Singlecelled

Single-celled organisms are organisms composed of a single cell that carries out all essential life processes. They can be prokaryotic, lacking a defined nucleus, or eukaryotic, containing a nucleus and organelles. The category includes bacteria, archaea, many protists, and some fungi such as yeasts.

Most are microscopic, typically measuring a few micrometers in size, and exhibit a range of shapes from spheres and rods to spirals. Internal structure varies; prokaryotes have no true nucleus, while many single-celled eukaryotes have specialized organelles. Reproduction is usually asexual, by binary fission, budding, or spores; genetic exchange can occur by conjugation or other processes.

Single-celled organisms occupy nearly every habitat, including soil, freshwater, oceans, and extreme environments such as hot springs and acidic mines. Their metabolism is diverse: autotrophs derive energy from photosynthesis or chemosynthesis, while heterotrophs obtain nutrients by consuming organic matter. Some grow as phototrophs, others as consumers or saprophytes. Many act as pathogens, but most contribute to nutrient cycling and ecological balance.

Ecologically and scientifically, unicellular life is foundational. Unicellular organisms were among the first life forms on Earth and remain essential models in genetics, biochemistry, and cell biology. They support industries such as fermentation and bioremediation and serve as study systems for cellular processes.

Examples include bacteria such as Escherichia coli and Bacillus subtilis; archaea such as Halobacterium; unicellular fungi such as Saccharomyces cerevisiae; and protists such as Paramecium, Amoeba, and Chlamydomonas.