The Structure Of Prokaryotes
An example of a typical prokaryote is the bacterial cell. Bacterial cells can be shaped like rods, spheres, or corkscrews. All prokaryotes are bounded by a plasma membrane. Overlying this plasma membrane is a cell wall, and in some bacteria, a capsule consisting of a jelly-like material overlies the cell wall. Many bacteria that cause illness in animals have capsules. The capsule provides an extra layer of protection for the bacteria, and often pathogenic bacteria with capsules cause much more severe disease than those without capsules.
Within the cytoplasm of prokaryotes is a nucleoid, a region where the genetic material (DNA) resides. This nucleoid is not a true nucleus because it is not bounded by a membrane. Also within the cytoplasm are numerous ribosomes. These ribosomes are not attached to any structure and are thus called "free" ribosomes.
Attached to the cell wall of some bacteria are flagella, whip-like structures that provide for movement. Some bacteria also have pili, which are short, finger-like projections that assist the bacteria in attaching to tissues. Bacteria cannot cause disease if they cannot attach to tissues. Bacteria that cause pneumonia, for instance, attach to the tissues of the lung. Bacterial pili greatly facilitates this attachment to tissues, and thus, like capsules, bacteria with pili are often more virulent than those without.
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