= Organism
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An <organism> is an individual living system, made of one <biological cell> or an integrated assembly of <biological cells>. An <Escherichia coli> cell is an entire <organism>; a <neuron> is normally a specialized part of an <animal>. <Metabolism>, regulated exchange through <cell membranes>, and inherited information connect these very different scales. See https://openstax.org/books/biology-2e/pages/1-2-themes-and-concepts-of-biology[OpenStax Biology 2e, 1.2: living systems and levels of organization].
Keep two questions separate: how many <biological cells> make the <organism>, and how those <biological cells> are organized. A <yeast> and an <Escherichia coli> are both <unicellular organisms>, but the <yeast> is a <eukaryote> and the <Escherichia coli> a <prokaryote>. A <plant> and an <animal> are <multicellular organisms> with <eukaryotic> cellular organization. Neither distinction ranks <organisms> as better or worse: <natural selection> acts on reproduction in particular environments.
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