Prokaryotic cells are generally smaller than eukaryotic cells, ranging from 0.25 × 1.2 m to
1.5 × 4 m. Each individual prokaryote is a single cell, but many types of prokaryotes are
usually seen in chains, small clusters, or even clusters containing hundreds of individuals.
# The plasma membrane encloses the cell, regulating the traffic of materials into and out of
the cell and separating it from its environment.
# The nucleoid contains the hereditary material (DNA) of the cell
The rest of the material enclosed in the plasma membrane is called the cytoplasm. The
cytoplasm is composed of two components: the more fluid cytosol and insoluble suspended
particles, including ribosomes.
* The cytosol consists mostly of water that contains dissolved ions, small molecules, and
soluble macromolecules such as proteins.
* Ribosomes are complexes of RNA and proteins about 25 nm in diameter. They are the
sites of protein synthesis.
CELL WALLS Most prokaryotes have a cell wall located outside the plasma membrane.
The rigidity of the cell wall supports the cell and determines its shape. The cell walls of
most bacteria, but not archaea, contain peptidoglycan, a polymer of amino sugars, cross-
linked by covalent bonds to form a single giant molecule around the entire cell. In some
bacteria, another layer—the outer membrane (a polysaccharide-rich phospholipid
membrane)—encloses the peptidoglycan layer. Unlike the plasma membrane, this outer
membrane is not a major permeability barrier.
Enclosing the cell wall in some bacteria is a layer of slime, composed mostly of
polysaccharides and referred to as a capsule. The capsules of some bacteria may protect
them from attack by white blood cells in the animals they infect. The capsule helps keep the
cell from drying out, and sometimes it helps the bacterium attach to other cells. Many
prokaryotes produce no capsule, and those that do have capsules can survive even if they
lose them, so the capsule is not essential to prokaryotic life.
We know that the flagella cause the motion of the cell because if they are removed, the cell
does not move.
Pili project from the surfaces of some groups of bacteria. Shorter than flagella, these
hairlike structures help bacteria adhere to one another when they exchange genetic
material, as well as to animal cells for protection and food.
Eukaryotic cells generally have dimensions up to 10 times greater than those of prokaryotes