Explain why viruses are not classified within any of the recognised domains of life.

IB DP Biology Standard Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Viruses are acellular and lack a cellular structure.
They do not possess the cellular organisation that defines the three domains of life.
They cannot carry out metabolism or other life processes independently.
They require a host cell’s machinery to replicate, so they cannot reproduce on their own.

Examiner tips

Use the exact terms ‘acellular’, ‘cellular organisation’, ‘metabolism’, ‘replicate independently’.
Show the logical link: acellular → not a domain; no metabolism → not life; no independent replication → not life.
Keep each point brief and separate with a line break or bullet.

Common mistakes

Confusing viruses with bacteria or other microbes; claiming they are a domain. Forgetting to mention the lack of metabolism. Using vague phrases like ‘not alive’ without explaining why.

Mark scheme (4 marks)

Viruses are acellular / not composed of cells
The three domains (Bacteria, Archaea, Eukarya) are defined by cellular organisation, so viruses do not fit any domain
Viruses cannot carry out metabolism / do not carry out life processes independently
Viruses cannot replicate independently / require host cell machinery for reproduction

Key terms in this question

domain