Lipoteichoic acids anchor the cell wall to the cell membrane. Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Up to 90 percent of the cell-wall in Gram-positive bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic acids. Gram-positive organisms typically lack the outer membrane found in Gram-negative organisms ( Figure 22.16). The different bacterial responses to the staining procedure are ultimately due to cell wall structure. The Gram staining method is named after its inventor, Danish scientist Hans Christian Gram (1853–1938). Note that all Gram-positive bacteria belong to one phylum bacteria in the other phyla (Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others) are Gram-negative. S-layer (surface layer) proteins are also present on the outside of cell walls of both Archaea and Bacteria.īacteria are divided into two major groups: Gram positive and Gram negative, based on their reaction to Gram staining. (Proteins normally have only L-amino acids as a consequence, many of our antibiotics work by mimicking D-amino acids and therefore have specific effects on bacterial cell-wall development.) There are more than 100 different forms of peptidoglycan. The chemical composition of the cell wall varies between Archaea and Bacteria, and also varies between bacterial species.īacterial cell walls contain peptidoglycan, composed of polysaccharide chains that are cross-linked by unusual peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine. It is located outside the cell membrane and prevents osmotic lysis (bursting due to increasing volume). The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. Therefore, the osmotic pressure within the cell is relatively high. The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Second, an ether bond instead of an ester bond connects the lipid to the glycerol. First, they have branched phytanyl sidechains instead of linear ones. Archaeal phospholipids differ from those found in Bacteria and Eukarya in two ways. Some archaeal membranes are lipid monolayers instead of bilayers ( Figure 22.15).įigure 22.15 Bacterial and archaeal phospholipids. In archaeal cell membranes, isoprene (phytanyl) chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Recall that the general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. Its selectively permeable nature keeps ions, proteins, and other molecules within the cell and prevents them from diffusing into the extracellular environment, while other molecules may move through the membrane. The prokaryotic plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the cell and separates the inside from the outside. of Energy scale-bar data from Matt Russell) The Plasma Membrane of Prokaryotes Stetter credit “Korarchaeota”: modification of work by Office of Science of the U.S. (credit “Halobacterium”: modification of work by NASA credit “Nanoarchaeotum equitans”: modification of work by Karl O. Archaea are separated into four phyla: the Korarchaeota, Euryarchaeota, Crenarchaeota, and Nanoarchaeota. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.įigure 22.14 Archaeal phyla. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces including the surfaces of other cells. The capsule enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some bacterial species have a capsule outside the cell wall. The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Most prokaryotes have a cell wall outside the plasma membrane. Their chromosome-usually single-consists of a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Recall that prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal membrane-bound structures ( Figure 22.10). David Cox scale-bar data from Matt Russell) The Prokaryotic Cell Richard Facklam, CDC credit c: modification of work by Dr. (credit a: modification of work by Janice Haney Carr, Dr. Prokaryotes fall into three basic categories based on their shape, visualized here using scanning electron microscopy: (a) cocci, or spherical (a pair is shown) (b) bacilli, or rod-shaped and (c) spirilli, or spiral-shaped. Figure 22.9 Common prokaryotic cell types.
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