Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
Simplest Living Organisms
Membranes and Cell Walls

Like Mycoplasmas, E. coli Cells are enclosed by a thin (~8 nm) membrane composed of Proteins (~50%) and Lipids (~50%)¹. Under an Electron microscope, a stained membrane (e.g., with permanganate) appears as two extremely thin (~2.0 nm) dark lines separated by an unstained layer (~3.5 nm) (Fig. 1-2, B). Membranes of approximately this thickness and staining pattern are found in all cells, both bacterial and eukaryotic.

The Cell membrane is much more than a simple sack; it regulates The transport of low-molecular-weight substances into and out of the cell. In Bacteria, Enzymes that catalyze oxidation processes are associated with the inner surface of the membrane. Bacterial membranes frequently form infolded regions that appear in cross-section as multilayered structures, known as mesosomes (Fig. 1-1 and 1-2, D). It is believed that specialized metabolic processes and METABOLISM/36.html">DNA Replication take place within these mesosomes. Although mesosomes are not always detectable in E. coli cells, DNA replication in this Organism apparently occurs at specific sites on the membrane surface and is regulated by membrane-bound enzymes. The formation of a new membrane (septum) between dividing cells occurs synchronously with DNA Synthesis.

A defining characteristic of true bacteria is the presence of a rigid Cell wall surrounding the cell membrane. In E. coli, this wall is ~40 nm thick and has a complex multilayered Structure (The chemical composition of the E. coli cell wall is discussed in Chapter 5). In some bacteria, the wall thickness can reach 80 nm, and it may also be surrounded by a thick capsule or slime layer. The primary function of The cell wall appears to be the Prevention of osmotic Swelling and cell lysis when bacteria enter a salt-free environment. If the ambient osmotic pressure is properly maintained, the bacterial wall can be dissolved, leaving the cell enclosed solely by its membrane. Such protoplasts can be obtained by treating Gram-positive bacteria, such as Bacillus megaterium, with the enzyme Lysozyme. Protoplasts serve as convenient models for biochemical research because the absence of a cell wall allows many substances to penetrate the cell faster and more easily. The term spheroplast is generally used for Gram-negative bacterial cells with a partially degraded cell wall. Penicillin (which disrupts cell wall synthesis in many bacteria) converts E. coli and certain other Gram-negative bacteria into spheroplasts. Bacterial strains lacking rigid walls are referred to as L-forms.

¹ Membranes of true bacteria do not contain Cholesterol, yet, paradoxically, cholesterol is present in the membranes of most mycoplasmas, just as it is in humans. Members of the genus Mycoplasma lead a parasitic lifestyle and acquire the necessary cholesterol from their host organism.