4.4: Prokaryotic Cells
Prokaryotes are small unicellular organisms that include the domains—Archaea and Bacteria. Bacteria include many common organisms, such as Salmonella and E. coli, while the Archaea include extremophiles that live in harsh environments, such as volcanic springs.
Like eukaryotic cells, all prokaryotic cells are surrounded by a plasma membrane, have genetic material in the form of single, circular DNA, a cytoplasm that fills the interior of the cell, and ribosomes that synthesize proteins. However, unlike eukaryotic cells, prokaryotes lack a nucleus or other membrane-bound intracellular organelles. Their cellular components float freely within the cytoplasm, although their DNA is clustered within a region called the nucleoid.
Inside the cytoplasm, many prokaryotes have small, circular, double-stranded pieces of DNA called plasmids. These are distinct from the cell's chromosomal DNA and carry just a few special genes that provide bacteria with survival advantages, such as antibiotic resistance. Plasmids are self-replicating and can be transmitted between prokaryotic cells.
Most prokaryotes have a cell wall made of peptidoglycan that lies outside of their plasma membrane. It physically protects the cell and helps it maintain osmotic pressure in different environments. Many prokaryotes also have a sticky capsule layer that covers their cell wall and allows them to stick to a substrate or each other, thus providing additional protection.
While prokaryotes do not have membrane-bound organelles, some have infoldings of the plasma membrane that carry out specialized functions—such as photosynthesis in cyanobacteria. Therefore, although prokaryotes are simple cells compared to eukaryotes, they do have some unique structures that help them carry out complex functions and allow them to live in a wide variety of environments.