Prokaryotes are a type of cell lacking a nucleus and other membrane-bound organelles.
The first cells on Earth were single-celled life forms and may have been prokaryotes. Prokaryotes are divided into two domains: Archaea and Bacteria. Scientists estimate that a 30-gram sample of humus-rich forest soil contains 1 million species of bacteria. Bacteria live on every continent and in every extreme environment, from the hottest, driest desert to the bitter cold wastelands of Antarctica. In addition to the basic cellular components, prokaryotic cells have other structures that help them survive and reproduce. Prokaryotes, being small and one-celled, can be quickly impacted by changes in their environment. Protective structures help prokaryotes survive in a changing environment. Directly outside the cell membrane is a cell wall, a rigid carbohydrate structure that provides overall support and protection for the cell. Most prokaryotes have a cell wall. The capsule is a polysaccharide layer tightly adhered to the outside of the cell wall that prevents cell dehydration and helps the cell stick to surfaces. The capsule of pathogenic bacteria protects the bacteria from their host's immune system, which is triggered by the presence of foreign substances within an organism. Not all prokaryotes have a capsule. Unicellular organisms need structures to enable them to move. The flagellum (plural, flagella) is a threadlike tail that allows some cells to move. It propels prokaryotes through liquid, allowing aquatic prokaryotes to move around their environment. Movement enhances the possibility of finding food. A pilus (plural, pili) is a short, hairlike projection found on the surface of many bacteria that helps cells stick together. The pili can also stick to surfaces and help the prokaryote move.
The nucleoid is the area inside a prokaryotic cell where genetic material (DNA) is found. The chromosome transfers from one organism to another in a reproductive process called conjugation. During conjugation, the DNA is transferred via a specialized sex pilus that is longer than regular pili. Many prokaryotes have additional small circular DNA molecules called plasmids. These molecules replicate independently and can be exchanged between individual prokaryotes. Many plasmids carry genes that confer antibiotic resistance to the bacterium.