Membranes and Cell Walls

Like the mycoplasma, the E. coli cell is bounded by an 8-nm membrane which consists of ~50% protein and 50% lipid. When “stained” (e.g., with permanganate) for electron microscopy, this single membrane appears as two very thin (2.0 nm) dark lines separated by an unstained center band (~3.5 nm) (Fig. 1-4; see also Fig. 8-4). Single membranes of approximately the same thickness and staining behavior occur in all cells, both of bacteria and of eukaryotes. A cell membrane is much more than just a sack. It serves to control the passage of small molecules into and out of the cell. Its outer surface carries receptors for recognition of various materials. The inside surface of bacterial membranes contains enzymes that catalyze most of the oxidative metabolism of the cells. Bacterial cell membranes are sometimes folded inward to form internal structures involved in photosynthesis or other specialized reactions of metabolism such as oxidation
of ammonia to nitrate. 2 In E. coli replication of DNA seems to occur on certain parts of the membrane surface, probably under the control of membrane-bound enzymes. The formation of the new membrane which divides multiplying cells proceeds synchronously with the synthesis of DNA. A characteristic of true bacteria (eubacteria) is a rigid cell wall which surrounds the cell membrane. The 40-nm-thick wall of E. coli is a complex, layered structure five times thicker than the cell membrane. Its chemical makeup is considered in Chapter 8. One of the layers is often referred to as the outer membrane. In some bacteria the wall may be as much as 80 nm thick and may be further surrounded by a thick capsule or glycocalyx (slime layer). 13 The main function of the wall seems to be to prevent osmotic
swelling and bursting of the bacterial cell when the surrounding medium is hypotonic. If the osmotic pressure of the medium is not too low, bacterial cell walls can sometimes be issolved, leaving living cells bounded only by their membranes. Such protoplasts can be produced by action of the enzyme lysozyme on gram-positive bacteria such as Bacillusmegaterium. Treatment of cells of gram-negative bacteria with penicillin (Box 20-G) produces sphero lasts, cells with partially disrupted walls. Spheroplasts and protoplasts are useful in biochemical studies because substances enter cells more readily when the cell wall is absent. Strains of bacteria lacking rigid walls are known as L forms.