Structure and Functions of Bacterial Cell Envelope
The outer layer or cell envelope provides a structural and physi-ological barrier between the protoplasm (inside) of the cell and the external environment. The cell envelope protects bacteria against osmotic lysis and gives bacteria rigidity and shape. The cell envelope primarily consists of two components: a cell wall and cytoplasmic or plasma membrane. It encloses the proto-plasm, which consists of (i) cytoplasm, (ii) cytoplasmic inclu-sions (mesosomes, ribosomes, inclusion granules, vacuoles), and (iii) a single circular DNA (Fig. 2-7).
Cell membrane or plasma membrane is a thin (5–10 nm) semipermeable
membrane that acts as an osmotic barrier. It lies beneath the cell wall
separating it from the cell cytoplasm. Cell membrane primarily contains
phospholipids and proteins. It also contains enzymes associated with DNA
biosynthesis, cell wall polymers, and membrane lipids. Bacterial plasma
membranes usually have a higher proportion of protein than eukaryotic
membranes. They usually differ from eukaryotic membranes in lacking sterols,
such as cholesterol, except in Mycoplasma.
The cell membrane has following functions:
It acts as a semipermeable membrane regulating the inflow and
outflow of metabolites to and from the protoplasm.
It helps in electron transport and oxidative phosphorylation.