Cells can be organized in two broad structural patterns: prokaryotic and eukaryotic.

A prokaryotic cell has no membrane-bounded nucleus. Its DNA occupies a region of the cytoplasm called the nucleoid. Bacteria and archaea have prokaryotic cell organization, although these major evolutionary lineages differ in important molecular details.

A typical prokaryotic cell contains:

• a plasma membrane;
• cytosol;
• ribosomes;
• one or more DNA molecules in the nucleoid;
• often a cell wall outside the membrane;
• sometimes additional surface appendages or internal membrane specializations.

A eukaryotic cell encloses most of its genomic DNA inside a membrane-bounded nucleus and contains additional membrane-bounded organelles. Animals, plants, fungi and protists are eukaryotes.

Eukaryotic cells use internal membranes to partition chemistry into specialized compartments. A mitochondrion, for example, maintains an internal environment different from the surrounding cytosol; the endoplasmic reticulum and Golgi apparatus create other distinct processing and transport environments.

Shared foundations

Prokaryotic and eukaryotic cells are not unrelated inventions. Both use DNA as genetic material, ribosomes for protein synthesis, plasma membranes for boundary control and catalyzed cellular reaction networks.

The main structural distinction is therefore compartmentalization by internal membranes, not the presence versus absence of fundamental cellular chemistry.

Size is not the definition

Prokaryotic cells are often smaller than eukaryotic cells, but size alone does not determine the category. A large bacterium remains prokaryotic if it lacks the characteristic eukaryotic nucleus and organelle system.

Similarly, the word prokaryote describes an organizational pattern shared by bacteria and archaea; it should not be interpreted as meaning that all prokaryotes form one simple or primitive biological group.

The distinction provides a map for cell biology: some processes can be understood from universal cellular components, while others depend on the internal compartment system specific to eukaryotic cells.