Viruses, Bacteria and Fungi: Structure and Classification
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Bacteria are prokaryotic cells with cell walls, cytoplasm and DNA free in the cytoplasm. Fungi are eukaryotic with chitin cell walls and saprophytic nutrition. Viruses are non-cellular: they consist of a protein coat surrounding DNA or RNA and can only reproduce inside a host cell.
This subtopic brings together the structural differences between three groups that students frequently confuse. Understanding their structures is essential for the classification and disease topics. Supplement students also need to know how pathogenic examples from each group cause disease.
Bacterial Cell Structure
Bacteria are prokaryotes: their key features:
No membrane-bound nucleus: DNA is a single circular chromosome in the cytoplasm.
Plasmids: small extra loops of DNA (Supplement).
Cell wall made of peptidoglycan (not cellulose, not chitin).
No mitochondria or endoplasmic reticulum: ribosomes are smaller (70S) than eukaryotic ones.
Some have a flagellum for movement and a slime capsule for protection.
Size: typically 1–5 μm. Much smaller than eukaryotic cells (10–100 μm).
Fungal Structure
Fungi like Mucor have:
Eukaryotic cells with a true nucleus.
Cell walls made of chitin.
A body made of hyphae: thread-like structures that form a network called a mycelium.
Saprophytic nutrition: they secrete enzymes onto organic matter and absorb digested products.
Reproduction via spores (asexual in Mucor: spores form in sporangia at the tips of hyphae).
Yeast is an exception: it is a single-celled fungus that reproduces by budding.
Virus Structure: Why Viruses Are Not Living
Viruses are not cells. They are much smaller than bacteria (20–300 nm) and consist of:
A protein coat (capsid).
A core of genetic material: either DNA or RNA, never both.
Some viruses have an outer lipid envelope (e.g. Influenza, HIV).
Why are viruses not classified as living?
They have no cytoplasm, no ribosomes, no organelles.
They cannot carry out metabolic reactions: no respiration, no nutrition.
They can only reproduce by injecting their genetic material into a host cell and hijacking its machinery.
Examples: tobacco mosaic virus (TMV) infects plants; influenza virus and HIV infect humans.
Try a focused question (3 marks)
State three structural differences between a bacterium and a virus. (3 marks)
Show the answer and marking guidance
A bacterium has a cell wall; a virus has a protein coat (capsid) / no cell wall.
A bacterium has cytoplasm and ribosomes; a virus has no cytoplasm or ribosomes.
A bacterium has its own DNA (circular chromosome); a virus has either DNA or RNA, and cannot replicate without a host cell.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying viruses are living organisms: they cannot carry out life processes independently.
Confusing bacterial cell walls (peptidoglycan) with plant cell walls (cellulose) or fungal cell walls (chitin).
Stating that bacteria have a nucleus: bacteria are prokaryotes with DNA free in the cytoplasm.