Prokaryotic Cell

    Prokaryotic cells are composed of mostly ribosomes and DNA.  They lack a nucleus and membrane-bound organelles.  The cytoplasm of most prokaryotes is unstructured, although it has a fine granular appearance due to its many ribosomes.  Prokaryotic cells generally range in size from 0.5 to 2 micrometers.  The outer layers of the cell are a cell membrane and a cell wall.

    The membrane is formed by a lipid bilayer.  The bilayer consists of hydrophilic heads and hydrophobic tails.  As shown in the picture, the hydrophilic heads are on the outside of the lipid bilayer, and the hydrophobic tails point toward the inside.


    Above a cell wall can be seen in a more extensive form.

     Surrounding the cell membrane is the cell wall.  The wall gives the different prokaryotes their shape.  Many of the prokaryotes have rigid cell walls, some have flexible walls, and only the mycoplasms have no cell walls.  Because most bacteria are hypertonic in relation to their environment, they would burst without the cell walls.

    The cell walls are complex and contain many molecules not found in eukaryotic cells.  Except for the Archaebacteria, the cell walls contain complex polymers known as peptidoglycans.  The types of cell walls are determined by their ability to combine firmly with dyes such as gentian violet.  In gram-positive cells, cells with walls that combine with the dye, the wall consists of a homogeneous layer of peptidoglycans and  polysaccharides that ranges from 10 to 80 nanometers in thickness.  In gram-negative cells, cells with walls that do not combine with the dye, the wall consists of two layers; an inner peptidoglycan layer, only 2 to 3 nanometers thick, and an outer layer of lipoproteins and lipopolysaccharides arranged  in the form of a bilayer,  about 7 to 8 nanometers in thickness, similar in structure to the cell membrane.  The cell wall also affects other characteristics of the bacteria such as susceptibility to antibiotics.  Gram-positive cells will be more susceptible than gram-negative cells.

    In some bacteria, a gluey polysaccharide capsule is present outside the cell wall.  The function of the capsule is not clear, but it is involved with pathogenic activity in certain organisms.  For example the encapsulated form of Streptococcus pneumoniae is virulent, whereas the nonencapsulated form is generally nonvirulent.  It appears that the capsule may interfere with phagocytosis by host white blood cells.

    Some prokaryotes have flagella and pili.  Flagella are used to move the organism.  A flagellum is a long, slender hair-like extension that uses a whip-like motion to propel the organism.  Flagella are usually about 12 to 18 nanometers in length.  Pili are used to attach an organism to something, such as a food source, the surface of a liquid, or, in conjugating bacteria, one another.  There are usually hundreds of pili around a single bacterium.  Pili are usually about 4 to 35 nanometers in length.  Pili are rigid, cylindrical rods whereas flagella are flexible, cylindrical rods.
 

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