12 exam-style questions (59 marks) in three tiers. Try each one on paper first, then check the model answer and mark scheme.
Chapter 3 notes12 questions · 59 marks
Showing 12 of 12 questions
Recall and simple application
Match each food class to its role: (a) carbohydrate, (b) fat, (c) protein.
Complete the food-test table.
| Reagent | Test for | Positive result |
|---|---|---|
| iodine | starch | ________ |
| Benedict's (heated) | ________ | brick-red precipitate |
| biuret | protein | ________ |
| ethanol + water | fat | ________ |
Name the small units from which (a) starch and (b) proteins are synthesised.
State what is meant by an enzyme and by denaturation.
Structured, exam-style
Four drinks were tested with heated Benedict's solution: P stayed blue, Q turned green, R turned orange, S gave a brick-red precipitate. (a) Rank the drinks by reducing-sugar content. (b) Explain the colour ladder you used. (c) State one extra step all four tubes needed.
Explain enzyme specificity using the lock-and-key hypothesis. Your answer should use the terms active site, substrate and enzyme–substrate complex.
A student adds iodine to a potato slice and to a slice of apple. Potato turns blue-black; apple stays brown. (a) State the conclusion for each. (b) The student then tests apple juice with Benedict's and it turns orange after heating. What can you conclude? (c) Explain why the apple's iodine result does not contradict (b).
Protein was digested by pepsin at three pH values: pH 2 — digested in 4 minutes; pH 6 — digested in 24 minutes; pH 10 — not digested in 60 minutes. (a) State pepsin's optimum pH from the data. (b) Calculate the rate (1 ÷ time) at pH 2 and pH 6. (c) Explain the pH 10 result.
Data, evaluation and explanation
A student claims: "Enzymes are killed by boiling, and at low temperatures enzymes stop working for the same reason." Evaluate both parts of the claim.
Starch digestion times at fixed enzyme concentration: 10 °C — 12 min; 20 °C — 6 min; 30 °C — 3 min; 40 °C — 1.5 min; 50 °C — no digestion. (a) Calculate all four digestion rates. (b) Describe the full trend. (c) Explain the trend using particle and active-site ideas.
A meal contains bread, butter and egg. (a) Name the main food class in each. (b) State the building units of the butter's food class. (c) Describe how you would confirm the presence of the egg's food class in the laboratory. (d) The bread's food class is digested to a small sugar — name it and state its role.
Design a controlled investigation to find the optimum temperature of amylase on starch. (a) State the independent and dependent variables. (b) Name two variables to control. (c) Describe how the result at each temperature would be measured. (d) Predict the shape of the graph of rate against temperature for a human enzyme.
Next step
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
30 marks · 40 min · 5 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 3 (roles of carbohydrates, fats and proteins; food tests; building units; lock-and-key enzyme action; temperature and pH effects).
Mark scheme
PROAnswers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.