Prokaryotic cells are unicellular organisms that lack organelles or internal membrane-bound structure. This means that they do not have a nucleus. However, it has a single chromosome which is piece of circular, double stranded DNA located in an area of cell called the nucleoid. Most prokaryotes have cell wall outside of plasma membrane. The composition of cell wall differs between the two domains of life into which prokaryotes are divided, bacteria and archae. Also its cell wall differs from the eukaryotic cell wall found in plants and fungi. The cell wall is a protective layer and responsible for the organism’s shape. Some bacteria has capsules outside the cell wall. This capsule is found in some species and enables it to attach to surfaces, protect from dehydration and attacked by phagocytic cells, and increase resistance to our immune system. Some of the prokaryotic cells have flagella which is used for locomotion and pili used for attachment to surface. The plasmid consists of extra-chromosal DNA and also presents in many species of bacteria and archae. The plasma membrane is a thin layer that completely surrounds cell and separates the inside from the outside. In the archael cell membrane, the isoprene chain linked to glycerol replace fatty acid linked to glycerol in a bacterial membrane. Some archael membrane are lipid monolayers than bilayers.