= Unicellular organism
{wiki}
A <unicellular organism> is an <organism> whose living body consists of one <biological cell>. That <biological cell> maintains itself, grows and participates in <reproduction>; it is not merely one specialized component of a larger body. <Escherichia coli> and ordinary free-living <Saccharomyces cerevisiae> illustrate <prokaryotic> and <eukaryotic> examples respectively. Cellular organization and the number of <biological cells> are separate questions. See https://openstax.org/books/biology-2e/pages/4-1-studying-cells[Clark, Douglas and Choi, Biology 2e, 4.1: cells as units of organisms].
How can one <biological cell> do enough to sustain an <organism>? Its <cell membrane> retains useful material while <membrane transport> allows exchange; <metabolism> transforms nutrients and supplies usable <energy>; <gene expression> makes <proteins> needed to maintain and rebuild the machinery. These functions operate together inside the same boundary. In <Saccharomyces cerevisiae>, for example, <glucose> can support <alcoholic fermentation> and production of <ATP>, while the <cell cycle> and <budding> produce descendants. A single <biological cell> therefore combines chemical processing, regulated exchange and inherited organization. See https://www.ncbi.nlm.nih.gov/books/NBK26864/[Alberts et al., Molecular Biology of the Cell (2002): universal cellular machinery] and the experimental accounts under <Saccharomyces cerevisiae> and <yeast>.
\Image[/unicellular-multicellular-organization.png]
{title=One <biological cell>, a <biological colony>, <clonal development> and <developmental integration>}
{description=Original conceptual schematic, not micrographs or an evolutionary ladder. Colors mark illustrative lineages in the middle panels and illustrative functions in the right panel; they are not measured traits. A <biological colony> can also be clonal, and <clonal development> need not produce specialized <biological cells>. The integrated panel illustrates a division of functions, not a particular <species>.}
Being a <unicellular organism> does not mean isolated or independent of all other life. <Unicellular organisms> can interact and cooperate in groups. A <biological colony> describes association; <clonal development> describes descent; <developmental integration> describes coordination of the developing whole. Neither shared ancestry nor a visible clump alone establishes the latter. This distinction makes single-cell experiments easier to interpret: measuring the growth of one <biological cell>, the expansion of a group and the <reproductive success> of a reproducing cluster can answer different questions. See https://pmc.ncbi.nlm.nih.gov/articles/PMC2781869/[Queller and Strassmann, Beyond society: the evolution of organismality (2009): cooperation and the limits of simple organism criteria].
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