# Explain why viruses are not classified within any of the recognised kingdoms or domains of life, and outline one piece of evidence that supports their exclusion from living classification systems.

> IB DP Biology Higher Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks

## Mark scheme (4 marks)

1. Viruses are acellular / do not possess a cell, which is a fundamental criterion shared by all recognised living organisms.
2. Viruses cannot carry out metabolism independently / have no metabolic activity outside a host cell, so they do not meet the metabolic criterion for life.
3. Viruses cannot self-replicate / reproduce independently; they require a host cell's ribosomes and enzymes to replicate their genetic material and assemble new particles.
4. Named evidence: viruses lack ribosomes (or any organelles), which are present in all cellular life, supporting their exclusion; OR viruses show no 16S/18S rRNA sequences that are used in universal phylogenetic trees to place all cellular organisms.

## Key terms

- [domain](https://www.gradenine.co.uk/glossary/domain)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-viruses-are-not-classified-5da84a78) · Published by Druglandscape Ltd.