Explain why viruses are not considered to be living cells, despite containing genetic material and being able to reproduce.

IB DP Biology Standard Level (2023 syllabus) — A2.1 Origins of cells · 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 not considered living cells because they lack a cell membrane and are not bounded by a membrane of their own. They cannot carry out metabolism and do not perform metabolic reactions independently. Viruses can only reproduce inside a host cell, making them obligate intracellular parasites. The cell theory states that all living things are made of cells, and viruses do not conform to this requirement.

Examiner tips

  • Use the four key points in the order given; each point is worth one mark.
  • Show clear understanding of what constitutes a cell (membrane, metabolism, reproduction).

Common mistakes

  • Confusing viruses with bacteria and claiming they have a cell membrane.
  • Mentioning that viruses can reproduce on their own, ignoring the host‑cell requirement.

Mark scheme (4 marks)

  1. Viruses lack a cell membrane / are not bounded by a membrane of their own
  2. Viruses cannot carry out metabolism / do not perform metabolic reactions independently
  3. Viruses can only reproduce inside a host cell / are obligate intracellular parasites
  4. The cell theory states all living things are made of cells / viruses do not conform to cell theory

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