AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2023: Question 6

✅ Correct answer

Starch molecules are large and insoluble, so they cannot be absorbed into the blood (or pass through the wall of the small intestine).

💡 Key knowledge

• Starch is too big to pass through the gut wall.
• Digestion breaks starch into smaller sugar molecules.
• Sugars are small/soluble, so they can be absorbed into the blood.

🧠 Exam technique

For 2 marks, you needed two linked ideas: one about starch being large/insoluble, and one about absorption not being possible.

❌ Common errors

• Saying only “starch is broken down” without explaining why.
• Writing “it is absorbed” instead of “it cannot be absorbed”.
• Confusing starch with sugar.

✅ Correct answer

10 cm³ dilute iodine solution and 1 cm³ water

💡 Key knowledge

At 0 g/dm³ starch, there is no starch present, so the test mixture should contain the same volume of liquid, but without starch. Water is used to replace the starch suspension.

🧠 Exam technique

Use the details from the question. The mark scheme accepts water or the full answer describing the diluted iodine mixture.

❌ Common errors

• Saying just “iodine” without showing it is diluted.
• Forgetting the water.
• Confusing the starch suspension with the iodine solution.

✅ Correct answer

The percentage of light passing through decreased as starch concentration increased. This is because the iodine mixture became darker / more cloudy, so more light was absorbed.

💡 Key knowledge

• More starch makes the mixture darker/more cloudy.
• Darker liquids let less light through.
• Colorimeters measure percentage of light transmitted.

🧠 Exam technique

To get all 3 marks, make a cause → effect chain:

more starch → darker mixture → less light passes through / more light absorbed.

❌ Common errors

• Describing the graph without explaining why it changes.
• Saying “less light passes through because the starch is darker” without linking it to the iodine mixture.
• Writing “light is made” or “light disappears”.

✅ Correct answer

To allow them to reach 37 °C / equilibrate / be at the optimum temperature.

💡 Key knowledge

Amylase works fastest near body temperature, around 37 °C. Temperature needs to be controlled so the test is fair.

🧠 Exam technique

For one mark, a short, direct answer is enough. Mention the temperature target or the idea of equilibration.

❌ Common errors

• Talking about “mixing” instead of temperature control.
• Saying “to make it warm” without stating why.
• Giving a vague answer like “to make the reaction better”.

📐 Calculations: step-by-step

1. Read the starch concentration at 0 minutes from Figure 8: 6.6 g/dm³ (accept 6.5 to 6.7 g/dm³)
2. Read the starch concentration at 3 minutes from the question: 0.3 g/dm³
3. Find the change in concentration: 6.6 - 0.3 = 6.3 g/dm³
4. Divide by time: 6.3 ÷ 3 = 2.1 g/dm³ per minute

✅ Correct answer

Mean rate = 2.1 g/dm³ per minute

🧠 Exam technique

• Use the correct units: g/dm³ per minute.
• Show both values used in the calculation.
• Use the graph accurately: 0 min is not exactly 6.0, it is about 6.6.

❌ Common errors

• Using 6.0 instead of 6.6 from the graph.
• Forgetting to subtract first.
• Writing the unit as just g/dm³, without “per minute”.
• Using percentage light values instead of starch concentration.

✅ Correct answer

At pH 1, the shape of the active site would be altered because the enzyme would be denatured. So starch would not be digested, and the percentage of light passing through would stay at 10 (or not change / increase).

💡 Key knowledge

• Enzymes have an optimum pH.
• Very acidic conditions can denature enzymes.
• If amylase does not work, starch remains in the mixture, so the light reading stays low.

🧠 Exam technique

This is a chain of 3 ideas: acid changes active site → enzyme stops working → result on the graph stays the same.

❌ Common errors

• Only saying “enzymes work best at pH 7” without applying it to this question.
• Missing the idea of denaturation or active site shape.
• Not linking the biology to the graph result.

💡 Key facts to remember

• Starch is large and insoluble.
• Amylase digests starch into sugars.
• Temperature and pH affect enzyme activity.
• Colorimeter: darker solution = less light passes through.
• Rate = change ÷ time.

🧠 How to score full marks

• Use the exact evidence from the graph.
• Write clear cause-and-effect statements.
• Include units in calculations.
• For enzyme questions, mention active site, denaturation, and the result.

❌ Biggest traps

• Mixing up starch and sugar.
• Describing a graph without explaining it.
• Forgetting the unit “per minute”.
• Not linking pH to enzyme shape.