Year 7 Science · Unit 1 · Lesson 5

Microorganisms

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Learning Goals

Trace how the influenza virus makes you sick, and how yeast makes bread rise.
Compare the sizes of microbes and explain why antibiotics do not work on viruses.
Explain why a fungus is not a plant.

Warm Up

Match each term to its meaning

Write the letter of the correct meaning next to each term.

Term	Letter	Meaning
Microorganism		A. An organism that breaks down dead plants and animals.
Bacterium		B. A single-celled organism that has a nucleus.
Pathogen		C. A living thing too small to see without a microscope.
Decomposer		D. The living cell that a virus gets inside and takes over.
Protist		E. A single-celled organism with no nucleus.
Host cell		F. A microbe that causes disease.

1. Name the four groups of microorganisms and give one example of each.

Recall 2 marks

Your Turn

Cause and effect: how the flu makes you sick

Complete the chain. Each box leads to the box after it. Write in the empty boxes.

Apply 3 marks

An influenza virus reaches the cells lining your airways.

Hundreds of new influenza viruses are made inside one cell.

The airway cells that did the copying are destroyed.

Order the steps: yeast and bread

Number the steps from 1 (first) to 6 (last) to show how yeast makes bread light and airy.

Step number	What happens
	Bubbles of carbon dioxide are trapped in the dough, so the dough rises.
	A baker mixes yeast into dough made of flour, water and a little sugar.
	The baked loaf is light and airy because the bubbles are set in place.
	The yeast releases carbon dioxide gas (CO₂) as it feeds.
	The risen dough is baked in a hot oven.
	The yeast cells feed on the sugar in the dough.

Go Deeper

Read the data

Scenario: drawing microbes to scale

Ari is making a poster of three microbes. The textbook drawings make them look about the same size, so Ari checks their real lengths. Use 1 μm = 1000 nm.

Table 1. Typical length of three microbes

Microbe	Group	Typical length	Made of cells?
Amoeba	protist	50 μm	yes
E. coli	bacterium	2 μm	yes
influenza	virus	100 nm	no

Source: typical sizes given in Lesson 5.

2. Ari draws E. coli 1 cm long. How many times longer than E. coli must the Amoeba be drawn? Show your working.

Apply 2 marks

Answer times

3. Convert the length of E. coli into nanometres. Then work out how many times longer E. coli is than the influenza virus.

Apply 2 marks

Answer times

4. Penicillin kills bacteria by stopping them building a cell wall. Use the last column of Table 1 to explain why penicillin has no effect on the influenza virus.

Analyse 3 marks

Show What You Know

5. A student says, “Viruses are just like bacteria: both cause disease the same way.” Explain one key difference in how bacteria and viruses cause disease in the body.

Apply 2 marks

6. Mia says: “A mushroom is a plant. It grows in the ground and it doesn’t move.” Explain why scientists do not agree with Mia. Compare how a mushroom and a gum tree get their food and what their cells are built from.

Analyse 6 marks

Answer Key (for teachers)

Match each term to its meaning

Microorganism → C; Bacterium → E; Pathogen → F; Decomposer → A; Protist → B; Host cell → D.

Question 1

Bacteria (E. coli), viruses (influenza), fungi (yeast), protists (Amoeba). 1 for all four groups named; 1 for a correct example of each. [2 marks]

Cause and effect

Box 1: its oily coat merges with the cell membrane and its RNA gets inside. Box 2: it takes over the cell’s machinery, which builds viruses instead. Box 3: the airways are damaged and you get the flu. [3 marks]

Order the steps

Reading down the table as printed, the correct step numbers are 4, 1, 6, 3, 5, 2. In order: yeast mixed in; yeast feeds on sugar; it releases CO₂; bubbles trapped, dough rises; dough baked; loaf light and airy.

Question 2

50 ÷ 2 (1) = 25 times (a 25 cm drawing) (1). [2 marks]

Question 3

2 μm = 2000 nm (1); 2000 ÷ 100 = 20 times (1). [2 marks]

Question 4

Influenza is not made of cells (1), so it has no cell wall (1); an antibiotic attacks a structure bacteria have, so in a virus there is nothing to attack (1). [3 marks]

Question 5

A virus hijacks host cells to copy itself, destroying them (1); bacteria break down body tissue or release waste chemicals the body reacts to (1). [2 marks]

Question 6

Marking guide: 1 staying put and growing is not enough; 3 feeding: a fungus digests food outside its body and absorbs it (1), it cannot photosynthesise (1), a gum tree makes its own food from light (1), which is why fungi are their own kingdom; 1 chitin vs cellulose; 1 evidence (grows in shade). [6 marks]

Model answer: Mia is right that a mushroom stays put and grows, but plants and fungi both do that, so it is not enough. A gum tree makes its own food from air, water and light by photosynthesis. A mushroom cannot photosynthesise: it releases digesting chemicals into what it grows on and absorbs the food. A plant cell wall is made of cellulose, but a fungal cell wall is made of chitin. So fungi are their own kingdom, not plants. Mushrooms come up in the shade, because sunlight is no use to them.

Model answer: A mushroom cannot photosynthesise; it digests food outside itself and absorbs it, while a gum tree makes food using light. Its walls are chitin, not cellulose. It is a fungus.

Common error: saying influenza carries DNA; it carries RNA.