Encellighet

Encellighet, or unicellularity, is the condition in which an organism is composed of a single cell that carries out all essential life processes. Most bacteria, archaea, many protists, and several fungi and algae are unicellular, though some species form colonies or transient aggregates without true multicellularity. In unicellular organisms, the entire metabolism, growth, response to the environment, and reproduction occur within one cell.

Cellular organization differs between prokaryotes and eukaryotes. Prokaryotic unicells have no nucleus, their genetic material resides in a simple cytoplasmic region, and organelles are limited. Eukaryotic unicells possess a nucleus and other membrane‑bound organelles, enabling more complex internal organization and diverse metabolisms.

Reproduction is often asexual, most commonly by binary fission in bacteria and archaea or by budding and simple division in yeasts. Some protists exhibit sexual processes or alternation of generations, which can increase genetic variation.

Ecologically, unicellular organisms play central roles in nutrient cycles, decomposition, and primary production. They can adapt rapidly to changing environments due to short generation times, but they face constraints related to surface area‑to‑volume ratios and limited opportunities for coordinated specialization.

From an evolutionary perspective, unicellularity is the ancestral condition for life on Earth. Several major lineages later evolved multicellularity, implying that complex organisms arose from ancestral single‑celled forms. Nevertheless, unicellular organisms remain widespread and diverse, and they continue to be essential model systems in biology and biotechnology.