1. 1
   A student investigated the breakdown of starch by amylase at 35°C and pH 7. She mixed starch solution with amylase and, every 30 seconds, tested a sample with iodine solution to see if starch was still present. After 3 minutes, no starch remained.

   (a)

   Which part of the amylase molecule allows it to break down starch?

   [1 mark]

   • AThe nucleus
   • BThe cell membrane
   • CThe active site
   • DThe vacuole

   (b)

   The student then repeated the experiment at 90°C. No breakdown of starch was detected, even after 10 minutes.
   
   What is the best explanation for this result?

   [1 mark]

   • AThe iodine solution stopped working at high temperature
   • BThe starch has been denatured and can no longer react
   • CThe amylase moved too quickly to collide with starch molecules
   • DThe amylase has been denatured and its active site no longer fits the starch

   (c)

   Explain why amylase can break down starch but cannot break down protein molecules.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician measured the activity of pepsin (a protein-digesting enzyme) at increasing pH values, from pH 1 to pH 9, keeping temperature and substrate concentration constant. Pepsin activity was highest at pH 2 and fell steadily as pH increased, reaching almost zero by pH 7.

   (a)

   What does this pattern suggest about pepsin's optimum pH?

   [1 mark]

   • APepsin's optimum pH is strongly alkaline, around pH 9
   • BPepsin's optimum pH is neutral, around pH 7
   • CPepsin's optimum pH is strongly acidic, around pH 2
   • DPepsin has no optimum pH and works equally at all values

   (b)

   Which statement correctly explains why pepsin activity fell as pH increased above pH 2?

   [1 mark]

   • ARising pH has no effect on protein structure
   • BRising pH increases the concentration of substrate available to pepsin
   • CRising pH increases the kinetic energy of pepsin molecules
   • DRising pH gradually changes the shape of pepsin's active site, reducing substrate binding

   (c)

   The stomach produces hydrochloric acid, which maintains a strongly acidic environment around pH 2. Suggest why this is important for pepsin's role in digestion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student carried out a core practical investigating the effect of substrate concentration on the rate of an enzyme-catalysed reaction. She used a fixed volume and concentration of enzyme and increased the concentration of substrate solution across five trials, keeping temperature and pH constant. She measured the time taken for the reaction to reach completion at each substrate concentration.
   
   Her results showed that as substrate concentration increased, the reaction time decreased, until above a certain substrate concentration the reaction time stopped changing.

   (a)

   Describe how the student should have controlled the variables in this investigation, other than substrate concentration, to make it a fair test.

   [3 marks]

   (b)

   Explain why the reaction time stopped decreasing above a certain substrate concentration, even though substrate concentration continued to increase.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food scientist is developing a calorimetry method to measure the energy content of two snack bars, Bar X and Bar Y. In her method, a small dried sample of each food is burned underneath a boiling tube containing a known volume of water, and the temperature rise of the water is recorded using a thermometer. She calculates the energy released per gram of food using the temperature rise, the mass of water and the mass of food burned. Bar Y contains a mixture of carbohydrates, proteins and lipids, and she also wants to confirm which nutrients Bar Y contains before testing it, and understand how enzymes digest these nutrients.

   (a)

   Using Bar X, she recorded a much smaller temperature rise than expected from the manufacturer's stated energy value on the packaging. Evaluate the calorimetry method described and explain possible reasons why the measured energy value for Bar X might be lower than the manufacturer's stated value.

   [6 marks]

   (b)

   Describe how the scientist could test a sample of Bar Y for starch, reducing sugars, protein and fat, and explain the role of different enzymes in breaking down these food groups into smaller molecules that the body can absorb.

   [6 marks]

   Total for question 4: 12 marks

End of questions