The Cell Envelope and Its Modifications in Prokaryotes

The outer boundary of a typical bacterial cell is not a single simple layer but a chemically complex structure made up of three distinct layers working together as a single protective and functional unit: an outer glycocalyx, a middle cell wall, and an inner plasma membrane. Collectively, these three layers are referred to as the cell envelope, and in many bacteria they are further extended into specialised structures such as mesosomes, flagella, pili and fimbriae.

The outermost layer, the glycocalyx, is built mainly of polysaccharides and varies considerably in its thickness and organisation from one species to another. In some bacteria it forms a thick, tightly organised, well-defined layer called a capsule; in others it forms a much looser, more diffuse, easily washed-off layer called a slime layer; and in still others it may be only loosely organised or barely present at all. Where a substantial capsule is present, it often helps protect the bacterium from desiccation and from being engulfed by the immune cells of a host organism.

The middle layer, the cell wall, lies just beneath the glycocalyx and is chiefly responsible for determining the shape of the cell and for providing it with the structural strength needed to withstand internal osmotic (turgor) pressure -- without a cell wall, a bacterial cell placed in a dilute (hypotonic) medium would take up water and burst. In most bacteria the wall is built from peptidoglycan (also called murein), a mesh-like polymer of repeating sugar units cross-linked by short chains of amino acids. The relative thickness of this peptidoglycan layer, and whether it is further covered by an outer membrane, is the structural basis of the Gram-staining reaction widely used to classify bacteria into Gram-positive and Gram-negative groups.

The innermost layer is the plasma membrane, chemically similar in its basic organisation to the plasma membrane of a eukaryotic cell (described later in this chapter), and acting as the selectively permeable interface across which the cell exchanges materials with its environment.

In addition to these three basic layers, the plasma membrane of many bacteria is further extended and folded inward into the cytoplasm to form specialised structures called mesosomes -- formed as extensions of the plasma membrane in the shape of vesicles, tubules, or lamellae (sheet-like folds). Because they greatly increase the surface area of membrane available within the cell, mesosomes are believed to assist with a range of functions: they may help in the formation of the new cell wall during cell division, in the replication of DNA and its orderly distribution to the two daughter cells, in respiration, in secretion, in increasing the surface area for enzyme activity, and in the formation of resistant spores under unfavourable conditions. …