.obex phospholipid proteins murein gel membrane proteins cytoplasm
Fig. 1.30. Molecular structure of a prokaryote cell wall. (A) Cell wall structure of archea, a Gram-positive cell and a Gram-negative cell. The chemical composition of a Gram-negative cell wall. (B) Details of a Gram-negative cell wall.
cell wall subdivisions are between bacteria that stain with the Gram stain (Gram positive) and those that do not (Gram negative). Grampositive bacteria have a murein cell wall (0.015 ^m) outside the cell membrane which consists of peptidoglycans with embedded teichoic acid derivatives. It has an external surface layer of proteins or polysaccharides. The murein wall can be enriched with lipidic molecules to increase hydrophobicity, as in Corynobacteria, Mycobacteria and Nocardia. In the Planctobacteria, the murein layer is replaced by a pro-teinaceous cell wall. The second type of cell wall is found in Gramnegative bacteria. They have a loose murein layer (~0.015-0.020 ^m thick), but with an additional outer membrane. The loose murein gel of Gram-negative bacteria is interlinked by lipoproteins and contains proteins from the cytoplasm. The outer membrane is chemically different from the cell membrane and contains lipopolysaccharide extensions, structural proteins and porins. The porin proteins allow the passage of molecules smaller than 600 Da molecular weight into and out of the murein gel. Differences between the exact molecular composition of the cell wall, between families and higher taxa, are useful in species identification. The third type of cell wall is found in the archea, where the outer membrane and the murein wall are absent. In some genera, there is a wall outside the cell membrane, but it is not composed of murein.
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