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Learn5090Paper 2 · TheoryClassificationMicroorganisms and viruses
Classification · Subtopic 2 of 2

Microorganisms and Viruses

Fungi (e.g. yeast, Mucor) have a chitin cell wall, feed by saprotrophic nutrition (secreting enzymes onto food), and many grow as thread-like hyphae forming a mycelium. Bacteria (prokaryotes) are single cells with no nucleus. Protoctists are mostly single-celled with a nucleus; some are animal-like (Amoeba), some plant-like (Chlorella), some cause disease (Plasmodium, malaria). Viruses are not cells: just genetic material in a protein coat, and they reproduce only inside living cells.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 30 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the features of fungi, bacteria and protoctists.
  • Describe the structure of viruses.
  • Explain why viruses are not classed as living organisms.
Pause and think

Is a virus alive?

Section 2 of 14

Core idea

Start hereThe invisible majority

A teaspoon of soil holds more microorganisms than there are people on Earth. Most are harmless or useful; a few cause disease.

The big idea

Fungi, bacteria and protoctists are living kingdoms with distinct features. Viruses are not cells and reproduce only inside other cells.

1. Comparing microorganisms

Features at a glance

Nucleus or not, wall or not.

Worked exampleSaprotrophic nutrition

Describe how bread mould feeds.

Goal
Mechanism.
Strategy
Extracellular digestion.
  1. hyphae secrete enzymes\text{hyphae secrete enzymes}
    Enzymes digest starch in the bread outside the fungus.
  2. absorb soluble products\text{absorb soluble products}
    Sugars diffuse into the hyphae.
AnswerExtracellular digestion and absorption.
✓ CheckThis is why mould rots food ✓.
Edge caseAlive or not? Viruses contain genes and evolve, but can't respire, feed or reproduce on their own, so they aren't placed in a kingdom.
Pause and think

Why do doctors not prescribe antibiotics for a cold?

Summary

  • Fungi: hyphae, chitin.
  • Bacteria: no nucleus.
  • Viruses: not cells.
Key words
Hyphae
Thread-like filaments of a fungus.
Saprotrophic
Feeding by secreting enzymes onto dead material.
Pathogen
A microorganism that causes disease.
HistoryPenicillin. In 1928 Alexander Fleming noticed a fungus, Penicillium, killing bacteria on a dish he had left out. It became penicillin, the first antibiotic.
Fun factThe biggest organism. A honey fungus in Oregon covers nearly 10 square kilometres underground and may be thousands of years old: one of the largest organisms on Earth.
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