Prokaryotes

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and other membrane-bound organelles. They form the two domains Bacteria and Archaea. Most prokaryotes are microscopic, typically 0.2 to several micrometers in size. Their genetic material is usually a single circular chromosome located in the nucleoid, and many also carry small DNA molecules called plasmids.

Prokaryotic cells have various cell envelopes. Bacteria commonly have cell walls containing peptidoglycan, while archaeal cell walls are composed of other polymers such as pseudopeptidoglycan. The plasma membrane holds essential metabolic processes, including respiration and photosynthesis in certain groups.

Prokaryotes reproduce primarily by binary fission, an asexual process. Genetic diversity arises through horizontal gene transfer, including transformation (uptake of DNA), transduction (gene transfer by viruses), and conjugation (cell-to-cell transfer).

Metabolically, prokaryotes display immense diversity. They include photoautotrophs that capture light energy, chemoautotrophs that fix inorganic carbon, and heterotrophs that rely on organic compounds. Some perform oxygenic photosynthesis; others fermentation or anaerobic respiration.

Ecologically, prokaryotes drive nutrient cycles, decompose organic matter, and form symbiotic relationships with other organisms. They inhabit nearly every environment, from oceans to soils to extreme habitats. They include both beneficial organisms and pathogens.

Evolutionarily, prokaryotes are older than eukaryotes and lack a nucleus and membrane-bound organelles. Their ribosomes are smaller (70S) than those of eukaryotes. The endosymbiotic theory describes how mitochondria and plastids arose from ancient prokaryotes that entered into mutualistic relationships with early eukaryotic cells.