A virus is a set of instructions in a protein box. No cytoplasm, no membrane of its own in many cases, almost no enzymes, and no way of doing anything at all until it gets inside a living cell. That is why it does not count as alive — and why it is so hard to kill.
📚 What you need to know
Viruses are non-cellular infectious particles. They are not classed as living organisms.
They have no cellular structures and no metabolism, so they are described as acellular.
They do not fit the features used to classify organisms, so they sit outside the three-domain system.
They measure roughly 20 to 300 nm, so only an electron microscope can show them.
Every virus has a nucleic acid core (DNA or RNA) inside a protein coat called a capsid, with attachment proteins on the outside.
Some also have a lipid envelope taken from the membrane of the cell they were made in.
They are parasitic: they can only reproduce inside a living host cell, using that cell’s ribosomes and energy.
Structure varies widely — shapes can be polyhedral, spherical, helical or complex — and each virus infects a specific host cell.
Why a virus is not alive
Go back to the seven functions of life. A virus fails almost all of them. It has no metabolism, so no chemical reactions of its own. It does not grow — a virus particle has a fixed size from the moment it is assembled. It cannot respond, cannot maintain internal conditions, and cannot reproduce without hijacking something else.
It has no cytoplasm and very few enzymes, if any. Even the energy used to build new virus particles is released by the host cell, because a virus does not respire. Being non-cellular, it also fails the second statement of cell theory: it is not made of cells, and it is not a cell itself.
Where do you put them, then? Living organisms are sorted into three domains — eubacteria, archaea and eukaryotes. Viruses have none of the features that system uses, so they are not placed in any domain. They sit outside the whole classification, which is unusual and worth saying in an exam answer.
The parts every virus has
The attachment proteins are the part that decides everything. They are the reason a given virus can infect one type of cell and no other.
Feature
Detail
Small size
Roughly 20 to 300 nm across — far smaller than a prokaryotic cell, and only visible with an electron microscope
Fixed size
A virus particle does not grow once it has been assembled
Nucleic acid core
The genome is DNA or RNA; it may be single or double stranded, and linear or circular
Capsid
A protein coat that encloses and protects the genetic material
Attachment proteins
Sit on the capsid surface and let the virus bind to and enter a host cell
No cytoplasm
Nothing inside except the genome and, in some viruses, a few proteins
Few or no enzymes
The virus relies on the host cell’s enzymes instead
Lipid envelope
Present in some viruses only, formed from the phospholipids of the host cell membrane, and used in cell recognition
Watch the wording on the genome. Cells always have double-stranded DNA. Viruses can have DNA or RNA, single or double stranded, linear or circular. If a question asks how viral genetic material differs from a cell’s, that variety is the answer.
How much they vary
Simple does not mean uniform. Virus shape and structure vary enormously, and the shapes have names you should recognise.
Shape is not decoration. A complex virus with a tail can inject its genome through a bacterial cell wall, which a simple sphere could never do.
Host specificity
A virus cannot infect just anything. Its attachment proteins have to fit receptor molecules on the surface of the host cell, rather like a key fitting one lock. If there is no match, the virus cannot get in.
HIV attaches to certain white blood cells.
Hepatitis viruses attach to liver cells.
🧠
Attachment decides everything
Which cells a virus can infect, which species it can jump to, and which drugs might block it — all of it comes back to the attachment proteins on the surface.
Three viruses to know
Bacteriophage lambda
A bacterial virus. It infects Escherichia coli, and it looks less like a ball and more like a landing craft.
The tail proteins contract, driving the tail through the bacterial cell wall so the DNA can be pushed straight into the cytoplasm. The rest of the phage stays outside.
Coronaviruses
A group of viruses that cause respiratory diseases in mammals and birds, spread through respiratory fluids. Examples include SARS-CoV-2 (the COVID-19 virus), MERS and SARS.
Genome: single-stranded RNA.
Shape: spherical.
An envelope outside the capsid.
Many glycoproteins projecting from the surface. Under an electron microscope these look like a ring around the particle, which is where the name comes from — corona is Latin for crown.
HIV
The human immunodeficiency virus is passed on only by direct exchange of body fluids, so transmission routes include sexual intercourse, blood donation, shared needles, from mother to child across the placenta, mixing of blood during birth, and breast milk.
Part of HIV
What it is and what it does
Genetic material
Two strands of RNA
Enzyme
Reverse transcriptase, which makes DNA from the viral RNA — the reason HIV is called a retrovirus
Capsid
A protein coat around the RNA and enzymes
Envelope
A lipid bilayer taken from the membrane of the host helper T cell the particle escaped from
Glycoproteins
Sit in the envelope and act as the attachment proteins
Why the envelope comes from the host. A new virus particle leaves by pushing out through the host cell membrane, wrapping itself in a piece of that membrane as it goes. So the envelope is host material with viral glycoproteins pushed through it — which is also why enveloped viruses are so good at not being noticed straight away.
Worked examples
WE 1
Explain why viruses are not classed as living
Explain why viruses are not considered to be living organisms. (3 marks)
Point 1: no cells
Viruses have no cellular structures and no cytoplasm, so they are acellular.
Point 2: no metabolism
They have very few or no enzymes and carry out no chemical reactions of their own, so they cannot respire or grow.
Point 3: cannot reproduce alone
They are parasitic and can only reproduce by infecting a living cell and using its ribosomes and energy.
Acellular, no metabolism, and unable to reproduce independently“acellular” and “no metabolism” are the two phrases mark schemes reward
WE 2
Compare viral and cellular genetic material
Describe how the genetic material of viruses differs from that of living cells. (3 marks)
Difference 1: the molecule
A virus genome may be DNA or RNA, whereas cells always use DNA.
Difference 2: the strands
Viral nucleic acid can be single or double stranded; cellular DNA is double stranded.
Difference 3: the shape
It can be linear or circular, and there is very little of it compared with a cell’s genome.
DNA or RNA, single or double stranded, linear or circulargive both sides of each difference, not just the viral half
WE 3
Explain host specificity
HIV infects certain white blood cells but not liver cells. Explain why. (3 marks)
Point 1: what does the bindingAttachment proteins on the surface of the virus bind to receptor molecules on the host cell membrane.
Point 2: the fit is specific
The shape of the attachment protein is complementary to only certain receptors.
Point 3: the consequence
Those receptors are present on some white blood cells but not on liver cells, so HIV cannot attach to or enter a liver cell.
No matching receptor means no attachment, so no infectionthe answer is about complementary shapes, not about the virus “choosing” a cell
💡 Exam tips
Use the words non-cellular, acellular and parasitic — they are the standard wording.
Quote the size range, 20 to 300 nm, and say an electron microscope is needed.
Every virus answer should mention nucleic acid, capsid and attachment proteins.
The lipid envelope is in some viruses only, and it comes from the host cell membrane.
For HIV, remember two RNA strands and reverse transcriptase, which makes it a retrovirus.
For coronaviruses, remember single-stranded RNA, spherical, enveloped and covered in glycoproteins.
⚠ Common mistakes
Calling a virus a cell or an organism. It is a particle, and it is not alive.
Saying all viruses have an envelope. Only some do.
Writing that viruses have DNA. Many have RNA instead.
Giving viruses ribosomes. They use the host cell’s ribosomes.
Saying a virus respires to get energy. The host cell releases the energy.
Placing viruses in a domain. They sit outside the three-domain system.
Up next: Replication in Viruses — the lytic pathway that destroys the host cell, and the lysogenic pathway that hides inside it for years.
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