1. 1
   A student is investigating enzymes involved in human digestion. She sets up test tubes containing starch solution, protein solution and lipid (fat) solution, each mixed with a different digestive enzyme extract, and incubates them at 37°C.

   (a)

   Which enzyme would break down the starch solution into simple sugars?

   [1 mark]

   • AAmylase
   • BProtease
   • CLipase
   • DBile

   (b)

   The protein solution in her experiment is broken down into small soluble molecules by a protease enzyme. What are these small soluble molecules called?

   [1 mark]

   • AFatty acids
   • BAmino acids
   • CSimple sugars
   • DGlycerol

   (c)

   Explain, using the lock and key theory, why each enzyme in her experiment only breaks down one type of food molecule and has no effect on the other two solutions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A patient has had their gall bladder removed, so bile can no longer be stored and released in large amounts during digestion.

   (a)

   Bile is alkaline. What is the main effect of bile entering the small intestine from the liver during digestion?

   [1 mark]

   • AIt absorbs digested food molecules into the bloodstream
   • BIt breaks down starch into simple sugars
   • CIt breaks down proteins into amino acids
   • DIt neutralises hydrochloric acid from the stomach and emulsifies fat

   (b)

   Without sufficient bile emulsifying the fat in the patient's food, which of the following would be the most likely direct consequence for lipid digestion?

   [1 mark]

   • AFat would be converted directly into glycerol without any enzyme action
   • BLipase would be denatured and stop working completely
   • CThe surface area of fat droplets would decrease, slowing the rate at which lipase can act
   • DThe stomach would become more acidic, increasing lipase activity

   (c)

   Describe where bile is produced and where it is stored before being released into the small intestine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician investigates the effect of pH on the rate of reaction of amylase using a continuous sampling technique with iodine reagent. She mixes amylase and starch solution at pH 7, kept at a constant 35°C in a water bath, and removes a drop of the mixture every 30 seconds to test with iodine solution. Iodine solution turns blue-black in the presence of starch and stays orange-brown once all the starch has been broken down.

   (a)

   Describe how she should carry out this investigation to find the time taken for the starch to be fully digested at pH 7.

   [3 marks]

   (b)

   She repeats the investigation at pH 3, pH 5, pH 9 and pH 11, keeping the temperature and volumes of amylase and starch solution the same each time. At pH 7 the starch was fully broken down in 90 seconds; at pH 3 and pH 11 the starch was not broken down at all, even after 10 minutes. Explain these results in terms of enzyme structure, and describe two variables (other than pH) that she must keep constant to make this a fair test.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A patient with a rare digestive disorder produces almost no lipase in their small intestine, but produces normal amounts of amylase and protease, and their liver and gall bladder function normally.

   (a)

   Explain, in detail, the roles that the digestive system's organs and enzymes normally play in breaking down and absorbing food, and how this patient's condition would specifically affect the digestion of a meal containing carbohydrate, protein and fat.

   [6 marks]

   (b)

   A doctor suggests that this patient could use qualitative reagent tests to check whether a meal has been properly digested, by testing samples taken from their gut at intervals. Describe how Benedict's test, the iodine test and the Biuret test could each be used to test for the presence of sugars, starch and protein in a food sample, and explain what the results of these three tests would suggest about this patient's digestion of the meal described above, several hours after eating it.

   [6 marks]

   Total for question 4: 12 marks

End of questions