Each cell is bounded by an extremely delicate, thin, elastic, trilaminar living membrane, celled plasma membrane. It is about ten thousandth of a millimeter ion thickness. In plant cells it is present on the inner side of the cell wall. Plasma membrane is made up of two layers of lipid molecules with protein molecules sandwiching it and embedded in it. It is a selectively permeable membrane which allows the flow of some substances into the cell and out of the cell.
Thus, it helps in maintaining the distinct chemical composition of the cell. Besides protecting the internal structures, it also gives shape and rigidity to cell. Plasma membrane has extremely small pores whose opening and closing are dependent upon the metabolic activities of the cell.
The following functions are performed by cell membrane or plasma membrane:
1. It gives a definite shape to the cell.
2. It provides protection to the internal contents of the cell.
3. It is selectively permeable.
4. It can take in solid and liquid materials by in folding.
5. In animal cells, it is involved in the formation of vesicles, cilia, flagella, microvilli, etc.