Fermentation is the release of energy from sugar by microorganisms without using oxygen. In yeast it produces ethanol and carbon dioxide. It appears in questions on bread, brewing, yoghurt and investigations with gas bubbles.
It links directly to gas exchange and respiration, so if anaerobic respiration is unclear, read that first.
What happens during fermentation?
Yeast is a single-celled fungus. When oxygen is limited, its cells break down glucose in a series of enzyme-controlled reactions and release a small amount of energy.
The word equation is:
glucose → ethanol + carbon dioxide (with energy released)
Some bacteria do something different. In yoghurt making, lactic acid bacteria ferment the sugar in milk (lactose) to lactic acid. The acid lowers the pH, which thickens the milk and gives the sour taste.
How do you turn this into an explanation?
Use a three-part chain: condition, process, consequence.
- Condition: what the microorganism has (sugar, warmth, little oxygen).
- Process: what the cells do (anaerobic respiration by enzymes).
- Consequence: what you observe (gas bubbles, a rising dough, a sour taste).
Worked example
The data below are invented for practice. A student places the same mass of yeast and glucose solution in three sealed flasks. Each flask is kept at a different constant temperature. The gas leaves through a tube into water and the student counts bubbles for 2 minutes.
| Temperature (°C) | Bubbles in 2 minutes |
|---|---|
| 20 | 12 |
| 30 | 30 |
| 40 | 44 |
Question: Calculate the rate at each temperature and explain the trend.
Step 1, rate = bubbles ÷ time. At 20 °C: 12 ÷ 2 = 6 bubbles per minute. At 30 °C: 30 ÷ 2 = 15 bubbles per minute. At 40 °C: 44 ÷ 2 = 22 bubbles per minute.
Step 2, describe the trend. The rate increases as temperature rises from 20 °C to 40 °C.
Step 3, explain. The bubbles are carbon dioxide from anaerobic respiration. Higher temperature gives the enzymes and sugar molecules more kinetic energy, so they collide more often and react faster, so more gas forms each minute.
Step 4, add the limit. These data stop at 40 °C, so they cannot tell us the optimum. Above the optimum the enzymes would denature and the rate would fall.
The mistake to watch for
Mistaken answer: “The yeast needs oxygen to make alcohol, so the flask must be open.”
Fermentation in yeast is anaerobic. Oxygen is not needed, and with plenty of oxygen yeast respires aerobically, producing carbon dioxide and water instead of ethanol.
The correction is to read the word “fermentation” as a cue for “no oxygen, enzymes, sugar as the starting material”. Then name the products for the organism in the question.
Check yourself
1. Write the word equation for fermentation in yeast.
Show answer
glucose → ethanol + carbon dioxide (energy is released).
2. In the experiment above, a fourth flask at 70 °C gives only 2 bubbles in 2 minutes. Explain this.
Show answer
The rate is 2 ÷ 2 = 1 bubble per minute. At 70 °C the yeast enzymes have denatured: their shape has changed, so sugar no longer fits the active sites and little respiration occurs.
3. A fifth flask produces 18 bubbles in 3 minutes. What is the rate?
Show answer
18 ÷ 3 = 6 bubbles per minute.
Where this leads next
Next, see how enzymes are used in industry, then practise mixed questions in the biotechnology practice set.
Explaining an experiment in full sentences is a skill teachers can watch and adjust in real time, which is what online one-to-one Biology tuition is built around. The wider route is on the module page.