Valonia ventricosa, a kind of alga, is among the largest unicellular species. Its diameter can reach 5 centimetres (2.0 in).

Unicellular organisms are organisms that have one cell. They are divided into two quite different types, from different classification kingdoms.

• The prokaryotes, bacteria and archaea, have cells with no nucleus and a simple cell structure.
• Eukaryotes have a nucleus, and a more complex cell structure.

The differences between the prokaryota and eukaryota are significant. Eukaryotes possess a nucleus and various subcellular organs called organelles, and prokaryotes do not.