The cytoplasm of bacterial cells is enclosed by the cell, or plasma, membrane that separates and protects the internal environment of the cell from the external environment. It also regulates what enters and exits the cell.
The plasma membrane is composed of a phospholipid bilayer (two layers) embedded with proteins and other molecules. The phospholipids are not fixed relative to each other and able to flow past each other, making the membrane fluid.
Phospholipids are molecules composed of two long, hydrocarbon “tails” and a phosphate group “head.” The hydrophilic (water-attracted) phosphate group “heads” form the internal and external surface of the plasma membrane. The hydrophobic (water-repelled) tails form the interior of the plasma membrane.
Plasma Membrane Proteins
Embedded in the plasma membrane are proteins that perform multiple functions, including the detection of chemicals in the cell's environment and the transportation of materials into and out of the cell.
Most bacteria have an external cell wall composed of the chemical peptidoglycan (murein). The cell wall strengthens the plasma membrane and prevents the cell from bursting.
Not all bacteria possess a peptidoglycan cell wall. Chlamydia lacks peptidoglycan in its cell wall, and the mycoplasmas lack a cell wall.