Unicells

Unicells, or unicellular organisms, are organisms that consist of a single cell. They include prokaryotes such as bacteria and archaea, as well as many eukaryotic organisms, including protists (single-celled algae and protozoa) and some fungi such as yeasts. By definition, their entire life processes occur within one cell, in contrast with multicellular organisms that rely on many specialized cells and tissues.

In unicellular organisms, all essential activities—energy production, growth, response to the environment, and reproduction—are carried out within a single cell. Most bacteria and archaea reproduce asexually through binary fission, while many unicellular eukaryotes divide by mitosis or reproduce via budding or spores. Some protists also engage in sexual stages or genetic exchange, adding genetic diversity to populations.

Unicells show diverse metabolisms and ecological roles. They include autotrophs that capture light or chemical energy to synthesize organic compounds, and heterotrophs that rely on external organic material. They inhabit nearly every environment on Earth, from oceans and soils to extreme habitats like acidic waters or high-salt environments. Sizes are typically microscopic, though some colonies or biofilms of unicells can appear larger.

Ecologically, unicellular organisms are foundational. They are major contributors to primary production and nutrient cycling, decompose organic matter, form symbiotic relationships, and contribute to biofilms. Some are pathogens affecting plants, animals, or humans, while others have important uses in industry and research, such as yeast in fermentation or unicellular algae in biotechnology.

Genetically, prokaryotic unicells have no nucleus, with their DNA in a nucleoid region, while many unicellular eukaryotes possess a defined nucleus and organelles. Their simplicity makes them essential models for studying fundamental biology and evolution.