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Structure and Functions of Plasma Membrane
Nature and Occurrence
Most cellular organelles such as mitochondria, chloroplasts, lysosomes, peroxisomes, Golgi apparatus, nucleus, and endoplasmic reticulum, are all enclosed by the unit membrane. The cell surface membrane or plasma membrane is the outer covering of each cell. It is present in the cells of plants, animals, and microorganisms.
Plasma Membrane Structure
The plasma membrane is a living, thin, delicate, elastic, selectively permeable membrane. It is about 7 nm (70A) thick. Under a light microscope, this merely appears as a single line. However, the development of electron microscopes has made it possible to investigate the detailed structure of biological membranes (i.e., plasma membrane and another membrane of cellular organelles).
Chemical analysis has shown the membrane to be 75 percent phospholipids. In addition, the membrane contains proteins, cholesterol, and polysaccharides. However, it is the phospholipids that form key elements in the structure of the plasma membrane.
In 1972 Singer and Nicolson suggested a model, called the fluid mosaic model, to explain the ultrastructure of the plasma membrane or any other membrane of the cell. According to them plasma membrane is made up of a bilayer (two molecule thick layers) of phospholipids. Two types of protein molecules ‘floated about’ in the fluid phospholipid layer: Intrinsic proteins, which completely cover the lipid bilayer, and extrinsic proteins, which occur either on the outer surface or on the inner surface of the lipid membrane. The fluid mosaic model of the membrane has been described as “a number of protein icebergs floating in the sea of lipids”. This model is the most accepted one, as it describes both properties and organization of the membrane.
The proteins are present not to give strength to the membrane but to serve as
- enzymes (catalyse chemical reactions within the membrane)
- transport proteins or permeases (for movement of water-soluble ions)
- pumps (for active transport)
- receptor proteins (for endocytosis).
The presence of lipids and proteins provides flexibility to the plasma membrane. This property of flexibility of the plasma membrane helps in endocytosis.
Advantages of Selective Permeability
Selective permeability of plasma membrane ensures that
- The useful molecules enter the cell
- The metabolic intermediates remain within the cell, and
- Secretions and wastes leave the cell.
Thus, selective permeability of the cellular membranes enables the cell to maintain homeostasis, i.e., a constant internal environment in spite of the changes outside it.
The substances generally drawn in the cell include:
- raw materials for metabolism, i.e., food, water, salts, and oxygen;
- regulatory substances, e.g., vitamins and hormones.
The substances generally ousted of the cells include:
- the products of metabolism, namely nitrogenous wastes and carbon dioxide; and
- secretions such as proteins, proenzymes, hormones, milk, tear, mucus, immunoglobulins (antibodies), etc.
Functions of Plasma Membrane
The plasma membrane permits the entry and exit of some materials in the cells. Therefore, the plasma membrane is called a selectively permeable membrane. Let us see how the movement of substances takes place in and out of a cell.
Studies on the function of the plasma membrane have shown that it performs certain physical activities, such as diffusion and osmosis for the intake of some substances. Also, certain biological or physiological activities such as active transport and endocytosis are performed by the plasma membrane.