The protoplasm of every living cell is enclosed by a plasma membrane. It holds true for both simple prokaryotic, as well as for the complex eukaryotic cells. This membrane not only serves as a protective covering for the cellular components, but also is a crucial structure for transportation of nutrients and communication between the cells. In order to explain the structure and functions of biological membranes, the fluid mosaic model was proposed in 1972 by the researchers, S.J. Singer and Garth Nicolson.
The model explains the structural components of biological membranes. Besides this hypothesis, several theories pertaining to the plasma membrane structure have been developed. But, none of them are as acceptable as the fluid mosaic model. According to it, the cell membrane contains different types of protein and carbohydrate molecules embedded in a phospholipid bilayer.
The plasma membrane is a unique component of both plant and animal cells. It serves as a barrier between the cell interior and its surrounding. With reference to the model, the structure of this biological membrane is such that it only allows entry and exit of certain substances. Hence, it is simply referred to as a semipermeable membrane. In addition to cellular transport, cell membrane functions include recognition, adhesion, and signaling of cells.
Regarding the term 'fluid mosaic model', the cell membrane is more like a fluid, rather than being a rigid or solid structure. This is because of the sideways and lateral movements of protein and lipid molecules throughout the membrane, as per requirements of the cell. Since the membrane contains various molecules (embedded protein, carbohydrate, cholesterol, etc), it is described as a mosaic. For your reference, the two integral components (lipid bilayer and proteins) of a cell membrane along with other substances are discussed below.
The Lipid Bilayer
In the membrane, the amphipatic lipid molecules arrange themselves in a specific manner. The phospholipid layer folds upon itself due to its hydrophobic nature. The result is a lipid bilayer with the polar and hydrophilic heads orienting outside, and the non-polar and hydrophobic tails pointing towards the inner side. Thus, the lipid bilayer is water repelling in nature, which allows the entry of only lipid soluble molecules. Formation of this bilayer is the base for the fluid mosaic model of the plasma membrane.
The integral membrane proteins are present within the cell membrane. As per the illustration of the model, almost 70% of the total proteins found in cell membrane are integral ones. Amongst these, there are large protein molecules that extend on both sides of the phopholipid matrix, and collectively, they are known as tunnel proteins. Since the integral proteins are present within the lipid bilayer, their extraction is not possible.
In contrary to the integral membrane proteins, peripheral ones are located at the periphery of the cellular membrane. If you analyze the model, you can identify them as those molecules that are projected slightly on the outer surface of the lipid bilayer. They are attached to the hydrophilic lipid heads by hydrogen bonds or electrostatic bonds. Since they are slightly exposed to the membrane parts of a cell, extraction of these proteins is possible through sophisticated laboratory procedures.
Lipids and Other Components
In addition to the integral and membrane proteins, another major component is lipid. It basically consists of glycolipid, phospholipid, and cholesterol. The percentage content of each of these components varies from one cell membrane to another. The overall plasma membrane functions (cell-to-cell recognition) depends on glycoproteins, and other carbohydrates present in the membrane.
In cell biology, the biological membrane anatomy and functions are studied in detail. And as we have seen, the structural components are explained by the fluid mosaic model of plasma membrane. The entry and exit of molecular substances, which are required for survival of a cell, are regulated by the cell membrane.