Prokaryoilla

Prokaryoilla, the Finnish term for prokaryotes, are single-celled organisms that lack a true nucleus and other membrane-bound organelles. They include the two domains Bacteria and Archaea and are among Earth's oldest and most diverse life forms. Most prokaryotes are microscopic, with cells typically about 0.2 to a few micrometers in diameter, and they occur in rods, spheres, and spirals. Their genetic material is concentrated in a region called the nucleoid rather than a defined nucleus. They reproduce mainly by binary fission.

Prokaryotic cell structure is relatively simple. A cytoplasmic membrane encloses the cell, and many have a rigid cell wall that provides shape and protection. In bacteria the wall usually contains peptidoglycan; in archaea it may contain pseudopeptidoglycan or other polymers. Some prokaryotes have external structures such as flagella for movement or pili for attachment and gene transfer. Prokaryotic ribosomes are 70S, and many carry plasmids—small DNA circles with accessory genes.

Metabolic diversity is high. Prokaryotes can be photoautotrophs, chemoautotrophs, or heterotrophs, deriving energy from sunlight, inorganic compounds, or organic compounds. They inhabit a wide range of environments, from oceans and soil to acidic springs and subsurface habitats. They drive essential ecological processes, including decomposition, nutrient cycling, nitrogen fixation, and methane production or consumption. Genetic exchange among prokaryotes occurs through horizontal gene transfer mechanisms such as transformation, transduction, and conjugation, enabling rapid adaptation.

Prokaryotes are of major importance to humans and ecosystems. Many prokaryotes are beneficial—fermenters, nitrogen fixers, and producers of vitamins or enzymes—while others are pathogens. In research and biotechnology they are used for production of medicines, biofuels, and industrial enzymes. Taxonomically, prokaryotes do not form a single clade; bacteria and archaea represent two distinct domains with separate evolutionary histories.