A key feature of nearly all prokaryotic cells is the cell wall, which maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment.
The cell walls of prokaryotes differ in structure from those of eukaryotes. In eukaryotes that have cell walls, such as plants and fungi, the walls are usually made of cellulose or chitin. In contrast, most bacterial cell walls contain peptidoglycan, a polymer composed of modified sugars cross-linked by short polypeptides.
Using a technique called the Gram stain, developed by the 19th-century Danish physician Hans Christian Gram, scientists can categorize many bacterial species according to differences in cell wall composition.
Gram-positive bacteria have simpler walls with a relatively large amount of peptidoglycan. Gram-negative bacteria have less peptidoglycan
and are structurally more complex, with an outer membrane
that contains lipopolysaccharides (carbohydrates bonded
to lipids).
The cell wall of many prokaryotes is surrounded by a sticky layer of polysaccharide or protein. This layer is called a capsule.
In another way of withstanding harsh conditions, certain bacteria develop resistant cells called endospores when they lack an essential nutrient. The original cell produces a copy of its chromosome and surrounds that copy
with a tough multilayered structure, forming the endospore. some prokaryotes stick to their substrate or to one another by means of hair like appendages called fimbriae (singular, fimbria) Fimbriae are usually shorter and more numerous than pili (singular, pilus), appendages that
pull two cells together prior to DNA transfer from one cell to the other.
pili are sometimes referred to as sex pili.
About half of all prokaryotes are capable of taxis, a directed movement toward or away from a stimulus. For example, prokaryotes that exhibit chemotaxis change their movement pattern in response to chemicals. They may move toward nutrients or oxygen (positive chemotaxis) or away from a toxic substance (negative chemotaxis).
The bacterial flagellum has three main parts (the motor, hook, and filament) that are themselves composed of 42 different kinds of proteins.
The cells of prokaryotes are simpler than those of eukaryotes
in both their internal structure and the physical arrangement
of their DNA. The genome of a prokaryote is structurally different from
a eukaryotic genome and in most cases has considerably less DNA. Prokaryotes generally have circular chromosomes, whereas eukaryotes have linear chromosomes. In addition, in prokaryotes the chromosome is associated with many fewer proteins than are the chromosomes
of eukaryotes. Also unlike eukaryotes, prokaryotes lack a nucleus; their chromosome is located in the nucleoid, a region of cytoplasm that is not enclosed by a membrane. In addition to its single chromosome, a typical prokaryotic cell may also have much smaller rings of independently replicating DNA molecules called plasmids most carrying only a few genes.
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