PROKARYOTE

also spelled procaryote, any self-contained cell or organism that lacks internal unit membranes. Bacteria are among the best-known prokaryotic organisms. Prokaryotes lack a nuclear membrane and most of the components of eukaryotic cells. The cell membrane consists of a phospholipid unit membrane and constitutes the cell's primary osmotic barrier. The cytoplasm includes ribosomes that carry out translation and protein synthesis. The nuclear region usually consists of circular, double-stranded deoxyribonucleic acid (DNA). Many prokaryotes also contain accessory, self-replicating genetic structures, called plasmids, with additional dispensable cell functions, such as encoding proteins to inactivate antibiotics. The flagella are distinct from those of eukaryotes in design and movement. The organelles that are present, such as storage vesicles, are surrounded by a nonunit membrane consisting principally of proteins. See also eukaryote; bacteria.