Plasma membrane or cell membrane is the outer most boundary of the cell. However, in most plant cells, it is covered by a cell wall. Cell membrane is chemically composed of lipids and proteins; 60 – 80% are proteins, while 20-40% are lipids. In addition there is a small quantity of carbohydrates.
Fig. 4.4 Unit membrane
Many biologists contributed to establish the structural organization of cell membrane. It was proposed earlier that cell membrane is composed of lipid bilayer sandwiched between inner and outer layers of protein (Fig.4.4). this basic structure is called the nut membrane and is present in all the cellular organelles. Te modern technology has revealed that lipid bilayers are not sandwiched between two protein layers.
The protein layers are not continuous and are not confined to the surface of the membrane but are embedded in lipid layers in a mosaic manner (fig. 4.5). this discovery led to the proposal of Fluid Mosaic Model. This model at present is the most accepted one. Cell membrane also contains charged pores through which movement of materials takes place, both by active and passive transport.
Fig. 4.5 Fluid Mosaic Model
Transport of materials is one of the vital roles it plays for the cell. It offers a barrier between the cell contents and their environment, allowing only selective substances to pass through it, thus it is known as differentially permeable or selectively permeable membrane. The substances which are lipid soluble cross it more easily than others, therefore, it regulates the flow of materials and ions to maintain a definite gradient. Many small gas molecules, water, glucose etc. being neutral can easily cross while ions, being charged particles, have some difficulty in crossing. Many substances which are not needed, constantly enter the cell by passive transport, ethers are taken up against the concentration gradient (they move from the area of low concentration to the area of high concentration). This uphill movement of materials requires energy and is termed as active transport. The energy used for this movement is provided by ATP.
In many animal cells, the cell membrane helps to take in materials by infolding in the form of vacuoles. This type of intake is termed as endocytosis which can be either phagocytosis (to engulf solid particles) or pinocytosis (to take in liquid material). In neurons (nerve cells) the cell membrane transmits nerve impulses from one part of the body to the other to keep coordination.