The tough, firm, thick and elastic membrane inside the capsule layer of bacteria is called cell wall. It is inert in nature. It is composed of mucopeptide or peptidoglycan or murine. However, cell walls also contain proteins, lipids and polysaccharides. It contains some amount of muramic acid and teichoic acid. It is 10-50 millimicrons thick. The cell wall has several pores at places. Each pore is 1 millimicron in diameter. Exchange of essential substances takes place through these pores. It gives specific shape to cells and protects them from external injury.