Viruses: Structure and Non-Living Status
The structure of a virus particle (genetic material, capsid, attachment proteins) and why viruses are described as acellular and non-living. Includes the 4-mark exam question, a worked model answer and the examiner trap on never calling a virus a cell.
Chapters (6)
Exam question [4 marks]
Describe the structure of a virus particle and explain why viruses are described as non-living.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Genetic material / nucleic acid (DNA or RNA) at the core | "RNA", "DNA", "nucleic acid", "genome" | "nucleus"; "chromosome in a nucleus" |
| 2 | Surrounded by a protein coat / capsid | "capsid", "protein coat" | "cell wall"; "membrane"; "capsule" |
| 3 | Attachment proteins on the outside that bind to (complementary) receptors on the host cell | "attachment protein", "spikes / glycoproteins that bind to host cell receptor" | "flagella"; "attachment proteins are inside the capsid" |
| 4 | Non-living because it is acellular / has no cell structure (no cytoplasm or organelles) and cannot carry out metabolism or reproduce outside a host cell (both ideas are needed for the mark) | "no cytoplasm, ribosomes or enzymes", "only replicates inside a host cell" | "a virus is a very small cell"; "it is not alive because it is small" |
Marks 1 to 3 are for structure, mark 4 for the explanation. Calling a virus a cell, or giving it cytoplasm or a cell wall, scores no mark for that point. A statement of "non-living" with no reason does not score.
Model answer (4/4)
A virus particle has genetic material, either DNA or RNA, at its core. This is surrounded by a protein coat called the capsid. Attachment proteins on the outside bind to complementary receptors on the host cell. Viruses are non-living because they are acellular, with no cytoplasm or organelles, and they cannot carry out metabolism or reproduce unless they are inside a host cell.
Examiner trap
- Calling viruses cells or 'living organisms'
- Saying a virus has a cell wall or cytoplasm
Key terms
- virus
- acellular
- capsid
- attachment protein
- genetic material
- non-living
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:19Acellular: no cell structure
Cells have a membrane, cytoplasm, ribosomes and usually a nucleus. A virus has none of these. The word for that is acellular, which means it is not made of cells and has no cell structure.
0:34Virus particle structure
Now draw a virus particle from the middle outwards. At the centre is the genetic material. That is a nucleic acid, either DNA or RNA, depending on the virus.
Around it is the capsid, a coat made of protein subunits. Its job is to protect the genetic material.
On the outside are attachment proteins. These stick out from the surface and bind to a complementary receptor on a host cell. That is how the virus gets in.
1:03Why viruses are non-living
Living things carry out metabolic reactions. A virus has no cytoplasm, no ribosomes and no enzymes of its own, so it cannot do that. It cannot reproduce on its own either. It only replicates once it is inside a host cell, using the host's machinery.
That is why we call viruses non-living.
1:54Examiner trap and recap
The trap. Never call a virus a cell, and never give it a cell wall or cytoplasm. Recap. Genetic material, capsid, attachment protein. Acellular, so non-living.
Next, we look at how we actually see cells: optical microscopes, and the two kinds of electron microscope.
Keep going
More from 3.2 Cells
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
- Rough ER, Golgi and Lysosomes Explained · AQA 7402 · 3.2.1.1
- Cell Wall, Vacuole and Specialised Cells · AQA 7402 · 3.2.1.1
- Prokaryotic vs Eukaryotic Cells · AQA 7402 · 3.2.1.2
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-026.