AQA GCSE Biology: Enzymes and the Digestive System — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Enzymes and the Digestive System study guide | Enzymes and the Digestive System revision notes

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Section A

1. Name the levels of organisation from cell to organism, in order. [3]

2. State the three main types of digestive enzyme and the food group each one breaks down. [3]

Section B

3. Explain how the “lock and key” model describes enzyme action, and state two factors that affect the rate of an enzyme-controlled reaction. [4]

4. Amylase, protease and lipase each convert food into small soluble molecules.

(a) Name the products of protein digestion and of lipid digestion. [2] (b) State where in the body amylase is produced. [1] (c) Explain why food must be digested into small soluble molecules before it can be used by the body. [2]

5. Bile is produced in the liver and stored in the gall bladder.

(a) State two functions of bile in digestion. [2] (b) Explain how each function you gave increases the rate of fat digestion by lipase. [2]

6. In Required Practical 4, a student tests an unknown food sample using Benedict’s test, the iodine test and the Biuret test.

(a) Describe the observation that would indicate a positive Benedict’s test for reducing sugar. [2] (b) Describe the observation that would indicate a positive iodine test for starch. [1] (c) Describe the observation that would indicate a positive Biuret test for protein. [1]

7. In Required Practical 5, a student investigates the effect of pH on the rate of reaction of amylase with starch, using iodine reagent to test samples every 30 seconds.

(a) Describe how the student would use this method to determine the time taken for starch to be completely digested at each pH. [3] (b) Identify one variable that must be controlled in this investigation, other than pH, and explain why. [2] (c) Sketch (in words) the expected shape of a graph of time taken to digest starch against pH, and explain the pattern around the enzyme’s optimum pH. [3]

8. Explain, in terms of organisation, why a person with a damaged pancreas might struggle to digest food, using the terms cell, tissue, organ and organ system in your answer. [4]

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Answers

1. Cell → tissue → organ → organ system → organism — tissue [1], organ [1], organ system [1] (cell and organism are given).

2. Carbohydrases (e.g. amylase) break down carbohydrates/starch [1]; proteases break down proteins [1]; lipases break down lipids/fats [1].

3. An enzyme’s active site has a specific shape that fits its substrate, like a key fitting a lock, so only the correct substrate can bind and react [1] [1]. Temperature and pH both affect the rate [1] [1] (any two named factors accepted).

4. (a) Protein digestion produces amino acids [1]; lipid digestion produces glycerol and fatty acids [1]. (b) Amylase is produced in the salivary glands, pancreas, and small intestine [1] (any one). (c) Large, insoluble food molecules cannot pass through the wall of the digestive system into the blood [1]; digestion breaks them into small, soluble molecules that can be absorbed [1].

5. (a) Bile is alkaline, neutralising stomach acid [1]; it emulsifies fat into small droplets [1]. (b) Neutralising the acid creates the alkaline conditions in which lipase works best [1]; emulsifying fat increases its surface area, so lipase can act on it faster [1].

6. (a) The solution turns from blue to green, yellow, orange or brick-red (a precipitate forms) when heated with Benedict’s solution [1] [1]. (b) Iodine solution turns from orange-brown to blue-black [1]. (c) Biuret reagent turns from blue to purple/lilac [1].

7. (a) A sample is tested with iodine reagent every 30 seconds [1]; the solution stays blue-black while starch is present [1] and the time is recorded when it no longer turns blue-black (starch fully digested) [1]. (b) Temperature must be controlled (using a water bath) [1], because temperature also affects enzyme activity and would otherwise confound the effect of pH [1]. (c) Time taken decreases towards the optimum pH, then increases again either side of it [1]; at the optimum pH the enzyme’s active site fits the substrate best, so the reaction is fastest and starch is digested most quickly [1]; away from the optimum, the enzyme’s shape begins to change, reducing the rate [1].

8. Cells in the pancreas produce digestive enzymes; groups of these cells form pancreatic tissue [1]. The pancreas is an organ that releases enzymes into the small intestine [1]. It works with the stomach, liver, gall bladder and intestines as part of the digestive organ system [1]. Damage to the pancreas would reduce enzyme production, impairing the whole organ system’s ability to digest food, even though the other organs are undamaged [1].

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Where marks are usually lost

• Missing a level out of the cell → tissue → organ → organ system → organism sequence.
• Naming bile as an enzyme — it is not; it is a substance that aids digestion by neutralising acid and emulsifying fat, but does not itself break down food chemically.
• Describing the Benedict’s test colour change starting from the wrong colour, or forgetting that it requires heating.
• In Required Practical 5, forgetting to control temperature, which is the most commonly missed control variable.
• Explaining the pH-rate graph shape without linking it to the enzyme’s active site changing shape away from its optimum.

Approaching required-practical method questions

Questions on Required Practicals 4 and 5 reward the specific method actually used, not a general description of “doing an experiment”. For Required Practical 4, know which colour change corresponds to which food group and that Benedict’s test needs heating while the iodine and Biuret tests do not. For Required Practical 5, be ready to describe the continuous sampling technique precisely: iodine reagent is used to test a fresh sample from the reaction mixture at regular time intervals, and the time recorded is when the sample no longer turns blue-black, at each of a range of pH values with temperature held constant using a water bath.