# Explain why viruses are not considered living organisms, with reference to their structure and dependence on host cells.

> IB DP Biology Higher Level (2023 syllabus) — A2.3 Viruses (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. Viruses lack cellular organisation / are not composed of cells
2. Viruses cannot carry out metabolism independently / have no metabolic activity of their own
3. Viruses can only replicate inside a living host cell, using the host's biosynthetic machinery
4. Viruses do respond to their environment in the sense that they can evolve (via mutation/natural selection), but they are not considered living because they do not meet the full set of criteria for life (e.g. homeostasis, growth, independent response to stimuli are absent)

## 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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