AQA GCSE Combined Science: Trilogy Biology Paper 1 (Higher), 2019: Question 2

💡 Key knowledge

• Starch is digested by amylase.
• Small intestine is where digested food is absorbed into the blood.
• Iodine tests for starch.
• Benedict’s solution tests for sugar.
• Enzymes break large molecules into smaller ones.

🧠 Exam technique

• Use the exact scientific term the mark scheme wants.
• For 2-mark explanations, give two linked points.
• When describing results, refer to what changed over time.

✅ Correct answer

Amylase

💡 Key knowledge

• Amylase is a carbohydrase.
• The mark scheme also allows carbohydrase.
• Do not write amylase as amylose — that is a starch molecule, not an enzyme.

❌ Common errors

• Writing amylase incorrectly spelled.
• Giving the source of the enzyme, such as salivary amylase or pancreatic amylase, instead of the enzyme name.
• Writing amylose — this is not accepted.

✅ Correct answers

Partially permeable tubing: small intestine

Water in the test tube: blood

💡 Key knowledge

• The tubing models a membrane that lets small molecules through but not larger ones.
• The small intestine absorbs digested food into the bloodstream.
• The mark scheme also allows plasma for the liquid in the test tube.

🧠 Exam technique

• Match the model to the body part by its function.
• Think: “Where are small digested molecules absorbed?”
• Use small intestine, not just “intestine”.

❌ Common errors

• Writing large intestine — this is wrong.
• Writing just intestine without specifying small intestine.
• Calling the liquid water in the answer instead of its body equivalent.

✅ Correct answers

Starch: iodine solution

Sugar: Benedict’s solution

💡 Key knowledge

• Iodine tests for starch.
• Benedict’s tests for reducing sugar.
• In GCSE questions, “sugar” usually means a reducing sugar such as glucose.

❌ Common errors

• Writing iodide instead of iodine.
• Writing only “iodine” is usually fine, but iodine solution is safest.
• Mixing up the tests: Benedict’s is for sugar, not starch.

🧠 Exam technique

If you are unsure, remember:

• Iodine → I for starch
• Benedict’s → B for biological sugar test

✅ Correct answer

The enzyme had not started to work, so none of the starch had been digested/broken down into sugar.

💡 Key knowledge

• At the start, the mixture contains starch and enzyme, but not sugar.
• Amylase must act on starch first to produce sugar.
• Before digestion begins, there is no product yet.

❌ Common errors

• Saying “because there was no sugar added” — too simple and does not explain the biology.
• Forgetting that the question asks about test 1, which is the starting mixture.
• Writing “the enzyme had not worked enough” without linking it to starch digestion.

🧠 Exam technique

For a 1-mark explanation, give one clear cause-and-effect statement:

enzyme not active yet → starch not broken down → no sugar

✅ Correct answer

The enzyme digested/broke down starch to form sugar, but not all the starch was digested.

💡 Key knowledge

• After 30 minutes, the mixture inside the tubing tested positive for starch and positive for sugar.
• This shows that digestion had started, but was incomplete.
• Amylase breaks starch into smaller sugar molecules.

🧠 Exam technique

• Use the results table in your answer.
• Link each result to what it means:
  • starch still present → not all starch broken down
  • sugar present → starch has been digested
• Top answers explain both parts of the result.

❌ Common errors

• Saying only “the enzyme worked” without mentioning sugar formation.
• Ignoring that starch is still present.
• Confusing digestion with absorption.

✅ Correct answer

Sugar molecules formed are small enough to pass through the tubing, but starch molecules are too large to pass through.

💡 Key knowledge

• Partially permeable membranes let small molecules through.
• Large molecules like starch cannot pass through.
• This models absorption of digested food into the blood.

🧠 Exam technique

• Use the size idea: small enough vs too large.
• Make sure you answer why sugar is in the test tube water but starch is not.
• Use the table as evidence: water after 30 minutes is positive for sugar but negative for starch.

❌ Common errors

• Saying sugar “diffused because it is lighter” — incorrect.
• Writing that starch passed through the tubing.
• Not mentioning that the tubing is partially permeable.

📐 Calculation-style thinking: not a maths question, but still step-by-step

1. Identify the starting material: starch + enzyme.
2. Identify the products: sugar formed after digestion.
3. Use the test results to show what is present in each sample.
4. Match the model to the body: tubing = small intestine, water = blood.

🧠 What distinguished top-level responses?

• They used the correct scientific terms: amylase, small intestine, blood, iodine, Benedict’s.
• They gave two-part explanations for the 2-mark items.
• They connected the result to digestion and movement of small molecules.

❌ High-risk traps

• Mixing up digestion and absorption.
• Calling the tubing the large intestine.
• Using the wrong test reagents.
• Giving vague answers like “it changed” without saying what changed.