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Date	November 2017	Marks available	2	Reference code	17N.2.HL.TZ0.01
Level	Higher level	Paper	Paper 2	Time zone	TZ0 / no time zone
Command term	Explain	Question number	01	Adapted from	N/A

Question

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.

[1]

Using the data in the graph, deduce whether hypoxia increases or decreases the endurance of the rats’ diaphragm muscle.

[2]

Using the data presented in this question, explain the effect of hypoxia on the body.

[2]

Analyse the graph to obtain two conclusions about the concentration of sodium–potassium pumps.

[2]

d.i.

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.

[1]

d.ii.

Outline the effect of hypoxia on the force of contraction of the diaphragm.

[1]

e.i.

Hypoxia caused a 13 % increase in the surface area to volume ratio of the diaphragm. Suggest a reason for this change.

[1]

e.ii.

Using all relevant data in the question, evaluate the effectiveness of the rats’ adaptation to hypoxia.

[3]

Discuss the advantages and disadvantages of using rats as models in this investigation.

[2]

Markscheme

Erythrocyte percentage increased AND body mass reduced/smaller increase in mass

a. increases endurance «in relation to the control»

b. higher force/endurance at every testing time/throughout

smaller decreases in force «over time»

c. the magnitude of the difference is similar throughout the five minutes experiment/testing

d. differences are «statistically» significant

e. endurance of control is «approximately» 35 % versus endurance of hypoxia «approximately» 55 % «after 5 minutes»

Accept ±5 % for both percentages

[Max 2 Marks]

a. diaphragm more endurance/stronger/generates more force for more ventilation/inspiration

b. right ventricle mass increases to pump more blood

c. erythrocyte percentage increases to transport oxygen

d. less growth/body mass which reduces oxygen demand

Reject “loss of body mass”

The physiological reason is required for each mark

[Max 2 Marks]

a. hypoxia increases the concentration of sodium–potassium pumps

b. nitric oxide needed for/stimulates «production of» sodium-potassium pumps

c. nitric oxide synthase inhibitor reduces the concentration of pumps
OR
concentration of pumps reduced by inhibiting nitric oxide production

Award up to [1] for a conclusion on lines labelled 1 and up to [1] for a conclusion on the lines labelled 2

[Max 2 Marks]

d.i.

a. resting potential restored faster

b. increases the «maximum» frequency/rate of contractions

can contract again sooner

Accept shorter refractory period for mpa

Do not accept faster contraction/depolarization/ repolarization

[Max 1 Mark]

d.ii.

reduces «force of» twitch AND peak tetanic contraction

e.i.

a. decrease in volume/atrophy/loss of cells/less muscle fibres/less tissue in the diaphragm

b. SA to volume ratio increased to make oxygen uptake into muscle/cells faster

Do not accept reduction in area of diaphragm

[Max 1 Mark]

e.ii.

a. not effective because body mass lost

b. effective because body mass still increases/rats still grow

c. not effective because contractions/force exerted by diaphragm decreases

d. effective because more sodium-potassium pumps so more/faster rate of diaphragm/muscle contractions

e. effective because endurance of diaphragm increases

f. effective because mass of right ventricle increases

g. effective because erythrocyte percentage increases

For each marking point the candidate must make it clear whether they are arguing for adaptation being effective or not. This can be done by giving the physiological benefit of a change, for example greater mass of right ventricle so more blood pumped.

[Max 3 Marks]

Advantages:

a. small size

easy to look after in research labs

b. short lifespan

study can extend over several generations

c. can be killed «to get experimental results» if benefits of research justify it

d. «mammalian» so similarities with humans

e. fewer ethical objections than if humans are used/not ethical to subject humans to hypoxia/does not cause harm to humans

Accept any one of the advantages

Disadvantages:

f. ethical objections