Definition · Plain-language
Prokaryotic cell
A prokaryotic cell is a simple, small cell with no nucleus and no membrane-bound organelles, such as a bacterium.
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A cell without a nucleus
The defining feature of a prokaryotic cell is that it has no nucleus. Instead of being held inside a nuclear membrane, its DNA sits loose in the cytoplasm as a single circular loop, sometimes with small extra rings called plasmids. The name comes from Greek words meaning "before a kernel", reflecting the idea that these cells existed before the nucleus evolved. Bacteria and archaea are the two groups of prokaryotes, and they are among the oldest and most numerous forms of life on Earth.
Small and simple inside
Prokaryotic cells are much smaller than eukaryotic cells, typically a few micrometres across, and far simpler inside. They have no membrane-bound organelles — no mitochondria, no chloroplasts, no nucleus. They do have ribosomes for building proteins, a cell membrane, cytoplasm, and usually a cell wall (made of a material called peptidoglycan in bacteria) that gives shape and protection. Some have extra features such as a flagellum for movement. Their simplicity lets them grow and reproduce very quickly.
Where prokaryotes fit in life
Although they are simple, prokaryotes are enormously important. Bacteria live almost everywhere — in soil, water, air and inside other organisms — and play essential roles, from breaking down dead matter as decomposers to helping animals digest food. Some cause disease, but many are helpful or harmless. All organisms you can see with the naked eye, by contrast, are made of the more complex eukaryotic cells. Understanding the prokaryote–eukaryote split is one of the most basic distinctions in biology.
Key facts
At a glance
- Definition: a simple cell with no nucleus and no membrane-bound organelles
- DNA: a single circular loop, free in the cytoplasm, plus plasmids
- Examples: bacteria and archaea
- Size: small, typically 1–5 micrometres
- Cell number: almost always single-celled
- Has: ribosomes, cell membrane, cytoplasm and usually a cell wall
Common misconceptions
What people often get wrong
Often heard: Prokaryotic cells have a nucleus, just a smaller one.
Actually: Prokaryotic cells have no nucleus at all. Their DNA floats freely in the cytoplasm as a single loop. Having the genetic material enclosed in a nucleus is exactly what defines a eukaryotic cell, not a prokaryotic one.
Often heard: Prokaryotic cells have no internal structures whatsoever.
Actually: They are simple but not empty. Prokaryotic cells contain ribosomes that build proteins, plus cytoplasm, a cell membrane and usually a cell wall. What they lack are membrane-bound organelles such as a nucleus and mitochondria.
Often heard: All bacteria are harmful disease-causing germs.
Actually: Most bacteria are harmless or helpful. Many recycle nutrients as decomposers, help animals digest food, or are used in making foods such as yoghurt. Only a minority cause disease.
Going deeper
