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Date	November 2020	Marks available	3	Reference code	20N.2.SL.TZ0.1
Level	Standard level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Evaluate	Question number	1	Adapted from	N/A

Question

A study was conducted to look at the short-term effects of a change in diet on the risk of disease in young adults. The table shows data on the habitual diet of the participants as well as the study diet followed for two weeks.

[Source: Horowitz, J.F., Ortega, J.F., Hinko, A., Li, M., Nelson, R.K. and Mora-Rodriguez, R.,
2018. Changes in markers for cardio-metabolic disease risk after only 1-2 weeks of a high saturated fat diet in
overweight adults. PLoS ONE, 13(6), e0198372.]

Total blood plasma cholesterol levels were measured before the study began and once a week after starting the study diet. Mean results are shown in the bar chart, including the standard deviation.

Control of blood glucose concentration was investigated using an oral glucose tolerance test. For this test, the person was given a concentrated glucose drink (at time zero) and then blood samples were taken every 15 minutes to determine the plasma insulin level. This test was done before the study diet and after two weeks on the study diet. Mean results are shown in the graph, including the standard deviation.

Comment on the total energy content of the two diets.

[1]

Distinguish between the two diets.

[2]

Calculate, showing your working, the percentage change in mean cholesterol level after one week on the study diet.

. . . . . . . . . . . . . . . . . . . .%

[2]

Compare the data for plasma insulin levels before and after the study diet.

[2]

d.i.

State which cells secrete insulin.

[1]

d.ii.

Outline the reason for plasma insulin levels changing in the first 30 minutes of the test.

[1]

d.iii.

The hypothesis made before the study was that saturated fats in the diet affected the risk of coronary artery blockage and diabetes. Using all the data in question 1, evaluate whether this hypothesis is supported by the study.

[3]

Markscheme

a. energy is not changed (between the two diets);

b. study diet slightly lower in energy than habitual diet (but means/SD overlap);

c. spread of values show more variation for habitual diet / higher SD in habitual;

a. they differ in percent of saturated and unsaturated fats (but not total fat);

b. percent of saturated fats is higher in study diet / lower in habitual diet;

c. (mono/poly) unsaturated fats decreased in study diet compared to habitual diet/more in habitual diet
OR
polyunsaturated fats in study diet only half of what they were in habitual diet;

d. (slightly) less carbohydrate in study;

Allow numerical points if they are a valid comparison using distinguishing terms.

((165–150) ÷ 150) × 100;

(=) 10 (%);

Allow up to 167 = 11.3 %

1 mark for correct working if above 167.

a. both show same pattern of rise, level and then decrease / show same trend;

b. both show same/similar levels of insulin (at all times) due to overlapping error bars;

c. both rise for 30/45 minutes;

Do not give credit for contrasts.

d.i.

β cells of pancreas/islets (of Langerhans);

d.ii.

as blood glucose rises, insulin rises/increases to reduce the level/OWTTE;

Blood glucose must be mentioned as well as a rise in insulin.

d.iii.

Hypothesis is partially supported
a. Increased saturated fats in study diet resulted in increase in cholesterol levels;

b. cholesterol level is risk for blockage of coronary arteries;

Hypothesis is not supported
c. high insulin levels are sign of (Type II) diabetes;

d. insulin levels were the same in both diets so no increased risk;

e. study only 2 weeks long;

Examiners report

Considering that all students must complete an Individual investigation as part of their Biology course, few understood the significance of the uncertainties shown in the first table or the overlap of the standard deviations in the graph in part d. Only the better candidates were able to say that the energy was extremely similar in both diets. A surprising number could not calculate a percentage change in part c.

In part b, the command term was ‘distinguish’ which means that only differences were looked for. Similarly in d(i), ‘compare’ means that only similarities were needed. If both were required, the command term is ‘compare and contrast’.

In d, few could state that there was no significant difference in the two curves due to the overlap of the error bars. Most knew that the beta cells of the pancreas secrete insulin, although a surprising number thought that they were produced by the red blood cells. In e, many gained two marks for linking saturated fats to cholesterol and its consequence.

[N/A]

d.i.

[N/A]

d.ii.

[N/A]

d.iii.

[N/A]