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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