(a) A student investigated the effect of lactase on three different liquids:
- milk
- lactose‑free milk
- sucrose solution.
The student used an indicator to test for the presence of glucose. A sample of each liquid was tested before and after treatment with lactase. The indicator turned brown in the presence of glucose. The indicator remained blue in the absence of glucose. Table 4.1 shows the results of the tests.
(i) Explain the results for the three liquids shown in Table 4.1.
(ii) The student kept the solutions at a temperature that was close to the optimum during the investigation. Using your knowledge of the effect of temperature on enzyme activity, explain why this was important.
(b) As part of a balanced diet, some governments recommend that children drink milk that has vitamin D added to it.
(i) Suggest the dietary reasons for this advice.
(ii) Describe what is meant by a balanced diet.
▶️ Answer/Explanation
Ans
(a)(i) The results show that:
- Milk: Lactase breaks down lactose into glucose (and galactose), causing the indicator to turn brown after treatment.
- Lactose-free milk: Already contains glucose due to prior lactase treatment, so the indicator remains brown before and after.
- Sucrose solution: Lactase does not act on sucrose (enzyme specificity), so no glucose is produced, and the indicator stays blue.
(a)(ii) Maintaining the optimum temperature ensures maximum enzyme activity. At this temperature, kinetic energy is ideal for enzyme-substrate collisions. Deviations (too high or too low) reduce activity—either by denaturing the enzyme or slowing molecular motion.
(b)(i) Vitamin D enhances calcium absorption, which is crucial for developing strong bones and teeth in children. It also prevents deficiencies like rickets.
(b)(ii) A balanced diet provides all essential nutrients (carbohydrates, proteins, fats, vitamins, minerals) in appropriate proportions to meet energy needs and maintain health.
(a)(i) Enzymes are catalysts. Describe what is meant by the term catalyst.
(ii) Enzymes are proteins. State the names of the chemical elements found in all proteins.
(b) Lipase is an enzyme found in the human digestive system. Fig. 1.1 shows the effect of pH on the percentage activity of lipase.
Describe and explain the change in percentage activity of lipase shown on Fig. 1.1.
(c)(i) Complete the word equation to show the substrate and products for lipase.
(ii) Lipase acts in the duodenum. Explain how the body provides a suitable pH for lipase activity.
▶️ Answer/Explanation
(a)(i) A catalyst is a substance that increases the rate of a chemical reaction without being permanently changed or used up in the process.
(a)(ii) Proteins contain the elements carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
(b) The graph shows:
- Lipase activity is highest at pH 7 (optimum pH).
- Activity decreases sharply below pH 5 and above pH 8.
- At extremes of pH (pH 3.8 and pH 10.2), activity stops due to enzyme denaturation (active site shape changes, preventing substrate binding).
(c)(i) The word equation for lipase action:
Fats/Oils + Water → Fatty Acids + Glycerol
(c)(ii) The body maintains a suitable pH for lipase in the duodenum by:
- Bile (produced by the liver and stored in the gall bladder) is alkaline.
- It neutralizes stomach acid (HCl), raising the pH to ~7–8, which is optimal for lipase.