1. 1
   After a meal, liver cells of a mammal take up excess glucose from the blood and convert it into glycogen. Between meals, the glycogen is broken down and glucose is released back into the blood. A single glycogen molecule can contain tens of thousands of α-glucose units, joined in chains that are highly branched, with a new branch every 8–12 glucose units.

   (a)

   Which type of reaction joins α-glucose molecules to form glycogen, and what is released?

   [1 mark]

   • AHydrolysis, releasing water
   • BCondensation, releasing water
   • CCondensation, releasing carbon dioxide
   • DHydrolysis, releasing oxygen

   (b)

   Why is the highly branched structure of glycogen an advantage when glucose must be released quickly?

   [1 mark]

   • ABranching makes glycogen soluble so it can diffuse out of the cell
   • BBranching lets chains be cross-linked by hydrogen bonds into strong fibres
   • CBranching gives many chain ends, so enzymes can remove glucose units from many places at once
   • DBranching means glycogen is built from β-glucose, which is hydrolysed faster

   (c)

   Explain why storing glucose as glycogen, rather than as free glucose, is an advantage to a liver cell.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A person's ABO blood group depends on glycoproteins in the plasma membrane of their red blood cells. Everyone has the same basic carbohydrate chain (the O antigen). In people of group A an extra N-acetylgalactosamine is added to this chain; in group B an extra galactose is added; group AB has both. Blood plasma contains antibodies against the A or B antigens that are not present on the person's own red blood cells. A patient of blood group A is accidentally given a transfusion of group B blood.

   (a)

   What is a glycoprotein?

   [1 mark]

   • AA protein with a carbohydrate chain attached
   • BA lipid with a carbohydrate chain attached
   • CA polysaccharide made of amino acids
   • DA protein that stores glucose in the cytoplasm

   (b)

   Red blood cells of which group could be given to a patient of any ABO group without an ABO antigen reaction?

   [1 mark]

   • AA
   • BB
   • CAB
   • DO

   (c)

   Explain why the transfusion of group B blood causes a reaction in the group A patient.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compared the fatty acids in three fats and oils, given as percentages of total fat. Butter (from cows): saturated 63%, monounsaturated 26%, polyunsaturated 4%. Olive oil: saturated 14%, monounsaturated 73%, polyunsaturated 11%. Sunflower oil: saturated 10%, monounsaturated 20%, polyunsaturated 66%. The student also found the melting points of three fatty acids that each have 18 carbon atoms: stearic acid (no C=C double bonds) 69 °C, oleic acid (one cis C=C double bond) 13 °C and linoleic acid (two cis C=C double bonds) −5 °C. At 20 °C butter is solid, while olive oil and sunflower oil are liquid.

   (a)

   Explain the relationship between the number of C=C double bonds and the melting points of the three fatty acids.

   [3 marks]

   (b)

   Using the data, explain why butter is solid at 20 °C whereas the plant oils are liquid, and suggest why endotherms and plants store different types of fatty acid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student writes in a revision summary: "The only function of lipids is to store energy. They are useful for this because they do not dissolve in water." The student's teacher points out that a harp seal can have a layer of blubber (adipose tissue) several centimetres thick beneath its skin, that cell membranes are built from phospholipids, and that oestradiol and testosterone are lipids.

   (a)

   Explain how the properties of triglycerides make them suitable for long-term energy storage and for thermal insulation in animals such as the harp seal.

   [6 marks]

   (b)

   Evaluate the student's statement that the only function of lipids is to store energy.

   [6 marks]

   Total for question 4: 12 marks

End of questions