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CellGuide - Study Guide to Cell Structure I Prokaryotic...

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Study Guide to Cell Structure I. Prokaryotic Cells  [pro = before; karyose = kernel or nucleus] A. Plasma Membrane :  Each cell is enclosed within a typical Singer fluid mosaic membrane which is composed of a phospholipid bilayer  embedded with proteins and altered proteins.  The primary function of the plasma membrane is to serve as a diffusion barrier between the interior  of the cell and the environment.  The surface of the cell may have prokaryotic  flagella  and/or  pili .  In more advanced prokaryotic cells, the  plasma membrane may be invaginated to form respiratory structures called mesosomes , or photosynthetic structures called  thylakoids . B. Nucleoid  [oid = resembling]:  The nucleoid is a region in the center of the cell containing a large DNA molecule.  The DNA is naked (not  complexed with histone proteins) and circular, and is not properly called a chromosome.  The nucleoid may also contain a variety of smaller circlets  of DNA called plasmids .  The specific selection of plasmids varies from cell to cell.  The nucleoid is not surrounded by a nuclear envelope or any  other membrane. C. Cytoplasm  (cyte = cell; plasm = fluid ground substance]:  Cytoplasm is a rich organic soup composed primarily of water, and containing a  variety of ions, inorganic and organic molecules.  The major organelles in the cytoplasm of a prokaryotic cell are ribosomes .  Ribosomes are  responsible for the process of translation (protein synthesis). D. Prokaryotic cells contain no membrane bound organelles (other than mesosomes and thylakoids, which are actually invaginations of the plasma  membrane). E. Cell Wall :  The prokaryotic cell is enclosed within a cell wall formed from a substance called peptidoglycan .  Peptidoglycan is constructed  from a long polysaccharide (glycol = sugar) cross-linked by short oligopeptide chains.  Prokaryotic cells are generally divided into two major  categories based upon the complexity of the cell wall.  Gram positive cells have a relatively simple cell wall, while gram negative cells have a more  complex, layered cell wall.  (Gram positive and gram negative are derived from the response of the cell to a staining protocol called the gram stain;  the more complex walls of gram negative cells prevent the dye from penetrating and staining the cells.) F. Capsule , or Slime Layer:  Some prokaryotic cells are embedded in a mucous-like substance called a capsule.  The capsule material is  hygroscopic  (meaning that it holds water) and thus the capsule serves to keep the cell hydrated.
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