AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2022: Question 3

✅ Correct answer

Salivary glands, pancreas and small intestine

This is the only option that matches the mark scheme exactly.

💡 Key knowledge

• Amylase is an enzyme that digests starch.
• It is made in the salivary glands, pancreas and small intestine.
• Different organs make enzymes for digestion at different stages of the gut.

🧠 Exam technique

For multiple choice questions, compare each option carefully with the biology you know. One correct line is enough for the mark.

❌ Common errors

• Choosing stomach instead of small intestine.
• Forgetting that the pancreas makes digestive enzymes.

✅ Correct answer

pH of the amylase solution / buffer

💡 Key knowledge

• The independent variable is the factor the student changes.
• Here they repeat the experiment at pH 5, 6, 7 and 8.

🧠 Exam technique

If a method says “repeat using different pH values,” the independent variable is the pH.

The mark scheme also accepts “hydrogen ion concentration” because pH is linked to H⁺ concentration.

❌ Common errors

• Saying amylase or starch instead of pH.
• Giving the dependent variable, such as time taken.

✅ Correct answer

Iodine solution would not turn blue-black.

Or: iodine would stay orange/brown.

💡 Key knowledge

• Iodine is the test for starch.
• If starch is present, iodine turns blue-black.
• If starch has been broken down, iodine stays orange/brown.

🧠 Exam technique

The question asks how you would know the starch has gone. You need the result of the iodine test, not just “the starch is digested”.

❌ Common errors

• Saying “the mixture turns colourless” — that is not the iodine test result.
• Writing “the iodine turns black” without saying blue-black or stays brown.

✅ Correct answer

6.8

The mark scheme allows 6.75 to 6.85.

💡 Key knowledge

• The optimum pH is the pH where the enzyme works fastest.
• On this graph, that is where the time taken is at its lowest.

🧠 Exam technique

Read the curve carefully. The optimum is not always an exact printed number on the axis; it may be between two values, so estimate from the graph.

For this question, the lowest point is just under pH 7.

❌ Common errors

• Choosing pH 5 or pH 8 because they are the plotted points.
• Ignoring that the optimum is the minimum time, not the highest time.

✅ Correct answer

82 seconds

Acceptable range: 80 to 84 seconds.

💡 Key knowledge

• In Figure 7, the reaction finishes when the curve levels off.
• At this point, no more sugar is being produced because all the starch has been used up.

🧠 Exam technique

Look for the point where the line becomes flat. Then read the time from the x-axis.

Examiner note: answer lines take precedence, so a clearly read value from the graph was credited even if the wording was brief.

❌ Common errors

• Using the first flat-looking point too early.
• Reading the y-axis instead of the time axis.
• Forgetting to give the answer in seconds.

📐 Calculations: step by step

1. Draw a tangent to the curve at 40 seconds.
2. Choose two clear points on the tangent line.
3. Find the change in sugar and the change in time.
4. Calculate gradient using rate = change in y ÷ change in x .
5. Convert to per minute by multiplying by 60.

Example from the mark scheme:

rate = 2.25 ÷ 60 = 0.0375 arbitrary units per second

0.0375 × 60 = 2.25 arbitrary units per minute

✅ Correct answer

2.25 arbitrary units per minute

Acceptable range: 2.1 to 2.5 arbitrary units per minute.

💡 Key knowledge

• Rate is the gradient of the graph.
• Use a tangent because the curve is changing at 40 seconds.
• Always include units: arbitrary units per minute.

🧠 Exam technique

• Marks are for the method as well as the final answer.
• If your tangent is sensible and your arithmetic is correct, you can gain most or all marks.
• Make sure your working shows how you went from the graph to the rate.

❌ Common calculation traps

• Using a secant line instead of a tangent.
• Forgetting to convert from per second to per minute.
• Reversing the gradient calculation.
• Leaving off units.

✅ What a full-mark answer should include

• Enzymes are protein molecules.
• They have a specific 3D structure.
• They have an active site with a shape that fits the substrate.
• Starch is the substrate for amylase.
• At the optimum pH, the active site has the correct shape, so more enzyme-substrate complexes form.
• At pH values above or below the optimum, the active site changes shape.
• At extreme pH, the enzyme is denatured.
• This means the substrate no longer fits the active site, so the reaction rate decreases.

💡 Key knowledge

Key idea: pH affects the bonds holding the protein’s shape together.

When the shape changes, the active site changes too. That is why amylase works best at one pH and less well at other pH values.

🧠 Exam technique

This is a 6-mark explanation, so you need a linked chain of ideas, not separate facts.

A strong answer should go something like:

1. Enzymes are proteins with a specific shape.
2. The active site matches starch.
3. At the optimum pH the shape stays correct.
4. At different pH values the active site changes shape.
5. Fewer enzyme-substrate complexes form.
6. At extreme pH the enzyme is denatured and stops working.

Top responses were the ones that explained why pH changes activity, not just that it “affects the enzyme”.

❌ Common errors

• Saying only “low pH slows it down” with no explanation.
• Confusing the substrate with the enzyme.
• Not mentioning the active site.
• Not using the word denatured at higher/lower extreme pH.

Key facts to remember

• Amylase is produced in the salivary glands, pancreas and small intestine.
• Iodine stays orange/brown when starch is gone.
• The optimum pH is where the time taken is lowest.
• Rate = gradient = change in amount ÷ change in time.
• Enzyme activity depends on active site shape.

How to score well

• Use the exact biology term the mark scheme wants.
• Read graphs carefully and estimate if needed.
• Show working clearly in calculations.
• For explanations, link cause → effect → result.