A prokaryote is an organism whose cells lack a membrane-bound nucleus. Prokaryotes comprise Bacteria and Archaea, two fundamentally distinct groups that share this broad cellular organization. They are predominantly unicellular, although cells may form colonies, filaments, or organized communities. Unlike eukaryotes, their genetic material is not enclosed within a nuclear envelope. The term describes a type of cellular organization rather than a single, exclusive evolutionary lineage. (openstax.org)
Cellular organization
Prokaryotic cells contain DNA, cytoplasm, a plasma membrane, and ribosomes that synthesize proteins. They generally lack the elaborate membrane-bound organelle system characteristic of eukaryotes. Their DNA occupies a region called the nucleoid, which is not bounded by a nuclear membrane. Most familiar prokaryotes are microscopic and smaller than typical eukaryotic cells; their high surface-area-to-volume ratio facilitates exchange of nutrients and wastes with the surroundings. (openstax.org)
Common shapes include spheres, rods, and spirals, but morphology alone does not establish evolutionary relationships. Many species possess a cell wall that maintains shape and helps prevent osmotic rupture. Bacterial walls generally contain peptidoglycan, whereas archaeal walls lack this material and may instead contain protein layers or other polymers. Some prokaryotes have protective capsules, surface-attachment structures, or appendages for movement. These features vary among species and are not universal defining characteristics. (openstax.org)
The absence of a nucleus does not mean that the cell interior is unorganized. Genetic material is compacted and spatially arranged, while membranes and molecular machinery carry out transport, biosynthesis, and energy conversion. Descriptions of prokaryotes as “simple” refer mainly to their broad architecture, not to an absence of biochemical specialization. (ncbi.nlm.nih.gov)
Genetic organization and reproduction
A prokaryotic genome commonly includes one main circular chromosome, but this arrangement is not universal: some species have multiple chromosomes or linear chromosomes. Additional genetic information may occur on plasmids, independently replicating DNA molecules that can carry traits such as specialized metabolic functions or resistance determinants. Chromosomal DNA is packaged with DNA-binding proteins rather than existing as an unstructured strand. (ncbi.nlm.nih.gov)
In bacteria, transcription and translation can be coupled: ribosomes may begin translating an RNA molecule before its synthesis is complete. This is possible because a nuclear envelope does not separate the two processes. Archaeal information-processing systems differ substantially from bacterial systems, and several components more closely resemble their eukaryotic counterparts. (ncbi.nlm.nih.gov)
Many prokaryotes reproduce by binary fission. After DNA replication, chromosome copies are segregated and the cell divides into two daughter cells. In many bacteria, the protein FtsZ helps organize the division machinery. Reproduction ordinarily transmits genetic information from parent to offspring, while mutations introduce heritable variation. (openstax.org)
Prokaryotes also exchange genetic information through horizontal gene transfer. In bacteria, major mechanisms include transformation, the uptake of environmental DNA; transduction, transfer mediated by bacterial viruses; and conjugation, transfer through contact between cells. These processes are distinct from reproduction and can spread traits across lineages, complicating attempts to represent every gene’s history with a single branching tree. (ncbi.nlm.nih.gov)
Metabolic diversity
Prokaryotic metabolism encompasses many combinations of energy sources, carbon sources, and environmental requirements. Phototrophs obtain energy from light, whereas chemotrophs obtain it from chemical compounds. Autotrophs build organic material using inorganic carbon, principally carbon dioxide; heterotrophs obtain carbon from organic substances. These categories describe separate aspects of nutrition, so light use does not necessarily imply carbon fixation. (openstax.org)
Photosynthesis occurs in several bacterial groups. Cyanobacteria perform oxygen-producing photosynthesis, while other photosynthetic bacteria use pathways that do not release oxygen. Prokaryotes also obtain energy through aerobic respiration, anaerobic respiration, or fermentation, depending on the organism and conditions. Oxygen supports the growth of some species but inhibits or damages others. This metabolic diversity allows prokaryotes to occupy environments ranging from oxygenated surface waters to anoxic sediments. (openstax.org)
Habitats and ecological roles
Bacteria and archaea occur in soils, freshwater, oceans, sediments, and associations with other organisms. Some tolerate extreme heat, salinity, acidity, or pressure, but neither group is restricted to extreme environments. Cells may live independently or assemble into biofilms: surface-associated communities embedded in material produced by their members. Biofilm organization influences attachment, interactions, and resistance to environmental stress. (openstax.org)
Prokaryotes sustain major ecosystem processes. Decomposers recycle organic matter, and photosynthetic or chemically powered producers supply organic carbon. Their activities contribute to the carbon cycle and transformations of nitrogen and sulfur. Certain bacteria and archaea perform nitrogen fixation, converting atmospheric nitrogen into biologically usable compounds. Prokaryotes also participate in partnerships with plants and animals, while particular bacterial species cause disease. Human uses include food fermentation, wastewater treatment, and biological removal or transformation of pollutants. (openstax.org)
Evolutionary and classificatory significance
Molecular studies revealed that bacteria and archaea, despite their superficially similar cell organization, differ deeply in membrane chemistry and genetic machinery. Research on evolution supports an origin of the eukaryotic nuclear lineage within Archaea. Consequently, “prokaryotes” is useful as a descriptive category but does not denote a group containing a common ancestor and all its descendants. (openstax.org)
The endosymbiotic theory identifies bacterial ancestors of mitochondria and chloroplasts, connecting prokaryotic lineages directly to the development of eukaryotic cells. The word’s Greek roots mean approximately “before nucleus”; they should not be interpreted as implying that living bacteria or archaea are unchanged ancestral organisms. (nature.com)