Unicellular Organisms

(also unicellulars), plant or animal organisms consisting of a single cell. There are two levels of organization among unicellular organisms: prokaryotic and eukaryotic. Prokaryotic unicellulars (bacteria and some blue-green algae) lack a differentiated nucleus and do not undergo mitosis; a nucleoid contains the apparatus for the process of genetic information. Eukaryotic unicelluar organisms (green and some other algae; among animals, protozoans) have a cell nucleus that divides by mitosis. Unicellulars resemble the cells of multicellular organisms in general structure and contain a similar set of organelles. Among unicellulars, there is varying complexity of morphological differentiation, from very simply arranged organisms (amoebas, chlorellas) to extremely complex ones (infusorians, Acetabularia). Unicelluar organisms may form colonies. (See alsoMULTICELLULAR ORGANISMS.)