HL Paper 2
Hypoxia is a condition in which tissues of the body are deprived of an adequate oxygen supply. A study was carried out in rats to examine the effects of continuing hypoxia on the structure of the diaphragm, and to determine whether nitric oxide is implicated in adaptation of the diaphragm to hypoxia. The diaphragm helps to supply oxygen to tissues and organs in the body by ventilating the lungs.
A group of 36 adult male rats were kept for 6 weeks in low oxygen while 36 adult male rats were kept in normal oxygen levels.
[Source: Reproduced with permission of the © ERS 2011. European Respiratory Journal June 2011, 37 (6) 1474–1481; DOI: 10.1183/09031936.00079810]
The graph shows the effect of hypoxia on the endurance of rats’ diaphragm muscle after 6 weeks. Endurance is the change in force measured as a percentage of the initial force.
[Source: Reproduced with permission of the © ERS 2011. European Respiratory Journal June 2011, 37 (6) 1474–1481; DOI: 10.1183/09031936.00079810]
The sodium–potassium pump plays a role in muscle activity. Nitric oxide may have a role in the recovery of hypoxic muscles. The production of nitric oxide can be blocked with an inhibitor of the enzyme nitric oxide synthase. The graph shows the concentration of sodium–potassium pumps in the diaphragm of control and hypoxic rats without and with nitric oxide synthase inhibitor.
[Source: Reproduced with permission of the © ERS 2011. European Respiratory Journal June 2011, 37 (6) 1474–1481; DOI: 10.1183/09031936.00079810]
Skeletal muscle contractions can take two different forms: if they are stimulated by a single action potential they take the form of a twitch and if they are stimulated by a series of action potentials the contraction is longer lasting (tetanic). The table shows the effects of hypoxia on the force of twitch and peak tetanic contraction in the diaphragm.
[Source: Reproduced with permission of the © ERS 2011. European Respiratory Journal June 2011, 37 (6) 1474–1481; DOI: 10.1183/09031936.00079810]
Outline the effect of hypoxia on body mass and erythrocyte percentage.
Using the data in the graph, deduce whether hypoxia increases or decreases the endurance of the rats’ diaphragm muscle.
Using the data presented in this question, explain the effect of hypoxia on the body.
Analyse the graph to obtain two conclusions about the concentration of sodium–potassium pumps.
Muscle fibres are stimulated to contract by the binding of acetylcholine to receptors in their membranes and the subsequent depolarization.
Suggest a reason for increasing the concentration of sodium–potassium pumps in the membranes of diaphragm muscle fibres.
Outline the effect of hypoxia on the force of contraction of the diaphragm.
Hypoxia caused a 13 % increase in the surface area to volume ratio of the diaphragm. Suggest a reason for this change.
Using all relevant data in the question, evaluate the effectiveness of the rats’ adaptation to hypoxia.
Discuss the advantages and disadvantages of using rats as models in this investigation.
Cholinergic synapses use acetylcholine as their neurotransmitter. They are widespread in the body, passing on signals to muscle cells. These synapses are affected by neonicotinoid pesticides.
The diagrams, which are not drawn to scale, show the synapse between two neurons and a detail of the synaptic cleft.
On the diagrams, label with a letter H the hydrophilic end of a phospholipid.
On the diagrams, label with a letter E a vesicle involved in exocytosis.
On the diagrams, label with a letter P a location where a neonicotinoid pesticide could bind.
Outline how depolarization of the membrane of an axon occurs.
Explain how acetylcholine initiates an action potential in a postsynaptic membrane.
State the action of the enzyme acetylcholinesterase.
Explain what happens to an enzyme if there is a change of pH.
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.
Compare and contrast the changes in mean ambient and body temperatures during 2012.
Explain the change in heart rate during the period of hibernation.
Distinguish between the changes in porosity of the bones in humans and bears as age increases.
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.
The researchers assessed age as a proportion of normal life span, rather than in years. Suggest one reason for this.
Describe what is happening to the bone during hibernation.
Suggest how the graph would differ for a human during a long period of inactivity.
Calculate the percentage increase in the mean concentration of osteocalcin from pre-hibernation to hibernation.
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.
Discuss how helpful these studies of bears can be in developing an understanding of osteoporosis in humans.