KS3 · Biology

Fermentation in microorganisms

Look closely at a slice of bread. Those little holes were made by living cells respiring without oxygen.

Sort it out

Two ways to respire without oxygen

When oxygen runs out, humans can still respire anaerobically. So can microorganisms like yeast and bacteria, and in them it has a special name: fermentation. For each feature, tick where it belongs: fermentation, humans, both, or neither.

  • A Anaerobic respiration in microorganisms (fermentation)
  • B Anaerobic respiration in humans
  1. Happens when oxygen is not available
  2. Glucose is the only reactant
  3. Happens in the cytoplasm, not the mitochondria
  4. Releases less energy than aerobic respiration
  5. Makes carbon dioxide
  6. Makes ethanol (a type of alcohol)
  7. Makes lactic acid
  8. Can cause muscle cramps
  9. Used to make bread and alcoholic drinks
  10. Uses oxygen as a reactant
  11. Produces water

Predict, then check

Two cylinders, each with the same mix of flour, sugar and water. Yeast goes into only one of them. Then both are left to stand.

What will you see, and why?

Inside the fermenter

A yeast cell up close
cell wallcytoplasmcell membranenucleusmitochondriacarbon dioxideethanol

Showing 2 layers: Yeast cell, Products leaving

Layers

Explore

tap a part, then peel the products layer

Carbon dioxide and ethanol are waste products. They would harm the cell if they built up, so both leave the cell by diffusion.

Tap each part of the cell. Then peel the products layer off and on to see what leaves.

What do you think?

Yeast with no oxygen

A batch of yeast is sealed in a container with sugary water and no air.

Which is closest to what you think the yeast does?
How sure are you?

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

In breadIn alcoholic drinks
Carbon dioxidea gas
Ethanola type of alcohol

Each cell hides a short answer and the reason behind it. Predict before you tap.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How yeast respires without oxygen, and why that puts the holes in your bread.

What you need to know

  • All living organisms, including microorganisms such as yeast and bacteria, carry out respiration to provide energy for life processes such as moving and reproducing.
  • Anaerobic respiration is respiration without oxygen ('an' means without, 'aero' means air). In microorganisms it is called fermentation.
  • The word summary for fermentation is glucose → carbon dioxide + ethanol. Glucose is the only reactant, and ethanol is a type of alcohol.
  • Fermentation and anaerobic respiration in humans both happen without oxygen, both use only glucose, both happen in the cytoplasm and both release less energy than aerobic respiration. They differ in their products: humans make lactic acid.
  • Carbon dioxide and ethanol are toxic to the cell if they build up, so they are removed from the cell by diffusion.
  • In bread, the carbon dioxide makes the dough rise. In drinks such as wine and beer, the ethanol makes them alcoholic.

The big picture

All living organisms respire to get energy for life processes, and microorganisms such as yeast and bacteria can do it without oxygen. Anaerobic respiration in microorganisms is called fermentation: glucose → carbon dioxide + ethanol. It is very like anaerobic respiration in humans: both happen without oxygen, use glucose as the only reactant, take place in the cytoplasm and release less energy than aerobic respiration. The difference is the products, because humans make lactic acid instead. People have used fermentation for thousands of years: the carbon dioxide makes bread rise, and the ethanol makes drinks such as wine and beer alcoholic.

Key points

1Fermentation = anaerobic respiration in microorganisms such as yeast.
2Fermentation: glucose → carbon dioxide + ethanol. Humans: glucose → lactic acid.
3Aerobic respiration is different again: it uses oxygen as a reactant and produces water.
4A yeast cell has a cell wall, cell membrane, cytoplasm, nucleus and mitochondria, but no chloroplasts and no permanent vacuole. Fermentation happens in the cytoplasm.
5Bread: yeast breaks down carbohydrate in flour into glucose, ferments it, and the carbon dioxide bubbles make the dough rise, leaving air holes. The small amount of ethanol evaporates when the bread is baked.
6Drinks: yeast ferments glucose from grapes (wine) or grains such as wheat and barley (beer). The ethanol makes the drink alcoholic, and the carbon dioxide makes some drinks, such as beer, fizzy.
7Humans have used fermentation for thousands of years to make bread, yogurt, wine, beer and chocolate. Yogurt is fermented by bacteria, which gives it its sour taste.

Worked example

Problem

Yeast is added to crushed grapes and left in a tank with very little air. A few weeks later the grape juice has become wine. Explain how the wine became alcoholic.

⚠ Watch out

Thinking all anaerobic respiration makes lactic acid. That's only true in humans. When yeast and other microorganisms respire without oxygen, they make carbon dioxide and ethanol instead.

🧠

Memory hook

'An' = without, 'aero' = air. Without air, yeast still respires and gives you bubbles and alcohol: the bubbles lift the bread, the alcohol goes in the drink. In humans, respiring without air makes lactic acid instead.

✓

Check yourself

Cover the page. Write the word summary for fermentation. Then give one way fermentation is the same as anaerobic respiration in humans, and the one way they differ.

Flashcards

(16)
What is fermentation?
Anaerobic respiration in microorganisms, such as yeast.
What does 'anaerobic' mean?
Without oxygen. 'An' means without and 'aero' means air.
Why do microorganisms such as yeast and bacteria respire?
To provide energy for life processes, such as moving and reproducing.
Word summary for fermentation
glucose → carbon dioxide + ethanol
What is ethanol?
A type of alcohol, made when yeast ferments glucose.
Where in a yeast cell does fermentation happen?
In the cytoplasm. The mitochondria are not involved.
Name the parts of a yeast cell.
Cell wall, cell membrane, cytoplasm, nucleus and mitochondria. No chloroplasts and no permanent vacuole.
How does a yeast cell get rid of carbon dioxide and ethanol, and why must it?
By diffusion. Both are toxic to the cell if they build up.
Give two ways fermentation is like anaerobic respiration in humans.
Any two: both happen without oxygen; glucose is the only reactant; both happen in the cytoplasm; both release less energy than aerobic respiration.
What does anaerobic respiration make in humans?
Lactic acid. Anaerobic respiration in humans can also cause muscle cramps.
Which two features belong to aerobic respiration, not anaerobic?
It uses oxygen as a reactant, and it produces water.
Why does bread dough with yeast rise?
The yeast ferments glucose and makes carbon dioxide gas. The bubbles make the dough rise and leave air holes.
What happens to the ethanol made in bread dough?
There is only a small amount, and it evaporates during baking.
What makes some drinks, such as beer, fizzy?
Carbon dioxide released by the yeast as it ferments.
Name five products humans have made with fermentation for thousands of years.
Bread, yogurt, wine, beer and chocolate.
What ferments yogurt, and what does it give it?
Bacteria. They give yogurt its sour taste.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 1 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.