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 μm (mycoplasmas) to 0.25 μm (chlamydiae) to 0.45 μm (rickettsiae) to about 2.0 μm (many of the bacteria). 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. 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 called plasmids. Genetic information in the plasmids is apparently not essential for the continued survival of the organism.
Prokaryotic ribosomes contain protein and ribonucleic acid (RNA) and are the locations where protein is synthesized. Prokaryotic ribosomes have a sedimentation rate of 70S, and are therefore known as 70S ribosomes. (Eukaryotic cells have 80S ribosomes.) Certain antibiotics bind to these ribosomes and inhibit protein synthesis.
Some prokaryotic cells that engage in photosynthesis have internal membranes called thylakoids where their chlorophyll pigments are located. These membranes are also the sites of enzymes for photosynthesis. Certain bacteria have granules of phosphorus, starch, or glycogen. Granules called metachromatic granules stain with methylene blue and are used in diagnostic circumstances. Some bacterial species also have magnetosomes, which contain magnetic substances to help orient the organisms to hospitable environments.
External cellular structures. Many prokaryotic cells have at their surface a number of external structures that assist their functions. Among these structures are flagella. Flagella are found primarily in bacterial rods and are used for motility. A bacterium may have a single flagellum (a monotrichous bacterium), or flagella at both ends of the cell (an amphitrichous bacterium), or two or more flagella at one end of the cell (a lophotrichous bacterium), or it may be surrounded by flagella (a peritrichous bacterium).
Flagella are long, ultrathin structures, many times the length of the cell. They are composed of the protein flagellin arranged in long fibers. A hooklike structure and basal body connect the flagellum to the cell membrane. Flagella rotate and propel the bacteria.
Spirochetes lack flagella, but they possess axial filaments. The axial filaments extend beyond the cell wall and cause the spirochete to rotate in a corkscrew fashion and thereby move.
Some bacterial species have projections called pili (singular, pilus). Pili are used for attachments to surfaces such as tissues. Many pathogens possess pili, which are composed of the protein pilin. Certain pili, known as conjugation pili, unite prokaryotic cells to one another and permit the passage of DNA between the cells. The term fimbriae is often used for the attachment pili.
Many bacteria, especially pathogens, are enclosed at their surface by a layer of polysaccharides and proteins called the glycocalyx. The glycocalyx, composed of a thick, gummy material, serves as a reservoir for nutrients and protects the organism from changes in the environment. When the glycocalyx is a tightly bound structure, it is known as a capsule. When it is a poorly bound structure that flows easily, it is known as a slime layer. The material in dental plaque is composed largely of the material from the slime layer.
Endospores. Bacteria of the genera Bacillus and Clostridium are able to form highly resistant internal structures called endospores, or simply spores. Spores are formed during the normal life cycle when the environment becomes too harsh (Figure ).