Cell Envelope and its Modifications

Most prokaryotic cells, and bacterial cells in particular, are wrapped in a chemically complex cell envelope. Although this envelope is built from several layers, they work together as one protective unit, and the outer covering can be modified in ways that give bacteria extra abilities.

The three-layered cell envelope

The envelope is a tightly bound structure of three layers, from outside inwards:

1. the glycocalyx (outermost)
2. the cell wall
3. the plasma membrane

Each layer performs its own distinct function, yet the three act together as a single protective coat.

Gram positive and Gram negative bacteria

Bacteria can be sorted into two groups according to differences in their cell envelopes and how they respond to a staining method developed by Gram:

• Gram positive bacteria take up the Gram stain.
• Gram negative bacteria do not take up the Gram stain.

The glycocalyx and its forms

The glycocalyx varies in composition and thickness from one bacterium to another. It may be:

• a loose sheath called the slime layer, or
• a thick, tough layer called the capsule.

The cell wall

The cell wall determines the shape of the cell and gives it strong structural support, preventing the bacterium from bursting or collapsing.

The plasma membrane

The plasma membrane is selectively permeable and interacts with the outside world. In structure it is similar to the plasma membrane of eukaryotic cells.

The mesosome

A special membranous structure, the mesosome, is formed by extensions of the plasma membrane pushing into the cell. These extensions take the form of vesicles, tubules and lamellae, and they help with several processes:

• cell wall formation
• DNA replication and the distribution of DNA to daughter cells
• respiration
• secretion
• increasing the surface area of the plasma membrane and boosting its enzymatic content

Chromatophores

In some prokaryotes such as cyanobacteria, there are additional membranous extensions into the cytoplasm called chromatophores, which contain pigments.

Flagella and motility

Bacterial cells may be motile or non-motile. Motile bacteria bear thin, thread-like extensions from the cell wall called flagella. The number and arrangement of flagella vary among bacteria. A bacterial flagellum has three parts: …