Microorganisms are living things or virus particles that are too small to see without a microscope. The main groups differ in structure, nutrition and what they do to us.
This lesson is part of microorganisms. Once you can compare the groups, the next lesson, interpreting growth-condition data, looks at what they need to grow.
How do the four groups differ?
The grid below gives the feature that separates each group. Check the examples against your textbook.
| Group | Structure | Nutrition | Example |
|---|---|---|---|
| Bacteria | Single cell, no nucleus | Absorb food, or make their own | Lactobacillus |
| Fungi | Cells with a nucleus and a cell wall | Absorb food from dead or living matter | Yeast, mould |
| Protozoa | Single cell with a nucleus, no cell wall | Take in food | Plasmodium, Amoeba |
| Viruses | Not cells: genetic material inside a protein coat | None; reproduce inside a host cell | Influenza virus |
The feature that separates viruses is that they need a host cell. Everything else in the table has its own cells.
Worked example: a two-sided comparison
Question: Compare bacteria and fungi. Give two similarities and two differences.
Step 1: find the shared features. Both are microorganisms made of cells, and both can act as decomposers.
Step 2: find the contrasts. Bacteria have no nucleus, whereas fungi do. Moulds form thread-like structures, but bacteria are single cells.
Step 3: write in two-sided sentences. Use “whereas” or “but” so each statement covers both groups.
A full answer is: “Bacteria have no nucleus, whereas fungi have a nucleus. Both can decompose dead matter.”
The mistake that costs marks
The slip is to describe one group and never mention the other. “Bacteria have no nucleus” is not a comparison, because the reader does not know what fungi have.
| Wrong | Right |
|---|---|
| Bacteria are very small | Bacteria are smaller than fungal cells |
| Viruses cause diseases | Viruses have no cells, but bacteria are single cells |
| Fungi grow on bread | Fungi have a nucleus, while bacteria have no nucleus |
A comparison answer always holds both sides in the same sentence, joined by “whereas”, “but” or “while”.
Linking groups to a setting
A single kitchen can show all four groups. Yeast makes the dough rise, bacteria sour the milk, mould spoils old bread, and a virus may pass between people through sneezing.
Use real settings to remember each group. The graph evidence and fair-comparison lab can help with later data questions about growth.
Check yourself
State one structural difference between a virus and a bacterium, and explain why a virus needs a host cell.
Answer
A bacterium is a single cell, whereas a virus is not a cell. It is only genetic material inside a protein coat.
A virus has no cell machinery of its own, so it can reproduce only by using the machinery of a host cell.
What to study next
Growth conditions explain why microorganisms spoil food in one place and not in another. Continue with interpreting growth-condition data, then test all four lessons in the microorganisms practice set.
To compare groups with a teacher, see online one-to-one Science tuition.