AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2021: Question 3

✅ Correct answer

Releases energy to the surroundings.

💡 Key knowledge

• “Exothermic” means energy is transferred out of the reaction.
• The mark scheme allows: releases energy or transfers thermal energy to the surroundings.

🧠 Exam technique

• For 1 mark, keep it simple and direct.
• Say what happens to energy, not just “it gets hot”.

❌ Common errors

• “Gives off heat” can be okay, but “transfers energy” without saying where is too vague.
• Don’t write “energy is used up” — that is not the definition.

✅ Correct answer

To keep oxygen out.

💡 Key knowledge

• Fermentation is an anaerobic reaction.
• The oil layer stops air/oxygen reaching the yeast.
• This also helps prevent aerobic respiration.

🧠 Exam technique

If you want to improve the answer, add the science idea behind it:

“To keep oxygen out because fermentation is anaerobic.”

❌ Common errors

• Writing “to stop gases escaping” is not the reason here.
• Don’t say it stops carbon dioxide entering — the point is to exclude oxygen.

✅ Correct answer

To allow the mixture / yeast / cells to reach 2 °C.

💡 Key knowledge

• The mixture must be at the same temperature before measurements begin.
• This makes the test fair and gives more reliable results.

🧠 Exam technique

• Always link this to temperature control in experiments.
• Good wording: “to equilibrate” or “to reach the water bath temperature”.

❌ Common errors

• “To let the yeast settle” is not the point.
• “To let the gas syringe warm up” is irrelevant.

✅ Full-mark explanation

At 2 °C, it is too cold for the enzymes in yeast to work properly, so no carbon dioxide is produced or fermentation is very slow. When the flask is moved to 35 °C, the enzymes become active, so carbon dioxide is produced.

💡 Key knowledge

• Enzymes work more slowly at low temperature because there is less kinetic energy and fewer successful collisions.
• At a warmer temperature, enzymes work faster, so more CO₂ is produced.

🧠 Exam technique

• Use the data in your explanation: 0 cm³ at 2 °C, then 2 → 22 cm³ after moving to 35 °C.
• For 3 marks, you usually need:
  • a description of the pattern,
  • a temperature/enzyme explanation,
  • and a link to carbon dioxide production / fermentation.

❌ Common errors

• Saying the gas is oxygen — the gas produced is carbon dioxide.
• Only describing the data without explanation.
• Writing “the yeast died at 2 °C” — that is usually too strong and not needed.

✅ Model answer

35 °C is a suitable temperature for yeast enzymes, so carbon dioxide is produced rapidly at first. After about 15 minutes, the rate of fermentation slows down because the glucose / sugar starts to run out or because the ethanol concentration increases and starts to damage the cells.

💡 Key knowledge

• 35 °C is close to the optimum temperature for yeast enzymes in this question.
• Fermentation rate can slow when the substrate runs out.
• Ethanol is a product of fermentation and can become harmful to yeast cells.

🧠 Exam technique

• To reach full marks, include all of these:
  • temperature explanation — enzymes work well at 35 °C,
  • data quote — rapid gas production at first,
  • slowdown after 15 minutes,
  • cause — sugar runs out or ethanol builds up.
• Use the graph/table trend, not just one number.

❌ Common errors

• Confusing the gas again — it is carbon dioxide, not oxygen.
• Saying “temperature causes more sugar” — incorrect.
• Not mentioning why the rate slows later on.

💡 Fermentation facts

• Yeast ferments glucose to produce ethanol and carbon dioxide.
• It happens without oxygen.
• Enzymes control the reaction rate.

🧠 How to score marks

• Use short, scientific phrases.
• Quote data from the table when asked to explain results.
• Always link temperature to enzyme activity.

❌ Biggest traps

• Wrong gas: it is carbon dioxide.
• Ignoring the table.
• Not explaining the slowdown at 35 °C.