Topic 11: Gas Exchange in Humans -- Challenge Exam 3
1 hour 15 minutes
80
7
75:00
0610
Instructions
Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.
Question Navigation
This paper covers the whole of Topic 11. Like a real Cambridge paper it ranges across every sub-topic — 11.1 gas exchange surfaces and the breathing system, 11.2 inspired and expired air, 11.3 ventilation, and 11.4 exercise, breathing control and clean airways — and it mixes them inside single questions. All three Topic 11 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — Two Moments, and Nobody Has Told You Which Is Which
Total: 12 marks
Fig. 1.1 shows the same chest drawn from the side at two different moments, labelled A and B. Nothing has been named for you and the drawings have deliberately not been put in order. Table 1.1 gives measurements taken at those same two moments. Atmospheric pressure outside the body was 101.3 kPa throughout.
moment
volume of the thorax / dm³
pressure inside the thorax / kPa
A
2.4
101.6
B
2.9
100.7
Table 1.1
(a)[3]
State which of the two moments, A or B, shows inhalation. Give one piece of evidence from Fig. 1.1 and one piece of evidence from Table 1.1 to support your choice.
Model Answer — 1(a)
B shows inhalation [1]
evidence from Fig. 1.1 — in B the ribs are raised, lying more horizontally, and the sternum has moved up and out, and the sheet of muscle across the floor of the thorax is flattened and lower instead of domed [1]
evidence from Table 1.1 — in B the volume of the thorax is larger (2.9 against 2.4 dm³) and the pressure inside, 100.7 kPa, is below the atmospheric 101.3 kPa, so air will flow in [1]
⚠ If you missed marks here: The commonest error in the whole topic is putting the diaphragm the wrong way round — a great many candidates believe it moves up on inhalation because they picture it pushing air in. It does the opposite: it contracts, flattens and moves down, which makes the space above it bigger. The second trap is quoting the pressure without comparing it to atmospheric. 100.7 kPa is only meaningful next to the 101.3 kPa outside; a number on its own earns nothing.
(b)[4]
Two sets of muscles lie between the ribs. State what each set is doing at moment A and what each set is doing at moment B.
Model Answer — 1(b)
at B (inhalation) the external intercostal muscles contract [1]
and at B the internal intercostal muscles relax [1]
at A (exhalation) the external intercostal muscles relax [1]
and at A the internal intercostal muscles contract [1]
the two sets are an antagonistic pair — whenever one contracts the other relaxes, because a muscle can only pull, never push
⚠ If you missed marks here: Most answers name only the external intercostals and leave the internal ones out entirely, which throws away half the marks in a question that asks about both sets. Remember they are an antagonistic pair: writing “the intercostal muscles contract” without saying which set cannot score, because at any moment one set is contracting and the other is relaxing. Note also that quiet exhalation is largely passive elastic recoil — the internal intercostals really earn their place in forced exhalation, such as blowing out a candle.
(c)[3]
Use the figures in Table 1.1 to explain how air comes to enter the lungs at moment B.
Model Answer — 1(c)
the volume of the thorax increases, from 2.4 to 2.9 dm³ [1]
so the pressure inside the thorax falls, from 101.6 to 100.7 kPa, which is now 0.6 kPa below the atmospheric pressure of 101.3 kPa [1]
air therefore moves down the pressure gradient, from the higher pressure outside to the lower pressure in the lungs — it is pushed in by the atmosphere, not sucked in [1]
⚠ If you missed marks here: “The lungs suck the air in” is the single most heavily penalised phrase in this topic. Nothing in the body can pull air; a low pressure does not reach out and grab anything. Air is pushed in by the atmosphere pressing on it, in exactly the same way that it is pushed out again when the pressure inside rises above 101.3 kPa. Getting the order right also matters: volume changes first, and the pressure change follows from it.
(d)[2]
A student writes: “The lungs pull themselves open to take air in.” Explain why this statement is wrong.
Model Answer — 1(d)
the lungs contain no muscle, so they cannot contract and cannot pull on anything — they are elastic bags [1]
they are stretched by the movement of the thorax around them (the ribs moving up and out and the diaphragm flattening); the lungs follow the chest wall, they do not lead it [1]
⚠ If you missed marks here: This is worth fixing properly because the same wrong picture produces “the alveoli pump” and “the lungs inflate themselves” further down the paper. The lungs are passive throughout. Their only active property is elastic recoil — having been stretched, they spring back, and that is what drives quiet exhalation.
Question 2 — Designing It Yourself: How Fast, and How Deep
Total: 12 marks
A student wants to find out how physical activity affects breathing. She knows that breathing can change in two separate ways — how often a breath is taken, and how big each breath is — and she wants to measure both. She has a step, a stopwatch, a large plastic bottle, a bowl of water and some rubber tubing.
(a)[3]
State the independent variable, state the two dependent variables she should measure, and state two variables that must be kept the same.
Model Answer — 2(a)
independent variable — the level of physical activity, measured as the number of step-ups per minute [1]
dependent variables — the breathing rate in breaths per minute and the depth of breathing, that is the volume of air in one breath, in cm³ [1]
any two controlled variables, e.g. the same person throughout, the same length of time exercising at each level, the same height of step, the same room temperature, the same rest period before starting [1]
⚠ If you missed marks here: “Exercise” on its own is not an independent variable, because it cannot be set to a value — you must say how it is to be measured and changed (step-ups per minute, or minutes of running). And the two dependent variables are genuinely different things: rate is breaths per minute, depth is cm³ per breath. A great many candidates write “how much they breathe”, which is neither, and scores nothing.
(b)[3]
Describe a method she could use to obtain reliable results.
Model Answer — 2(b)
sit still for several minutes first and take a resting measurement, counting breaths for a whole minute with a stopwatch, to give the value at zero activity [1]
then do step-ups at a set rate for a fixed length of time (for example 20 step-ups per minute for three minutes), measuring rate and depth immediately afterwards, and repeat at increasing rates of step-ups [1]
repeat each level at least three times and calculate a mean, allowing the breathing to return to the resting value between trials [1]
⚠ If you missed marks here: Two marks are lost more often than any other in method questions. First, the resting reading is part of the experiment, not a preliminary — without it there is nothing to compare the exercise values with. Second, the recovery period between trials: breathing does not drop back to its resting value the instant exercise stops, so starting the next level too early makes every reading after the first one too high.
(c)[2]
Describe how she could use the bottle, the bowl of water and the tubing to measure the depth of one breath.
Model Answer — 2(c)
fill the bottle completely with water and invert it in the bowl of water, then lead the tubing up inside its neck, so that air blown along the tube collects at the top of the bottle and pushes water out [1]
breathe out one normal breath through the tube; the volume of water displaced equals the volume of that breath, which can be read from a scale marked on the bottle in cm³ (calibrate the bottle beforehand by pouring in measured volumes of water and marking the levels) [1]
⚠ If you missed marks here: Depth of breathing is a volume, so any valid method has to end with a reading in cm³ or dm³. Answers such as “watch how far the chest moves” or “see if they are breathing deeply” describe the right idea but produce no measurement, and Cambridge will not credit them. The other half of the mark is the displacement principle itself: the air does not vanish, it takes the place of water, so measuring the water measures the air.
(d)[2]
Table 2.1 shows her results for one person. Identify the anomalous result and suggest one reason for it.
step-ups per minute
breathing rate / breaths per minute
volume of one breath / cm³
0 (at rest)
14
500
20
19
900
40
25
1400
60
20
1500
80
36
2400
Table 2.1
Model Answer — 2(d)
the anomaly is the breathing rate of 20 at 60 step-ups per minute — every other value rises as the activity rises, but this one is lower than the 25 recorded at 40 step-ups, even though the volume of each breath went up as expected [1]
any sensible reason, e.g. the counting was started too late after stopping, so the rate had already begun to fall; the person did not keep to the set stepping rate for the whole three minutes; a miscount, or breaths counted for only 15 seconds and multiplied up; the person had not fully recovered or talked during the count [1]
⚠ If you missed marks here: Look for the value that breaks the trend, not the largest or smallest value — 36 at 80 step-ups is the biggest number in the table but it is exactly what the pattern predicts. Notice also the clue in the third column: the depth kept rising at 60 step-ups, so the person really was working harder, which tells you the fault lies in the way the rate was measured. And “human error” on its own scores nothing; you must say which human did what.
(e)[2]
Suggest two improvements she could make to her investigation, other than repeating it more times.
Model Answer — 2(e)
any two of the following, one mark each [2]:
measure the breathing during the exercise rather than immediately after it, because the rate begins to fall as soon as the person stops
record the chest movements on video, or use a chest belt sensor, and count from the recording — this removes counting errors and stops the person changing their breathing because they know it is being watched
test several different people and calculate a mean, so the conclusion is not limited to one individual
use more, smaller intervals of activity (every 10 step-ups per minute) so the shape of the relationship can be seen properly
use a proper spirometer or gas syringe rather than an inverted bottle, giving a more precise volume
check the explanation as well as the effect — bubble a fixed number of her breaths through limewater at rest and again straight after the exercise, and record how many breaths each takes to turn it cloudy, giving evidence that more carbon dioxide really is being produced
⚠ If you missed marks here: An improvement must attack a named weakness of this particular method. “Be more careful” and “use better equipment” are not improvements because they do not say what was wrong. The strongest answer here is measuring during rather than after the exercise: it removes a fault that affects every single reading, not just the anomalous one, and that is the sort of systematic problem examiners most want you to spot. The limewater route is worth having in mind too: it is the only suggestion here that tests the reason for the change rather than measuring the change again more accurately.
Question 3 — Marking Somebody Else’s Write-Up
Total: 12 marks
A student investigated the difference between the air breathed in and the air breathed out. This is his complete write-up, exactly as he handed it in.
METHOD
I poured 10 cm³ of limewater into boiling tube A and 25 cm³ of limewater into boiling tube B. I blew out through a straw into tube A twenty times. Then I used a syringe to push room air through tube B five times.
RESULT
The limewater in tube A went white. The limewater in tube B stayed the same.
CONCLUSION
Breathing makes carbon dioxide. There is no carbon dioxide in the air we breathe in.
(a)[4]
Identify four faults in this write-up.
Model Answer — 3(a)
different volumes of limewater in the two tubes, 10 cm³ against 25 cm³, so the comparison is not fair — the larger volume would need more carbon dioxide to change it anyway [1]
different amounts of air passed through the two tubes, twenty breaths against five pushes of the syringe, so the two tubes were not given the same chance to change [1]
the observation is wrongly described — limewater turns cloudy or milky, never “white”; and no time or number of breaths is recorded for when the change happened, so there is nothing to compare [1]
the conclusion is not supported — the result cannot show that inspired air contains no carbon dioxide, only that it contains less; the concentration of the limewater is also never stated, so nobody could repeat the investigation [1]
⚠ If you missed marks here: Notice that only two of the four faults are about the apparatus. The other two are about language and logic, and those are the ones candidates walk past. “Went white” will not be credited anywhere in an IGCSE mark scheme — the word is cloudy or milky. And “there is no carbon dioxide in the air we breathe in” is simply false: inspired air contains 0.04%, a small amount but not nothing.
(b)[2]
Describe how the investigation should be set up so that the comparison is a fair one.
Model Answer — 3(b)
put the same volume of limewater, at the same concentration, into both boiling tubes [1]
use a T-piece with two one-way valves and a single mouthpiece, so that the same person draws room air in through tube A and blows expired air out through tube B; then count the same number of breaths through each and record how many breaths it takes for each tube to turn cloudy [1]
⚠ If you missed marks here: The T-piece with two one-way valves is not decoration — it is what makes the tube of room air a proper control. Both tubes then get air from the same mouth, at the same rate, for the same number of breaths, and the only difference left is whether the air has been inside the lungs. If you set the two tubes up separately, you have changed more than one thing and the comparison collapses.
(c)[2]
Write a conclusion that the corrected investigation would actually support.
Model Answer — 3(c)
the limewater through which expired air was blown turned cloudy after far fewer breaths than the limewater through which room air was drawn [1]
therefore expired air contains more carbon dioxide than inspired air — not that inspired air contains none, and not that the body creates carbon dioxide out of nothing [1]
⚠ If you missed marks here: A conclusion has to stay inside what the results can prove. This experiment compares two samples of air, so the only conclusion available is a comparison: more carbon dioxide in one than the other. It says nothing at all about where the carbon dioxide came from, and nothing about oxygen — limewater does not test for oxygen.
(d)[4]
State four ways in which expired air differs from inspired air. Give figures where you can, and explain one of the four differences.
Model Answer — 3(d)
oxygen falls from 21% to 16% [1]
carbon dioxide rises from 0.04% to about 4% [1]
nitrogen is unchanged at 78%, because it is not used by the body [1]
expired air is saturated with water vapour and is warmer, at about 37 °C, which is body temperature [1]
explanation of one difference, e.g. expired air is warmer and wetter because it has been in contact with the warm, moist lining of the alveoli, and water evaporates from that lining into the air
⚠ If you missed marks here: Learn 21 and 16 as a pair, because the second number is the surprising one. Expired air still contains 16% oxygen — more than three quarters of the oxygen you breathe in comes straight back out again, which is exactly why mouth-to-mouth resuscitation works at all. Anyone who writes “expired air has no oxygen” has made the topic impossible to understand. The other easy mark is nitrogen: it stays at 78% precisely because nothing happens to it.
Question 4 — Fewer, Bigger Bags
Total: 12 marks
Emphysema is a lung disease in which the thin walls between neighbouring alveoli slowly break down. Where there were once many small alveoli there are then fewer, larger air spaces. The disease does not block the airways, and the person can still move air in and out. Table 4.1 compares the lungs of a healthy adult with those of an adult with advanced emphysema.
healthy adult
adult with emphysema
number of air spaces in the lungs / millions
400
100
total surface area of the air spaces / m²
70
25
breathing rate at rest / breaths per minute
14
24
Table 4.1
(a)[3]
Calculate the percentage decrease in the total surface area of the air spaces, and calculate the mean surface area of one air space in each person. Show your working.
healthy: 70 ÷ 400 000 000 = 1.75 × 10−7 m² per air space [1]
emphysema: 25 ÷ 100 000 000 = 2.5 × 10−7 m² per air space [1]
⚠ If you missed marks here: Percentage decrease is always the change divided by the original value, so the denominator is 70, not 25. Dividing by 25 gives 180%, which should look wrong immediately — you cannot lose 180% of something. And watch the units in the column heading: the numbers are given in millions, so 400 means 400 000 000. Missing that gives an answer six orders of magnitude too big.
(b)[2]
Use your answers to (a) to explain what has happened inside the lungs of the person with emphysema.
Model Answer — 4(b)
each individual air space is now larger (2.5 × 10−7 against 1.75 × 10−7 m²), because walls have broken down and neighbouring alveoli have merged into single bigger spaces [1]
but because there are only a quarter as many of them, the total surface area has fallen by about two thirds — one big sac has a much smaller surface area to volume ratio than the many small sacs it replaced [1]
⚠ If you missed marks here: The trap in this question is assuming that bigger air spaces must be better because they hold more air. They do hold more air — and that is beside the point. Gas exchange happens across a surface, not inside a volume, and merging many small sacs into one big one destroys surface without destroying volume. It is the same surface area to volume argument you met with the villus and the root hair cell, running in reverse.
(c)[3]
Explain why this person becomes breathless when climbing a single flight of stairs.
Model Answer — 4(c)
there is a much smaller surface area for gas exchange, so less oxygen can diffuse into the blood, and less carbon dioxide out of it, in a given time [1]
climbing stairs makes the muscles respire faster, so they use up oxygen and produce carbon dioxide much more quickly than at rest [1]
the lungs cannot supply that extra oxygen or remove the extra carbon dioxide fast enough, so the carbon dioxide concentration in the blood rises and the person feels short of breath and has to stop [1]
⚠ If you missed marks here: Breathlessness questions need both sides of the balance: demand going up and supply unable to follow. An answer that only says “they have less surface area” does not explain why the problem appears on the stairs and not in the chair. Be careful too with “they cannot get enough air” — the stem tells you the airways are not blocked, so air still moves freely; what has failed is the exchange, not the ventilation.
(d)[2]
Explain why this person has a raised breathing rate of 24 breaths per minute even when sitting still.
Model Answer — 4(d)
even at rest the reduced surface area cannot remove carbon dioxide quickly enough, so the concentration of carbon dioxide in the blood rises [1]
this is detected by the brain, which sends nerve impulses to the diaphragm and the intercostal muscles to increase the rate and the depth of breathing, so that more carbon dioxide is removed [1]
⚠ If you missed marks here: The trigger is high carbon dioxide, not low oxygen. It is worth saying that sentence out loud, because almost everyone assumes it is the other way round: you breathe faster because carbon dioxide has built up, and the brain is what detects it. Say “detected by the brain” explicitly — “the body notices” will not be credited.
(e)[2]
Suggest how the percentages of oxygen and of carbon dioxide in this person’s expired air would compare with those of a healthy adult. Explain your suggestion.
Model Answer — 4(e)
the oxygen would be higher than 16% and the carbon dioxide lower than 4% — that is, the expired air would be closer in composition to the inspired air [1]
because with so much less surface area, less oxygen diffuses out of each breath into the blood and less carbon dioxide diffuses into it before that air is breathed out again; the fast, shallow breathing also means more of each breath never reaches an exchange surface at all [1]
⚠ If you missed marks here: The tempting answer is “less oxygen in the expired air, because they are short of oxygen”. Follow the gas, not the feeling. The person feels short of oxygen precisely because the oxygen is failing to leave the air and enter the blood — so it stays in the air and is breathed straight back out. Less exchange always means expired air that looks more like inspired air.
Question 5 — The Tubes, and What Happens When One Closes
Total: 10 marks
Air reaches the alveoli through a branching set of tubes, and those tubes are not all built the same way. During an asthma attack three things happen at once in the smallest of them: the ring of muscle in the wall contracts, the lining of the tube swells, and extra mucus is produced.
(a)[3]
Describe the cartilage found in the wall of the trachea and explain why it is needed.
Model Answer — 5(a)
the wall contains C-shaped rings of cartilage, stacked one above another along its length [1]
they hold the trachea open and stop it collapsing [1]
this matters because during inhalation the pressure inside the thorax, and so inside the airway, falls below atmospheric pressure, and a soft tube would be squashed shut by the higher pressure outside — the rings are incomplete at the back so that the oesophagus behind can still bulge when food is swallowed [1]
⚠ If you missed marks here: Two errors cost marks here. The rings are cartilage, not bone — bone would be rigid and could not bend as the neck moves. And “to protect the trachea” is far too vague: the mark comes from saying it is held open, against a pressure that would otherwise close it. Notice how this joins up with Question 1 — the low pressure inside the thorax that pulls air in is the very thing that would flatten an unsupported tube.
(b)[2]
Bronchioles have no rings of cartilage. Suggest one advantage and one disadvantage of this.
Model Answer — 5(b)
advantage — without rigid rings the bronchioles are flexible, and their diameter can be changed; they can widen to let more air through during exercise, and they can bend and stretch as the lungs change shape and volume with every breath [1]
disadvantage — there is nothing to hold them open, so they can be narrowed or closed altogether, for example when the muscle in the wall contracts or the lining swells [1]
⚠ If you missed marks here: A “suggest” question is asking you to reason from what you already know, not to recall a fact. The chain here is short: cartilage holds a tube at a fixed width, therefore no cartilage means a tube whose width can change — and a width that can change is useful when you want more air, and dangerous when something makes it narrow.
(c)[3]
Explain the effect of the three changes described in the introduction on the ventilation of the lungs.
Model Answer — 5(c)
all three changes — muscle contracting, lining swelling and extra mucus — narrow the lumen of the bronchioles, and mucus may block some of them completely [1]
so there is much greater resistance to the flow of air, and far less air moves in and out of the lungs with each breath; ventilation therefore falls even though the thorax is still moving normally [1]
the person has to work much harder to move air, especially to breathe out, since exhalation is normally passive; breathing becomes fast, shallow and wheezy [1]
⚠ If you missed marks here: Use the word ventilation here and keep it separate from gas exchange — this part is about air failing to move along tubes, and nothing at all has yet gone wrong at the alveolus. A very common answer says “the lungs cannot expand”, which is not what happens: the ribs and diaphragm move exactly as usual, and that is precisely why the effort feels so useless.
(d)[2]
Explain the effect of these changes on gas exchange at the alveoli.
Model Answer — 5(d)
less air reaches the alveoli, so the air already in them is not replaced with fresh air — ventilation of the exchange surface is poor [1]
so the concentration gradients are not maintained: the oxygen concentration in the alveolar air falls towards that in the blood and the carbon dioxide concentration rises, so less oxygen diffuses into the blood and less carbon dioxide diffuses out [1]
⚠ If you missed marks here: This is the mark that tells an examiner you understand what “good ventilation with air” is for. Ventilation does not give the blood oxygen — it keeps the alveolar air fresh, and fresh air is what maintains the concentration gradient. Say the words “concentration gradient”; without them the answer is only a description of feeling breathless.
Question 6 — The Athlete and the Rest of Us
Total: 12 marks
Table 6.1 shows measurements taken from one trained endurance athlete and one untrained adult of the same age, height and mass, first sitting at rest and then during hard exercise. Ventilation is the total volume of air moved in and out of the lungs in one minute; it is the breathing rate multiplied by the volume of one breath. One value has been left out.
untrained, at rest
untrained, hard exercise
athlete, at rest
athlete, hard exercise
breathing rate / breaths per minute
15
40
12
45
volume of one breath / cm³
500
2000
550
3000
ventilation / dm³ per minute
7.5
80.0
?
135.0
oxygen in expired air / %
16.5
15.0
16.0
14.0
Table 6.1 (1 dm³ = 1000 cm³)
(a)[2]
Calculate the missing value. Show your working and give the unit.
Model Answer — 6(a)
12 × 550 = 6600 cm³ per minute [1]
6600 ÷ 1000 = 6.6 dm³ per minute [1]
⚠ If you missed marks here: The unit conversion is where this mark is usually lost. The two measurements are in cm³ but the row is headed dm³ per minute, so the answer must be divided by 1000. Leaving 6600 in the box is a wrong answer even though the arithmetic was right — and you can check it instantly against the untrained column, where 15 × 500 gives 7500 cm³, printed as 7.5.
(b)[2]
Calculate the percentage increase in ventilation from rest to hard exercise for the untrained adult. Give your answer to the nearest whole number.
⚠ If you missed marks here: Two habits to fix. First, divide the change by the starting value, not the final value by the starting value — 80.0 ÷ 7.5 × 100 gives 1067%, which is the answer to “what percentage of the resting value is the exercise value”, a different question. Second, do not be alarmed by an answer over 100%: ventilation really does rise about tenfold, and a percentage increase of nearly 1000% is simply another way of saying that.
(c)[3]
Describe three differences between the athlete and the untrained adult shown in Table 6.1. Use figures from the table in your answer.
Model Answer — 6(c)
at rest the athlete breathes more slowly but more deeply — 12 breaths per minute against 15, but 550 cm³ per breath against 500 — so their resting ventilation is actually lower, 6.6 against 7.5 dm³ per minute [1]
during exercise the athlete reaches a far greater ventilation, 135.0 against 80.0 dm³ per minute, and achieves this mostly through a much greater depth (3000 against 2000 cm³) rather than a much faster rate (45 against 40) [1]
the athlete’s expired air contains a lower percentage of oxygen at both levels — 16.0% against 16.5% at rest, and 14.0% against 15.0% during exercise [1]
⚠ If you missed marks here: “Describe” with a table in front of you always means quote the figures; a comparison in words alone will be given at most half the marks. The result people get wrong is the resting one — the athlete’s resting ventilation is lower, not higher, and it is very easy to assume the fitter person must be doing more of everything. Read the row, not your expectations.
(d)[3]
Explain why the athlete’s expired air contains a lower percentage of oxygen than the untrained adult’s.
Model Answer — 6(d)
a lower percentage left in the expired air means a greater proportion of the oxygen in each breath has diffused into the blood before that air is breathed out again [1]
the athlete breathes more deeply, so a larger fraction of each breath actually reaches the alveoli instead of staying in the trachea and bronchi, and more of the alveolar surface is properly ventilated [1]
their muscles are respiring faster and their blood supply to the lungs is better, so oxygen is carried away from the alveoli quickly by the haemoglobin in the red blood cells, which keeps the concentration gradient steep and diffusion rapid [1]
⚠ If you missed marks here: Beware of the sentence “the athlete has more oxygen” — the percentages measure what is left over, so a lower number means more was taken up, not less. And note what a good blood supply does: it does not “give more oxygen” and blood never carries oxygen to the alveoli. It removes oxygen from the blood side quickly, which is what maintains the concentration gradient.
(e)[2]
A magazine article claims: “These results prove that athletes take in more oxygen than the rest of us all day long.” Evaluate this claim.
Model Answer — 6(e)
the claim is not supported at rest — the athlete’s resting ventilation is lower (6.6 against 7.5 dm³ per minute), so they move less air per minute while sitting; the amount of oxygen a resting body needs is set by how fast its cells are respiring, not by how good its lungs are, and the athlete simply meets that same demand more efficiently [1]
and the evidence is too thin to prove anything — only one person of each kind was measured, with no repeats and no mean; the table also covers only two moments, at rest and at maximum effort, not “all day long” [1]
⚠ If you missed marks here: An evaluation needs two kinds of criticism: what the data actually show, and how good the data are. Most candidates offer one and stop. Watch for the word prove in a claim — a sample of one against one can suggest, never prove, and the marks are there for saying so.
Question 7 — Three Words That Are Not Interchangeable
Total: 10 marks
Candidates lose more marks in this topic to loose vocabulary than to anything else. Ventilation, gas exchange and respiration name three completely different processes, happening in three different places, and a mark scheme will not accept one in place of another. This question is about keeping them apart.
(a)[3]
Define ventilation, gas exchange and respiration. In each case make clear where the process happens.
Model Answer — 7(a)
ventilation — the movement of air into and out of the lungs, brought about by the ribs, the intercostal muscles and the diaphragm; it happens in the thorax as a whole [1]
gas exchange — the diffusion of oxygen and carbon dioxide in opposite directions across the alveolus wall, between the air in the alveolus and the blood in the capillary; it happens at the alveoli [1]
respiration — the chemical reactions in cells that break down nutrient molecules and release energy; it happens inside every living cell of the body, not in the lungs [1]
⚠ If you missed marks here: The definition to guard most carefully is respiration. In everyday English it means breathing; in Biology it never does. Writing “respiration is breathing in and out” will cost you marks in every paper you ever sit. Fix the locations in your head: ventilation is the whole chest, gas exchange is the alveolus wall, respiration is inside a cell.
(b)[5]
Describe the journey of one molecule of oxygen from the air in a room until it is used inside a leg muscle cell. Name in order every structure it passes through, and name the process that moves it at each stage.
Model Answer — 7(b)
it is carried in by ventilation through the nose or mouth, past the larynx, down the trachea, into a bronchus, then a bronchiole, and into an alveolus [1]
it then diffuses down its concentration gradient across the wall of the alveolus, which is one cell thick, and the wall of the capillary, which is also one cell thick — a total distance of about 1 µm; this is gas exchange [1]
it enters a red blood cell and combines with haemoglobin to form oxyhaemoglobin [1]
it is carried in the blood through the pulmonary vein to the left atrium, then the left ventricle, out through the aorta and along arteries into a capillary in the leg muscle [1]
the oxyhaemoglobin releases the oxygen, which diffuses out of the capillary and through the cell membrane into the muscle cell, where it is used in respiration to release energy [1]
⚠ If you missed marks here: Two structures are dropped from almost every answer: the bronchiole (candidates jump straight from bronchus to alveolus) and the capillary wall (they cross the alveolus wall and stop). Both are worth marks. Watch the direction of the blood vessel too: oxygen leaves the lungs in the pulmonary vein, which is the one vein in the body carrying oxygenated blood, and any answer that sends it out through the pulmonary artery has the whole journey running backwards.
(c)[2]
A person has stopped breathing but their heart is still beating. Explain, using two of the three words from (a), why blowing your own expired air into their lungs can keep them alive.
Model Answer — 7(c)
expired air still contains 16% oxygen — only about a quarter of the oxygen breathed in is actually absorbed, so more than three quarters comes straight back out again [1]
so the rescuer is supplying the ventilation that the casualty can no longer perform, and 16% is still far more oxygen than there is in the blood arriving at the alveoli, so the concentration gradient is maintained and gas exchange continues normally — the beating heart then delivers the oxygen to the cells [1]
⚠ If you missed marks here: If you believe expired air contains no oxygen, this question is unanswerable — and that is exactly why it is asked. The figure to hold on to is 21% in, 16% out. Notice too which process has failed: the casualty’s ventilation has stopped, while their gas exchange and their respiration are both still perfectly capable of working. That is the whole reason the rescue works.
Self-Assessment
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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