Viruses: Acellular Structure Revision Notes | A Level AQA Biology

1. What Makes a Virus Different from Everything Else?

Viruses sit in a strange category — they are not quite alive, yet they can hijack living cells and reproduce in enormous numbers. Understanding why biologists call them acellular and non-living is the foundation of this entire topic.

Every living organism you have studied so far — bacteria, plant cells, animal cells — is made of at least one cell. A cell has a membrane, cytoplasm, ribosomes, and its own metabolic machinery. Viruses have none of these. Strip a virus down and you find a remarkably minimal structure: some genetic material wrapped in a protein coat, with a few proteins on the outside. That is it.

This minimal design is not a flaw — it is precisely what makes viruses so effective. They carry only the instructions needed to take over a host cell's machinery. Everything else, they borrow from you.

2. Viruses Are Acellular and Non-Living

The term acellular means "not made of cells". Viruses do not have a cell membrane, no cytoplasm, no nucleus, and no organelles such as mitochondria or ribosomes. Because the cell is the basic unit of life, something that lacks cells entirely falls outside the definition of a living organism.

Think about what a living cell can do on its own: it can generate ATP through respiration, synthesise proteins using its ribosomes, and divide by mitosis or binary fission. A virus particle sitting on a surface can do none of these things. It has no metabolic activity whatsoever until it enters a host cell.

This is a genuinely contested area in biology — some scientists argue viruses occupy a grey zone. For AQA A Level purposes, the position is clear: viruses are acellular and non-living, and you should be able to justify that with reference to their structure and their dependence on host cells.

3. The Structure of a Virus Particle

Now that you know what viruses lack, it is time to focus on what they do have. Picture a virus particle as a tiny delivery package: the contents are the genetic instructions, the packaging is the protein coat, and the address label is the attachment protein on the outside.

Every virus particle — also called a virion — contains three core components that AQA requires you to know: genetic material, a capsid, and attachment proteins. Each component has a precise function, and examiners will expect you to link structure to function.

• Genetic material — DNA or RNA (single- or double-stranded), carrying the instructions for viral replication
• Capsid — a protein coat made of repeating subunits called capsomeres, which protects the genetic material
• Attachment proteins — surface proteins that bind to specific receptors on host cells, determining host range

3a. Genetic Material

The genetic material inside a virus can be DNA or RNA. This is one of the key differences between viruses and all cellular life — some viruses use RNA as their hereditary molecule, which is unusual. The influenza virus, for example, carries single-stranded RNA, while the herpes virus carries double-stranded DNA.

The genetic material encodes the proteins the virus needs — including the capsid proteins and attachment proteins — but it cannot be expressed without the host cell's ribosomes, tRNA, and energy supply. This dependency is central to why viruses are non-living.

3b. The Capsid

The capsid is the protein coat that surrounds and protects the viral genetic material. It is built from repeating protein subunits called capsomeres, which self-assemble into a precise geometric shape — most commonly a helix or an icosahedron (a 20-faced polyhedron).

The capsid serves two functions: it protects the nucleic acid from degradation by enzymes in the environment, and it plays a role in the initial stages of infection by helping the virus attach to and enter a host cell. The shape and protein composition of the capsid are highly specific to each virus type.

Some viruses — such as HIV and influenza — have an additional outer layer called an envelope, derived from the host cell's membrane. This is beyond the core AQA specification requirement, but worth noting if you encounter it in exam contexts: the envelope is not present in all viruses, and the capsid alone is sufficient for non-enveloped viruses.

3c. Attachment Proteins

Attachment proteins are located on the outer surface of the virus — either on the capsid itself or, in enveloped viruses, embedded in the envelope. They are the key that fits a specific lock: the receptor proteins on the surface of a host cell.

This specificity explains why different viruses infect different host species and different cell types within a host. The HIV virus, for instance, carries attachment proteins (gp120) that bind specifically to CD4 receptors found on T-helper lymphocytes. A virus cannot infect a cell that lacks the matching receptor — the attachment protein will not bind.

From an exam perspective, this structure-to-function link is a high-value explanation. Attachment proteins determine host range and tissue tropism (which tissues within a host are susceptible), and understanding this helps explain why some viruses are species-specific.

4. Comparing Virus Structure with Cellular Life

Earlier, we established that viruses lack the features of cells. Now that you know the three components of a virus particle, it is worth making that comparison explicit — because AQA examiners frequently ask you to compare viral structure with prokaryotic or eukaryotic cells.

The table makes the contrast stark. Viruses share the possession of genetic material with all cellular life, but every other feature that defines a living cell is absent. The capsid and attachment proteins are unique to viruses — no cellular organism has an equivalent protein coat surrounding its genetic material in this way.

5. Applying Viral Structure — How the Components Work Together

Knowing the three components in isolation is recall-level knowledge. To reach the higher marks in AQA questions, you need to explain how these components interact during infection. Here is how the sequence works.

Worked example: How Viral Structure Enables Infection: A Structural Walkthrough

Explain, with reference to the structure of a virus particle, how a virus is able to infect a specific host cell and replicate.

1. The attachment protein on the virus surface is complementary in shape to a specific receptor protein on the host cell membrane. — This is a lock-and-key interaction. If the host cell lacks the matching receptor, the virus cannot attach — this determines host range and tissue tropism.
2. Following attachment, the viral genetic material (DNA or RNA) is injected into or released into the host cell's cytoplasm. The capsid may remain outside (as in bacteriophages) or be taken in whole. — The capsid has done its job — it protected the nucleic acid during transit through the environment and facilitated entry into the host.
3. The viral nucleic acid is read by the host cell's ribosomes. The host's ATP, amino acids, and enzymes are used to synthesise viral proteins, including new capsid proteins and attachment proteins. — This is why viruses are non-living — they have no ribosomes or metabolic machinery of their own. They are entirely dependent on the host cell at this stage.
4. Newly synthesised capsid proteins self-assemble around copies of the viral genetic material to form new virions. — The attachment proteins are incorporated into the new capsid surfaces, ensuring the next generation of virions can also recognise and bind to host cells.
5. New virus particles are released from the host cell — often destroying it in the process (lysis) — and can go on to infect further cells. — Each of the three structural components — genetic material, capsid, and attachment proteins — is faithfully reproduced, maintaining the virus's ability to infect.

Answer: The attachment protein binds to a complementary host cell receptor, allowing entry of viral genetic material. The host cell's machinery replicates the nucleic acid and synthesises new capsid and attachment proteins. These self-assemble into new virions, which are released to infect further cells.

6. Exam Technique and Common Mistakes

Questions on viral structure appear regularly in AQA papers, and the marks are very achievable — but only if you use precise language. Vague answers that say "the virus attaches to the cell" without mentioning attachment proteins or receptors will not score the structure-to-function marks that examiners are looking for.

When a question asks you to describe the structure of a virus, give all three components with a brief function for each. When it asks you to explain why viruses are non-living, you must link to the absence of cellular structures and the inability to carry out independent metabolism — not just say "they need a host".

For compare questions between viruses and cells, structure your answer as a direct comparison: "Viruses have a capsid, whereas cells do not; cells have a cell membrane, whereas viruses do not." Parallel sentence structure picks up comparison marks efficiently.

Across this lesson, every structural feature of a virus — the genetic material, the capsid, and the attachment proteins — connects back to the central idea: viruses are acellular, non-living particles that can only function by exploiting the machinery of a living host cell. That single principle underpins every AQA question on this topic.