BSC 310 Lecture Notes - Lecture 9: Methylene Blue, Dental Plaque, Basal Body
Prokaryotic Cells
The characteristics of prokaryotic cells apply to the bacteria and cyanobacteria
(formerly known as blue green algae), as well as to the rickettsiae, chlamydiae, and ‐
mycoplasmas.
Size and shape.Prokaryotes are probably the smallest living organisms, ranging in size
from 0.15 m (mycoplasmas) to 0.25 m (chlamydiae) to 0.45 m (rickettsiae) to about μ μ μ
2.0 m (many of the bacteria). Certain prokaryotes, such as bacteria, occur in spherical μ
forms called cocci (singular, coccus) or in rodlike forms
called bacilli (singular, bacillus). Some bacteria have a comma shape ( vibrio), or a
flexible, wavy shape ( spirochete), or a corkscrew shape ( spirillum).
Some prokaryotes have a variety of shapes and sizes and are said to
be pleomorphic. Rickettsiae and mycoplasmas are examples of pleomorphic
microorganisms.
When certain prokaryotes divide, they cling to each other in a distinct arrangement.
A diplococcus, for example, consists of a pair of cocci, while a streptococcus consists
of a chain of cocci, and a tetracoccus consists of four cocci arranged in a cube. A
grapelike cluster of cocci is called a staphylococcus. Bacilli sometimes form long
chains called streptobacilli.
The cell wall and cell membrane. With the exception of mycoplasmas, all bacteria
have a semirigid cell wall. The cell wall gives shape to the organisms and prevents
them from bursting, especially since materials in the cytoplasm exert osmotic
pressures.
The chief component of the prokaryotic cell wall is peptidoglycan, a large polymer
composed of N acetylglucosamine and N acetylmuramic acid. Gram positive bacteria ‐ ‐ ‐
have more peptidoglycan in their cell wall, which may account for their ability to retain
the stain in the Gram stain procedure. Gram negative bacteria have more lipids in their ‐
cell wall. Polymers of teichoic acid are commonly associated with the peptidoglycan in
Gram positive bacteria.‐
In addition to the cell wall, Gram negative bacteria have a very thin surrounding layer ‐
called the outer membrane. Lipopolysaccharides known as endotoxins are part of this
outer membrane. A space called the periplasmic space separates the cell wall from
the outer membrane and contains a substance called periplasm.
All prokaryotes have cytoplasm surrounded by a cell membrane, also known as
the plasma membrane. The cell membrane conforms to the fluid mosaic model, which
means that its proteins float within a double layer of phospholipids. Respiratory
enzymes are located at the cell membrane of prokaryotes, and the membrane assists
DNA replication and has attachment points for bacterial flagella.
The cytoplasm. The cytoplasm of prokaryotic cells contains ribosomes and various
other granules used by the organism. The DNA is contained in the nuclear region
(the nucleoid) and has no histone protein to support it. Prokaryotic cells have in their
cytoplasm a single, looped chromosome, as well as numerous small loops of DNA
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Document Summary
The characteristics of prokaryotic cells apply to the bacteria and cyanobacteria (formerly known as blue green algae), as well as to the rickettsiae, chlamydiae, and mycoplasmas. Size and shape. prokaryotes are probably the smallest living organisms, ranging in size from 0. 15 m (mycoplasmas) to 0. 25 m (chlamydiae) to 0. 45 m (rickettsiae) to about. Certain prokaryotes, such as bacteria, occur in spherical forms called cocci (singular, coccus) or in rodlike forms called bacilli (singular, bacillus). Some bacteria have a comma shape ( vibrio), or a flexible, wavy shape ( spirochete), or a corkscrew shape ( spirillum). Some prokaryotes have a variety of shapes and sizes and are said to be pleomorphic. Rickettsiae and mycoplasmas are examples of pleomorphic microorganisms. When certain prokaryotes divide, they cling to each other in a distinct arrangement. A diplococcus, for example, consists of a pair of cocci, while a streptococcus consists of a chain of cocci, and a tetracoccus consists of four cocci arranged in a cube.
