any organism that lacks a distinct nucleus and other organelles due to the absence of internal membranes. Bacteria are among the best-known prokaryotic organisms. The lack of internal membranes in prokaryotes distinguishes them from eukaryotes (eukaryote). The prokaryotic cell membrane is made up of phospholipids and constitutes the cell's primary osmotic barrier. The cytoplasm contains ribosomes, which carry out protein synthesis, and a double-stranded deoxyribonucleic acid (DNA) chromosome, which is usually circular. Many prokaryotes also contain additional circular DNA molecules called plasmids (plasmid), with additional dispensable cell functions, such as encoding proteins to inactivate antibiotics. Some prokaryotes have flagella (flagellum). Prokaryotic flagella are distinct in design and movement from the flagella found on some eukaryotes. See also eukaryote; bacteria.