1. 1
   A student uses molecular models to build a short peptide. She joins four different amino acids one at a time, removing the atoms of one water molecule from the models each time two amino acids are joined. She then compares her model with the hormone insulin, which contains 51 amino acids arranged in two separate chains of 21 and 30 amino acids.

   (a)

   Which groups react with each other each time the student joins two amino acids?

   [1 mark]

   • AThe amine group of one amino acid and the R-group of the other
   • BThe carboxyl group of one amino acid and the amine group of the other
   • CThe alpha carbon atoms of the two amino acids
   • DThe hydrogen atoms attached to the two alpha carbon atoms

   (b)

   How many water molecules are released when the two chains of insulin are each assembled from free amino acids?

   [1 mark]

   • A51
   • B50
   • C49
   • D2

   (c)

   Outline the structure of a generalized amino acid.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A dietitian is planning meals for a 60 kg vegan athlete. The recommended intake of the essential amino acid lysine is 30 mg per kg of body mass per day. Wheat protein contains 25 mg of lysine per gram of protein but is relatively rich in the essential amino acid methionine. Lentil protein contains 70 mg of lysine per gram of protein but is relatively poor in methionine. The athlete currently eats 50 g of wheat protein and no lentils each day.

   (a)

   What makes lysine an essential amino acid?

   [1 mark]

   • AIt is needed in larger amounts than other amino acids
   • BIt is found only in animal proteins
   • CIt can be made from other amino acids in the liver
   • DIt cannot be synthesized in the body and must be obtained from food

   (b)

   What is the athlete's current daily shortfall in lysine?

   [1 mark]

   • A550 mg
   • B1250 mg
   • C1800 mg
   • D3050 mg

   (c)

   Calculate the mass of lentil protein that must be added to the athlete's daily diet to remove the lysine shortfall, and suggest why combining lentils with wheat is better than replacing all the wheat with lentils.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   To investigate denaturation, a student heated test tubes of a clear 1% egg albumin solution in water baths for 10 minutes at 30, 40, 50, 60, 70 and 80 °C, using one tube at each temperature. She then measured the percentage of light transmitted through each solution: a solution of native albumin is clear and transmits almost all light, whereas denatured albumin clumps together and makes the solution cloudy. Transmission was 98% at 30 °C, 97% at 40 °C, 95% at 50 °C, 61% at 60 °C, 18% at 70 °C and 12% at 80 °C. After cooling to 20 °C for one hour, the tubes from 70 °C and 80 °C were still cloudy.

   (a)

   Describe and explain the effect of temperature on the albumin solution.

   [3 marks]

   (b)

   The student concluded that albumin denatures at 60 °C and that the denaturation is permanent. Evaluate this conclusion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Oxytocin, a hormone that stimulates contraction of the uterus, is a single chain of 9 amino acids. Insulin, a hormone that regulates blood glucose, contains 51 amino acids. Titin, a protein that gives muscle its elasticity, is a single chain of about 34 000 amino acids. All three are built from the same 20 amino acids that are coded for in the genetic code. Pepsin, an enzyme that digests proteins in the stomach at about pH 2, loses its activity permanently if the pH is raised to 8.

   (a)

   Explain why an almost infinite variety of polypeptides is possible. Include a calculation using the information given.

   [6 marks]

   (b)

   Explain how changes in temperature and pH affect the structure of proteins, and discuss the consequences for the function of proteins such as pepsin.

   [6 marks]

   Total for question 4: 12 marks

End of questions