane by “Nagelli” and “Cramer” but was called plasmalemma by “Plowe”.
Chemical composition of Plasma Membrane:
Biochemical analysis of the plasma membrane showed that plasma membrane is chemically formed of these chemical which are giving below:
Lipid : 42% Most common lipid components are phospholipid (55-70% of which main phospholipid is lecithin). Other fatty substances are sterols example of sterol is cholesterol, sphingolipids, glycolipids and cerebrosides.
Proteins : 58-59%.
Carbohydrates : About 1% (mainly hexose, hexosamine, sialic acid, fructose).
Enzymes : Enzymes types (the most common enzyme is ATPase which helps in active transport). Other commonly found enzymes are phosphatases, nucleases etc.
Structure of Plasma Membrane:
Different models for explaining the structure of plasma membrane has been put forward from time to time depending upon the information available at a particular time. Some of them are as following:
1. Lipid and lipid bilayer models:
It was proposed by ‘Ernest Overton‘ in 1900. He was the first to suggest the existence of a lipid membrane in the cell. He proposed that cell membranes are composed of one layer of lipids. “ Evert Gorter ” and “François Grendel” in 1925 extracted lipids from RBC (erythrocytes) ghost (haemdysed empty RBC) and suggested that cell membrane are composed of two layers of liquid in which hydrophobic interface inwardly.
2. Lamellar models:
These models are based on the presence of both proteins and lipids as distinct layers in cell membrane.
(i) Sandwich Model:
“Danielli” and “Davson” on the basis of their surface tension and physiological studies suggesting that a cell membrane has four layers of molecules, two central of phospholipids and two external of proteins as P-b-L-P. These two layers of phospholipids are sandwiched on either side by a layer of hydrated alpha-globular proteins. The non-polar hydrophobic ends of the two lipid layers are towards the centre.
(ii) Unit Membrane Model:
J. David Robertson studied the ultrastructure of a number of biomembranes (cell membrane, nuclear membrane etc.) and realized that all these membranes are similar in their ultrastructure except in their thickness. The thickness depends upon the proportion and types of phospholipids and relative arrangement of phospholipids and proteins. This gives specificity to each membrane. He coined the term ‘unit membrane’ and stated that each biomembrane is formed of one (e.g. plasma membrane, lysosomal membrane) or more unit membrane (e.g. mitochondrial membrane , nuclear membrane and plastidial membranes).
This model states that biomembrane is a dipoproteinous and trilaminar. The proportion of lipids to proteins varies from 1 : 0-08 to I : 4. Protein molecules are extended (beta-proteins or fibrous) and not-globular. Each phospholipid is a polar molecule with a hydrophilic head of glycerol and hydrophobic tail of two fatty acid chains. Heads of phospholipid molecules are directed in opposite directions while tails of two layers face each other. Average thickness of biomembranes is 75-100A in which each protein layer is 15-30A while phospholipid bilayer is 35-45A. Out of phospholipid bilayer, the polar head is about 5A while the non-polar tails are about 25A thick.
3. Fluid Mosaic model:
Singer and Nicholson (1975) stated that plasma membrane is formed of “protein icebergs in a sea of lipids. It is lipoproteinous and trilaminar. Phospholipid bilayer is fluiding in nature in which phospholipid molecules show two types of movements.
(i) Transition movements (molecules change their position within the same layer).
(ii) Flip-flop movements (molecules of two layers can be interchanged).
Proteins are globular and are of two types;
(i) Extrinsic or peripheral proteins : These lie outside the outer and inner surfaces of phospholipid bilayer. These are loosely attached to phospholipid molecules so are easily separable. These form 70% of the proteins e.g. Acetyl-cholinestrase, ATPase etc.
(ii) Intrinsic or integral proteins : These are partially or completely embedded in the phospholipid bilayer. These are strongly held to phospholipid molecules so are not easily separable. These form 30% of the proteins e.g. cytochrome oxidase (mitochondrial membrane), rhodopsin (retinal rod cells) etc.
Phospholipid molecule is a polar molecule and has two specific ends – hydrophilic head of glycerol and hydrophobic tail of two fatty acid chains. Heads of phospholipid molecules of two layers are directed in opposite directions while tails of two layers face each other. Carbohydrate molecules (especially oligosaccharides) are also associated with the outer surface of cell membrane.
Structure of Plasma Membrane:
(i) Microcilli : They are thin projections from the free surface of plasma membrane. These increase the surface area.
(ii) Desmosome : Two lateral cells where unites, many fibres arise and spread themselves in cytoplasm. These are called desmosome.
(iii) Cilia : These are small projections at the outer surface of plasma membrane. They are divided into three parts –Stalk, Basal plate and Basal body.
Stalk has two central and nine peripheral fibres.
Functions of Plasma Membrane:
Plasma membrane or plasmalemma performs a variety of functions. It controls cellular semipermeability, resorption, excretion and secretion. cell membrane represents a selective pathway for transport of substances into and outside the cell, controlling cell form and cell activity. Besides transport, cell membrane also plays an important role in cell recognition and in making cell joints (connection between cells). These functions are as follows.
The osmosis is the diffusion of water molecules through cell membrane from low osmotic pressure to high osmotic pressure. Plasma membrane acts as a semi permeable membrane allowing the movement of water molecules in and out.
Pinocytosis and Phagocytosis:
Pinocytosis and Phagocytosis are invagination of plasma membrane to form a vacuole, which passes into the cell to form food vacuole. The “ingestion of solid particles by a cell through cell membrane is called as phagocytosis, while in pinocytosis, the liquid substances are incorporated into the cell by plasma membrane.
In active transport, the movement of molecules is against the concentration, gradient, tie. the molecules or ions move from lower concentration towards higher concentration. But in passive transport, movement of molecules ions from higher to lower concentration and here no energy is required
Plasma membrane reduces the surface tension.
Lipid molecules give membrane the electrical property.
Biogenesis (or Origin of Plasma Membrane):
Two interpretations have been put forward to explain the origin of plasma membrane.
- The plasma membrane is regarded to be an independent organelle which enlarges autonomously with the growth of the cell.
- More accepted view-The plasma membrane is formed as a result of self assembly of its chemical components, the lipids and the proteins. The monomers or the building block of lipids and proteins from the cytoplasm undergo polymerization forming large molecules. These macromolecules assemble together and organize to form the plasma membrane.
The above theory gets support from the observation that a new plasma membrane is formed wherever it is destroyed.
Transport across Cell Membrane:
Permeability and Transport:
The transport of a given substance across the plasma membrane depends upon : (i) the size of pores in the plasma membrane and (ii) the size and charge of the molecules of that substance. A membrane is said to be:
(i) Permeable to a given substance if that substance passes readily through the membrane.
(ii) Impermeable if the substance does not pass through the membrane.
(iii) Semipermeable (or selectively permeable) if the membrane allows some but not all to pass through it.
All biological membranes are selectively permeable. Responding to the environmental conditions (or cellular needs), the cell membrane may act as a barrier to a particular substance at one time but may actively promote its passage some other time. By regulating chemical traffic into the cell, the plasma membrane performs three main functions:
(i) Controls intracellular physiological conditions ideal for cell functioning.
(ii) Maintains variation in the concentration of substances and ions inside intracellular fluid and outside (extracellular fluid) of the cell.
(iii) Maintains balance between osmotic pressure of intracellular fluid and interstitial fluid.
Transport of substances across plasma membrane:
Transport of substances across plasma membrane into the cell can be achieved by following methods:
A. Transport of water by 1. Osmosis
B. Transport of ions and molecules by 2. Passive Transport
(i) Simple diffusion
(ii) Facilitated diffusion
C. Transport of large molecules by 4. Endocytosis
D. Transport of solid particles by (i) Pinocytosis
E. Release of substances by 5. Eneiocytosis.
If you are interested in biology also read this post – Structure of a Sperm Cell, Essay on Spermatogenesis