State the change in blood calcium levels in normal mice 10 minutes after the administration of calcium, with and without the addition of vitamin D.

With vitamin D:

Without vitamin D:

Compare the changes in blood calcium levels in normal mice and in mutant mice after the administration of calcium.

Explain, using graph B, the changes in blood calcium levels for the mice with different diets.

Discuss whether the scientists were able to support their hypothesis that rickets caused by the defective vitamin D receptor can be prevented by the intake of large amounts of calcium.

with vitamin D: (from 0.0 to) 1.7 μmol (units required – allow answers in the range of 1.65 to 1.75 μmol)

without vitamin D: (from 0.0 to) 1.1 μmol (units required – allow answers in the range of 1.05 to 1.15 μmol)

Both needed to award the mark.

a. both increase with time;

b. normal mice have a greater increase in blood calcium levels than mutant mice (after ten minutes);

c. normal mice have a maximum change of 1.1 μmol while mutant mice have a maximum change of 0.6 μmol;

d. mutant mice show gradual increase while normal mice show rapid increase followed by a plateau;

a. mice with low calcium diets have a greater increase in blood calcium levels (after calcium administration) because their body absorbs more calcium;

b. if they have had a high calcium diet they do not need to absorb so much calcium / vice versa;

c. probably receptors are all occupied/inhibited / less receptors;

a. hypothesis supported as blood calcium levels increased in mutant mice after intake of calcium (graph A);

b. but less than in normal mice / perhaps not enough to cure disease/rickets;

c. administering vitamin D also shows an increase in blood calcium levels (graph A);

d. but no good administering vitamin D as the receptor is defective;

e. in a high calcium diet, less absorption occurs (graph B), so might not be the solution;

f. should have tested mutant mice with different diets;

Most candidates read the graphs correctly to obtain the correct numerical values but a surprising number lost marks for not including units.

Most candidates were able to get two marks for recognizing that although both mutant and normal mice showed an increase in calcium levels over time, the normal mice had a greater increase which was rapid and then plateaued while the mutant mice had a more gradual increase. Candidates need to be careful that when they answer a compare question, that they actually compare the two items requested rather than simply describe each.

Candidates struggled with this question as they did not “explain” the changes in blood calcium levels; they simply stated what was in the graph. Those who did attempt an explanation, understood that the mice with a low calcium diet would need to absorb more calcium and therefore the increase in blood calcium levels would be more. Very few commented on that fact that receptors were involved in some way.

Candidates also struggled with this question. Stronger candidates were able to get two marks for the support of the hypothesis shown in graph A or that it was not supported as shown in the lower absorption of calcium in high calcium diets in graph B. No candidates commented on the idea that that the investigation did not test mutant mice on different diets.