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Date May 2022 Marks available 2 Reference code 22M.2.HL.TZ1.1
Level Higher level Paper Paper 2 Time zone Time zone 1
Command term Discuss Question number 1 Adapted from N/A

Question

In winter when temperatures drop, brown bears (Ursus arctos) enter a cave and hibernate. The graph shows the mean values for the body temperature, heart rate and activity of 14 brown bears throughout the year. The grey shaded areas indicate the periods when the bears are in transition between hibernation and normal activity.

[Source: Adapted from Evans, A.L., Singh, N.J., Friebe, A., Arnemo, J.M., Laske, T.G., Fröbert, O., Swenson, J.E. and Blanc,
S., 2016. Drivers of hibernation in the brown bear. Frontiers in Zoology, 13(7). This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).]

It was observed that during hibernation bears are not susceptible to osteoporosis, a condition characterized by a decrease in the density of bone, resulting in porous and fragile bones. This condition may develop in humans during long periods of inactivity and with increasing age. The graph shows the porosity of the tibia bones of black bears (Ursus americanus) and humans at different ages. Age is expressed as the proportion of normal life span.

[Source: Adapted from Journal of Biomechanics, 39(8), Donahue, S.W., McGee, M.E., Harvey, K.B.,
Vaughan, M.R. and Robbins, T., Hibernating bears as a model for preventing disuse osteoporosis,
pp. 1480–1488. Copyright (2006), with permission from Elsevier.]

Healthy bone is constantly being broken down (bone resorption) and at the same time being rebuilt (bone formation). Bone mass should therefore not normally change, nor bone diseases occur. To test whether these processes occurred during hibernation in black bears, blood serum was tested for the markers ICTP (indicating bone resorption) and PICP (indicating bone formation).

[Source: Republished with permission of Company of Biologists Ltd, from Parathyroid hormone may maintain bone formation
in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis. Donahue, Seth W; Galley, Sarah A; Vaughan,
Michael R; Patterson-Buckendahl, Patricia; Demers, Laurence M; Vance, Josef L; McGee, Meghan E, Journal of experimental
biology, 01 May 2006, Vol. 209, Issue Pt 9, pages 1630–1638; permission conveyed through Copyright Clearance Center, Inc.]

Osteocalcin is a peptide hormone that causes calcium to bind in the bones, so is involved in bone formation and regeneration. Research shows that changes in the mean concentration of osteocalcin in blood serum occur before and after hibernation in bears. In this research, concentration of both osteocalcin and parathyroid hormone were measured in the blood serum of bears. Results are shown in the bar chart and graph.

[Source: Republished with permission of Company of Biologists Ltd, from Parathyroid hormone may maintain bone formation
in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis. Donahue, Seth W; Galley, Sarah A; Vaughan,
Michael R; Patterson-Buckendahl, Patricia; Demers, Laurence M; Vance, Josef L;McGee, Meghan E, Journal of experimental
biology, 01 May 2006, Vol. 209, Issue Pt 9, pages 1630–1638; permission conveyed through Copyright Clearance Center, Inc.]

Estimate the difference between the highest and lowest mean body temperatures.

[1]
a.

Compare and contrast the changes in mean ambient and body temperatures during 2012.

[2]
b.

Explain the change in heart rate during the period of hibernation.

[2]
c.

Distinguish between the changes in porosity of the bones in humans and bears as age increases.

[1]
d.

The life expectancy of a human at the time of the study was 80 years. Estimate the porosity of the bones of the individual who was approximately 32 years old.

[1]
e.

The researchers assessed age as a proportion of normal life span, rather than in years. Suggest one reason for this.

[1]
f.

Describe what is happening to the bone during hibernation.

[2]
g.

Suggest how the graph would differ for a human during a long period of inactivity.

[1]
h.

Calculate the percentage increase in the mean concentration of osteocalcin from pre-hibernation to hibernation.

[1]
i.

A hypothesis has been proposed that an increase in parathyroid hormone concentration causes an increase in osteocalcin in bears. Evaluate the evidence for this hypothesis provided by the data.

[2]
j.

Discuss how helpful these studies of bears can be in developing an understanding of osteoporosis in humans.

[2]
k.

Markscheme

5 °C;

Units required. Accept answers in the range 4.5 to 5.5 °C.

a.

  Accept one similarity:

  1. both rise and then fall / both fall with hibernation and rise with activity
  2. both reach minimum during hibernation and maximum during activity;
  3. both lowest in January/February / both rise from January/February;

    Accept one difference
    :

  4. one peak of ambient temp but body temp has two peaks / OWTTE;
  5. body temp remains maximal for longer/plateaus whereas ambient peaks;
  6. body temperature is always higher than ambient temperature;
  7. ambient range is greater than body temperature range / OWTTE;
b.
  1. decreased/slower heart rate because bears less active/use less energy;
  2. less (cell) respiration / lower (rate of) metabolism;
  3. less oxygen/glucose required / less CO2 produced/needing to be removed;
  4. less muscle contraction/muscles require less blood;
  5. conserves energy;
c.

porosity increased in humans and decreased in bears;

Both needed.

d.

6 %;

Accept answers in the range 6.0 to 6.5 %.
Percentage sign required.

e.
  1. to allow comparison of bears and humans;
  2. bears have a different life span to humans / bears do not live to 80 years;
  3. because they age at different rates;
f.
  1. resorption/breaking down occurs and formation/rebuilding occurs;
  2. at similar rates / more resorption at most times;
  3. no/little (overall) change (in bone mass);
  4. lag between bone resorption rising and formation rising / OWTTE;
  5. bone resorption rising towards end as formation dropping / OWTTE;
g.
  1. more resorption than formation;
  2. PICP/bone formation (always) lower (than in bears);
  3. ICTP/bone resorption (always) higher (than in bears);
  4. ICTP above PICP by a greater amount in humans (than in bears);
h.

250 % (Allow 240 to 260 %)

i.
  1. (hypothesis supported by)
    positive/direct correlation/direct relationship (between parathyroid hormone and osteocalcin)
    OR
    osteocalcin rises as parathyroid hormone rises/vice versa;
  2. no evidence for causal link / causal link cannot be assumed / correlation does not prove causation;
    OR
    no evidence that parathyroid hormone causes change in osteocalcin;
    OR
    other factors may cause change in osteocalcin;
j.

  Accept one reason for the studies being helpful:

  1. (helps us understand how)) bears avoid osteoporosis;
  2. bone structure of bears and humans is similar / both are mammals;
  3. suggests that hormones/osteocalcin/parathyroid hormone might be a (preventative) treatment;

    Accept one reason for the studies not being helpful
    :
  4. humans do not hibernate / are not inactive for long periods;
  5. humans live for much longer;
k.

Examiners report

70 % of candidates were able to read two temperatures from the graph and subtract the higher from the lower correctly.

a.

Many candidates realized that they were expected to give similarities and differences between the curves, but most struggled to do this clearly. Rather than make eclectic statements about the data that may be correct but are not significant, the aim with a question such as this is to make comments that would allow someone who has not been shown the curves to sketch them. Some students confused maxima or minima with increases and decreases, so for example stating that ambient body temperature increased in July, when it reached a maximum in July and stopped increasing.

b.

Many candidates mentioned lack of activity as a reason for lower heart rate and some went on to mention reduced respiration rates or other aspects of physiology. Other candidates failed to offer an explanation and instead just described the data. A few claimed wrongly that bone porosity was directly proportional to age, in which case all those of a certain age would have the same degree of osteoporosis, which is plainly not the case.

c.

Well answered with most candidates referring to the positive and negative correlations.

d.

This question was very unusual in that the discrimination index was negative — stronger candidates were slightly less likely to answer correctly than weaker candidates, for reasons that are not obvious! Only a minority gave the expected answer of 6 %. A very common answer was 7 %, which is reached by calculating the age proportion correctly as 0.4 but using the trend line rather than the data point for the individual who was 32 years old. Careful reading of the question was needed to avoid this mistake.

e.

Most candidates were successful here, using the argument that humans and bears have different life spans.

f.

In this question candidates were expected to deduce the changes to bone from the concentrations of the markers. Given that neither of them reaches zero, we can assume that bone formation and resorption both continue throughout the hibernation period. Few candidates made this point clearly. The other 'best' answer was to comment on relative overall amounts of formation and resorption, either by stating that they are approximately equal or that there is slightly more resorption.

g.

Candidates were expected to suggest a difference between the bears and humans in the concentration of one or both of the markers, that would result in loss of bone mass. A common fault was to predict changes in the markers during a period of inactivity in humans, but changes over time were not the issue — it was differences between humans and bears that were relevant to the question.

h.

A very small proportion of candidates correctly calculated the percentage difference. There were many different incorrect answers, with 350 % being the most popular, which is the amount of osteocalcin during hibernation as a percentage of the amount pre-hibernation.

i.

This type of question is increasingly well answered with more candidates understanding that two variables being positively correlated does not prove a causation. The situation would have been different if an experiment had been done with the levels of parathyroid hormone as the independent variable controlled by the researchers.

j.

Answers here were very varied. The best included the idea that an understanding of how bears maintain bone mass despite inactivity during hibernation might lead to preventative treatments for humans. Because this was a discuss question, some counterargument was expected, based on differences between bears and humans.

k.

Syllabus sections

Core » Topic 6: Human physiology » 6.6 Hormones, homeostasis and reproduction
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