ABSTRACT
Prokaryotes are the most numerous and widespread organisms on earth, and are so classified because they have no defined membrane-bound nucleus. Prokaryotes generally range in size from 0.1 to 10 μm, and have one of three basic shapes: spherical, rod-like or helically coiled. To protect the cell from mechanical injury and osmotic pressure, most prokaryotes are surrounded by a rigid 3–25 nm thick cell wall. Prokaryotes comprise two separate but related groups: the bacteria and the archaea. The archaea mainly inhabit unusual environments such as salt brines, hot acid springs, bogs and the ocean depths, and include the sulfur bacteria and the methanogens, although some are found in less hostile environments. Many bacterial cells have one or more tail-like appendages known as flagella. By rotating their flagella, bacteria can move through the extracellular medium towards attractants and away from repellents, so called chemotaxis.
