The cell is the basic structural and functional unit of life. All known living organisms are composed of one or more cells, and all cells arise from preexisting cells by division. This is cell theory, one of the foundational principles of biology, established by Matthias Schleiden and Theodor Schwann in the 1830s and extended by Rudolf Virchow in 1855 with the principle omnis cellula e cellula — every cell from a cell.
Cells fall into two major categories. Prokaryotic cells (bacteria and archaea) lack a membrane-bound nucleus; their DNA floats in the cytoplasm. They are small, structurally simple, and ancient — the earliest fossil evidence of prokaryotes dates to approximately 3.5 billion years ago. Eukaryotic cells (protists, fungi, plants, animals) have a membrane-bound nucleus containing their DNA, along with specialized organelles — mitochondria, endoplasmic reticulum, Golgi apparatus, and in plants, chloroplasts. Eukaryotic cells are larger and more structurally complex than prokaryotes.
The distinction between prokaryote and eukaryote is not merely structural. Lynn Margulis’s endosymbiotic theory demonstrated that mitochondria and chloroplasts originated as free-living prokaryotes that entered into symbiotic relationships with ancestral cells. The eukaryotic cell is, in its evolutionary origin, a collaborative assemblage — not a single lineage’s invention but the product of merger between distinct organisms.
A cell maintains itself through metabolism — the coordinated set of chemical reactions that extract energy from the environment, build cellular components, and dispose of waste. It is bounded by a selectively permeable membrane that regulates what enters and exits. It contains DNA encoding the information needed to produce proteins. And it reproduces by dividing, passing copies of that DNA to daughter cells.
Related terms
- DNA — the molecule encoding genetic information within cells
- Metabolism — the chemical processes that sustain cellular life
- Organism — a living system composed of one or more cells
- Symbiosis — the endosymbiotic origin of eukaryotic organelles