⚡ Challenge Paper Preparation

Challenge Prep: Gas Exchange in Humans

IGCSE Biology 0610 — Topic 11 — Extended

Topic 11 looks like the easiest topic in the syllabus and gives away marks like the hardest, because almost every mark in it is a word. Three processes that people use interchangeably are three different things in three different places. Air is never sucked in. The diaphragm flattens when it contracts. Expired air still contains 16 % oxygen. Breathing speeds up because carbon dioxide rises, not because oxygen falls. Limewater goes cloudy. Goblet cells make the mucus and cilia move it, never the other way round. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below, each aimed at a place where a perfectly sensible sentence earns nothing at all.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 11 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Saying that respiration takes place in the lungs

The Trap“The lungs are where respiration happens.” It feels right because the lungs are obviously where the oxygen goes in and the carbon dioxide comes out. And because breathing is sometimes loosely called respiration in everyday speech, the habit is very hard to break.

The TruthThree separate words. Ventilation is the movement of air in and out of the lungs. Gas exchange is the diffusion of oxygen and carbon dioxide across the alveolus wall. Respiration is the chemical reaction inside cells that breaks down nutrient molecules and releases energy. The lungs do the first two. Respiration happens in every living cell in your body.

Why It MattersAny question that asks you to distinguish the three, or that offers all three as options, is testing this and nothing else. It also protects you in 11.2: the carbon dioxide detected by limewater was produced by respiring cells all over the body and merely delivered to the lungs.

Example Question“Distinguish between ventilation, gas exchange and respiration. [3]”

⚠️ TRAP

Trap 2: Writing that the lungs suck air in

The Trap“The lungs expand and suck the air down the trachea.” It is the picture almost everyone has, and it is wrong twice over: the lungs cannot expand themselves, and nothing can pull on a gas.

The TruthThe lungs contain no muscle. The diaphragm and the external intercostal muscles enlarge the thorax; the elastic lungs are stretched by it. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure, and the air outside is pushed in down the pressure gradient.

Why It MattersEvery ventilation mark scheme is written in volume and pressure. An answer that names the muscles and then jumps to “so air goes in” has skipped the two marks in the middle, and an answer that says “sucked” is usually refused outright.

Example Question“Explain how air is made to enter the lungs during inhalation. [4]”

⚠️ TRAP

Trap 3: Making the diaphragm move up when you breathe in

The Trap“The diaphragm contracts and moves up, making room for the air.” The word contract makes people imagine something shrinking upwards, and half of all candidates write this at some point.

The TruthAt rest the diaphragm is a dome bulging upwards into the chest. When its muscle contracts it flattens, so the middle of it moves down, and the volume of the thorax increases. When it relaxes it springs back into its dome and moves up, reducing the volume.

Why It Matters“Moves down” on its own is usually worth less than the full mark; the marking point is contracts and flattens. Get the pair together and you also cannot go wrong on exhalation, where it relaxes and domes.

Example Question“Describe what happens to the diaphragm during inhalation and during exhalation. [2]”

⚠️ TRAP

Trap 4: Thinking expired air contains no oxygen

The Trap“You breathe in oxygen and breathe out carbon dioxide.” Every primary school in the world teaches this sentence, and it is the reason candidates pick “0 %” or “4 %” for the oxygen content of expired air.

The TruthInspired air is 21 % oxygen; expired air is 16 %. You keep about five parts in every hundred, which is under a quarter of what you took in. Expired air also still contains far more oxygen (16 %) than carbon dioxide (4 %), and 78 % of it is nitrogen either way.

Why It MattersIt is the reason mouth-to-mouth resuscitation works, which is a favourite “suggest” question. It also stops you accepting any data table in which expired oxygen is near zero.

Example Question“Explain, using figures, how mouth-to-mouth resuscitation can supply a casualty with oxygen. [3]”

⚠️ TRAP

Trap 5: Saying breathing speeds up because oxygen runs low

The Trap“During exercise your muscles need more oxygen, so the low oxygen makes you breathe faster.” The first half is true, the second half is the wrong mechanism, and the whole sentence is refused.

The TruthThe trigger is an increased concentration of carbon dioxide in the blood, detected by the brain. The brain then sends impulses to the diaphragm and intercostal muscles, increasing both the rate and the depth of breathing.

Why It MattersThe syllabus names carbon dioxide and the brain explicitly, so those two words are the marks. There is a good reason for it too: carbon dioxide is produced in proportion to the work being done and its concentration changes sharply, whereas the oxygen content of blood barely moves during ordinary exercise.

Example Question“Explain the link between physical activity and the rate and depth of breathing. [4]”

⚠️ TRAP

Trap 6: Writing that the limewater went white

The Trap“The limewater turned white”, or “changed colour”, or “went chalky”. All three describe what it looks like and none of them is the accepted word.

The TruthLimewater turns cloudy, or equivalently milky. It does not gain a colour — it loses its transparency. And it is a test for carbon dioxide alone: it tells you nothing about oxygen, water vapour or temperature.

Why It MattersThis is a one-word mark that costs nothing to secure. It also anchors the comparison you are really being asked for: the tube through which expired air is bubbled turns cloudy in far fewer breaths.

Example Question“State the result you would expect in each tube and explain what it shows. [3]”

⚠️ TRAP

Trap 7: Giving the cilia the goblet cells’ job

The Trap“The cilia produce mucus which traps the bacteria.” The two cell types sit side by side in the same lining, so their jobs get merged into one sentence — and that sentence loses a mark every time.

The TruthGoblet cells secrete the mucus. The mucus traps pathogens and dust particles. Ciliated cells carry cilia which beat and sweep the mucus up the trachea, away from the lungs, to the back of the throat, where it is swallowed. Cilia produce nothing; goblet cells move nothing.

Why It MattersA three-mark question here is usually one mark per stage, so a merged answer collects one. It also explains smoking cleanly: smoke destroys the cilia, the goblet cells carry on regardless, and the mucus therefore collects.

Example Question“Explain the roles of goblet cells, mucus and ciliated cells in protecting the breathing system. [3]”

⚠️ TRAP

Trap 8: Naming only “the intercostal muscles”

The Trap“The intercostal muscles contract and the ribs move up.” At Core level that is fine. At Extended it is incomplete, because there are two sets and they do opposite things.

The TruthInhalation: external intercostals contract, internal intercostals relax, ribs move up and out. Forced exhalation: internal contract, external relax, ribs move down and in. They are an antagonistic pair because a muscle can only pull, never push.

Why It MattersIdentifying both layers in a diagram, and stating what each is doing, is Supplement content that Cambridge names explicitly. A question that says “name the muscles labelled X and Y” is testing exactly this.

Example Question“Name the muscles labelled A and B and state what each is doing during inhalation. [3]”

⚠️ TRAP

Trap 9: Explaining a good blood supply as “so oxygen can be carried round the body”

The TrapIt is a true sentence about a different topic. In a gas exchange question it describes a consequence rather than giving the reason the feature increases the rate of exchange.

The TruthThe dense capillary network carries oxygen away as fast as it arrives, so the concentration of oxygen in the blood at the alveolus stays low and the concentration gradient is maintained. Good ventilation does the same job from the other side, keeping alveolar oxygen high and carbon dioxide low.

Why It MattersTwo of the four features are gradient arguments, so the phrase “maintains the concentration gradient” earns marks twice in the same question. It is the single most reusable sentence in the topic.

Example Question“Explain how each of the four features of the alveoli increases the rate of gas exchange. [4]”

⚠️ TRAP

Trap 10: Treating “breathes faster” as the whole answer

The Trap“During exercise you breathe faster.” True, and worth about half of what was on offer, because the syllabus asks for the effect on the rate and the depth of breathing.

The TruthRate is breaths per minute — about 12–16 at rest, up to 40–50 in hard exercise. Depth is the volume per breath — about 500 cm³ at rest, up to 2500–3000 cm³. Multiply them and ventilation rises more than tenfold, which neither figure shows on its own.

Why It MattersIt is two marks rather than one on a written paper, and on a graph question it is the difference between reading peaks that are closer together and peaks that are taller. Challenge papers deliberately change one without the other.

Example Question“Describe two changes in the trace between 2 and 6 minutes and state what each shows. [4]”

⚠️ TRAP

Trap 11: Saying that cartilage rings are found all the way down to the alveoli

The Trap“Rings of cartilage hold the airways open.” Which airways? People extend them to the bronchioles, or call them rings of bone, or give them a job they do not have.

The TruthCartilage rings are found in the trachea and the bronchi. Bronchioles have none — they are held open by the surrounding lung tissue and have muscle in their walls so they can change diameter. Cartilage is firm but flexible, not bone, which is why you can bend your neck and still breathe.

Why It MattersThe function question is worth one clean mark: the rings hold the trachea open so it does not collapse when the pressure inside falls below atmospheric during inhalation. Any answer about warming, moistening or filtering the air is a different structure’s job.

Example Question“State the function of the cartilage in the trachea, and suggest why bronchioles do not contain any. [3]”

⚠️ TRAP

Trap 12: Claiming that the limewater experiment proves respiration happens in the lungs

The TrapThe limewater goes cloudy when you breathe on it, so the carbon dioxide must have been made where the air came from. It is a reasonable-sounding inference and it is the wrong one.

The TruthThe carbon dioxide was produced by respiring cells all over the body and carried to the lungs in the blood. At the alveolus its concentration in the blood is higher than in the alveolar air, so it diffuses out and is breathed away. The lungs are where carbon dioxide leaves, not where it is made.

Why It MattersEvaluation questions love this one, because it makes you separate what the data show from what you already believe. The experiment shows expired air is richer in carbon dioxide — not how much richer, and not where it came from.

Example Question“A student concludes that this experiment shows respiration occurs in the lungs. Evaluate that conclusion. [3]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — A Pressure Trace With No LabelsA sensor records the pressure inside a person’s thorax over one breathing cycle. Atmospheric pressure is 101.3 kPa. At moment P the reading is 100.5 kPa; at moment Q it is 101.3 kPa; at moment R it is 102.2 kPa. The volume of the thorax at P is 2.9 dm³ and at R it is 2.3 dm³. (a) Identify what is happening at P, Q and R. [3] (b) State what both sets of intercostal muscles are doing at R. [2] (c) Explain why the volume figure at R is smaller than at P. [3]

The reference value is 101.3, and it is given to you

P is below atmospheric (100.5 against 101.3), so air will be pushed into the thorax: P is inhalation. R is above atmospheric (102.2), so air is pushed out: R is exhalation. Q is equal to atmospheric, so there is no pressure gradient and no air is flowing — the moment between one movement and the next.

Nearly a kilopascal above atmospheric means this is forced

Quiet exhalation is close to passive — elastic recoil raises the pressure only slightly. A reading almost 1 kPa above atmospheric means muscles are actively squeezing. So at R the internal intercostal muscles contract, pulling the ribs down and in, while the external intercostal muscles relax. The diaphragm is relaxing and returning to its dome.

Volume is the cause; pressure is the effect

At R the diaphragm has relaxed and domed upwards and the ribs have moved down and in, so the volume of the thorax has decreased from 2.9 to 2.3 dm³. The same air now occupies a smaller space, so the pressure rises above atmospheric and air is pushed out. Do not write it the other way round: the air did not leave and cause the volume to shrink.

Three questions, in this order, for every ventilation data question

Is the pressure above or below atmospheric? That gives you the direction of airflow. How far above or below? That tells you whether it is quiet or forced. What must the volume have been doing to produce that pressure? That gives you the muscles. Answer them in that order and you can never get the muscles the wrong way round.

Full Mark-Scheme Answer(a) P — inhalation, because the pressure is below atmospheric so air is pushed in [1]; Q — no air movement, because the pressure equals atmospheric so there is no pressure gradient [1]; R — exhalation, because the pressure is above atmospheric so air is pushed out [1]. (b) Internal intercostal muscles contract [1]; external intercostal muscles relax [1]. (c) The diaphragm has relaxed and returned to its domed shape and the ribs have moved down and in [1], so the volume of the thorax has decreased [1], which raises the pressure inside above atmospheric so that air is forced out [1].

Walkthrough 2 — Two People, Same Ventilation, Different AnswerTwo students each ventilate 24 dm³ of air per minute. Student A takes 40 breaths of 600 cm³; student B takes 12 breaths of 2000 cm³. In both students, 150 cm³ of each breath remains in the trachea and bronchi, where no gas exchange takes place. (a) Show that both ventilate 24 dm³ per minute. [1] (b) Calculate the volume of air reaching the alveoli each minute in each student. [3] (c) Suggest which pattern of breathing is better and explain why. [2]

Ventilation = rate × depth, then one conversion at the end

Student A: 40 × 600 = 24 000 cm³ = 24 dm³ per minute. Student B: 12 × 2000 = 24 000 cm³ = 24 dm³ per minute. Identical, which is the whole point — the question is designed so that the obvious comparison tells you nothing.

Subtract 150 cm³ from every single breath

Student A wastes 150 cm³ on each of 40 breaths: 40 × 150 = 6000 cm³. Air reaching the alveoli = 24 000 − 6000 = 18 000 cm³ = 18 dm³.

Student B wastes 150 cm³ on each of only 12 breaths: 12 × 150 = 1800 cm³. Air reaching the alveoli = 24 000 − 1800 = 22 200 cm³ = 22.2 dm³.

The waste is a fixed cost per breath, so fewer breaths cost less

Student B gets 4.2 dm³ more air to the alveoli every minute for exactly the same total effort. Deep, slow breathing is more efficient because the volume left in the airways is a fixed amount per breath, so the more breaths you take, the more times you pay it.

Why the body deepens breathing rather than only speeding it up

This is the reason the syllabus insists on rate and depth. If exercise only raised the rate, an ever-larger share of each breath would be wasted on the airways. By increasing the depth as well, the body raises the proportion of every breath that actually reaches an alveolus. Being able to say that is what turns a two-mark answer into a full one.

Full Mark-Scheme Answer(a) A: 40 × 600 = 24 000 cm³ = 24 dm³; B: 12 × 2000 = 24 000 cm³ = 24 dm³ [1]. (b) A wastes 40 × 150 = 6000 cm³ [1], so 18 000 cm³ (18 dm³) reaches the alveoli [1]; B wastes 12 × 150 = 1800 cm³, so 22 200 cm³ (22.2 dm³) reaches the alveoli [1]. (c) Student B [1], because the air left in the airways is a fixed volume per breath, so fewer and deeper breaths waste less and deliver more air to the gas exchange surface [1].

Walkthrough 3 — A Composition Table With One Row MissingA student measures the composition of inspired and expired air and writes down: inspired — oxygen 21 %, carbon dioxide 0.04 %, nitrogen 78 %; expired — oxygen 16 %, carbon dioxide 4 %, nitrogen 78 %. She writes “water vapour: no difference” and “nitrogen: used up by the body”. (a) Correct both of her statements. [3] (b) Calculate the percentage of the inhaled oxygen that is actually absorbed. [2] (c) Explain why the carbon dioxide figure changes by so much more than the oxygen figure. [3]

Water vapour rises; nitrogen is simply ignored by the body

Water vapour increases. The lining of the airways and alveoli is moist — it has to be, because gases must dissolve before they can diffuse across a membrane — and water evaporates from that surface into the air. Expired air leaves saturated with water vapour.

Nitrogen is not used up. The percentage is unchanged because the body neither uses nitrogen gas nor produces it. Her table already shows 78 % in both columns, so her own data contradict her statement — always check that.

5 out of 21, not 5 out of 16

Oxygen falls from 21 % to 16 %, so 5 percentage points are absorbed out of the 21 that entered. That is 5 ÷ 21 × 100 = 23.8 %, so roughly a quarter of the oxygen inhaled is actually taken up. Dividing by 16 answers a question nobody asked.

Carbon dioxide changes by less in absolute terms, and far more as a ratio

Oxygen changes by 5 percentage points; carbon dioxide changes by 3.96 percentage points — slightly less, which surprises people. As a ratio, though, carbon dioxide goes up 100 times while oxygen falls to about three quarters of its value. The difference is entirely because carbon dioxide starts at the tiny figure of 0.04 %.

A big percentage change of a small number is still a small change

Say it explicitly. The near-equality of the two absolute changes is itself good biology: it is consistent with roughly one molecule of carbon dioxide being produced for each molecule of oxygen used. Examiners are looking for a candidate who can hold both facts — the huge ratio and the small absolute change — in the same answer.

Full Mark-Scheme Answer(a) Water vapour increases [1] because water evaporates from the moist lining of the airways and alveoli into the air [1]; nitrogen is not used up — the body neither uses nor produces it, which is why the figure is unchanged [1]. (b) 21 − 16 = 5 [1]; 5 ÷ 21 × 100 = 23.8 % (about a quarter) [1]. (c) Carbon dioxide starts from a very low value of 0.04 % so a rise to 4 % is a hundredfold increase [1], whereas oxygen starts from 21 % so a fall of 5 points is a much smaller proportion [1]; in absolute terms the two changes are of a similar size, consistent with carbon dioxide being produced as oxygen is used [1].

Walkthrough 4 — The Graph That Comes Down SlowlyA student rests for 2 minutes, exercises hard from 2 to 8 minutes, then rests again. Her breathing rate is 14 breaths per minute at rest, reaches 45 by 7 minutes, and is still 23 at 11 minutes, returning to 15 by 14 minutes. (a) Describe the changes in her breathing rate. [3] (b) Explain the rise between 2 and 7 minutes. [4] (c) Suggest why her breathing rate is still 23 breaths per minute three minutes after she stopped. [2]

Three marks means three stages with figures attached

Stage one: the rate is steady at 14 breaths per minute for the first 2 minutes. Stage two: it rises steeply once exercise begins, from 14 to 45 breaths per minute by 7 minutes. Stage three: after exercise stops it falls gradually, still 23 at 11 minutes and back to 15 by 14 minutes. Note the asymmetry — the rise takes about 5 minutes and the fall takes about 6.

Start in the muscle, not in the lungs

Her muscles are contracting more, so the muscle cells respire faster [1]. Faster respiration produces more carbon dioxide, so the concentration of carbon dioxide in her blood rises [1]. This rise is detected by the brain [1], which sends impulses to the diaphragm and intercostal muscles, increasing the rate and the depth of breathing [1].

The control system responds to a concentration, not to a decision

Stopping the exercise stops the extra production of carbon dioxide, but the carbon dioxide already dissolved in her blood is still there and is still above its resting concentration. Her brain is monitoring that concentration, so breathing stays raised until enough carbon dioxide has been removed. The rate falls as the concentration falls, which is why the recovery is a gradual curve rather than a step.

Avoid “the brain has not noticed” and avoid oxygen

Do not write that the brain takes time to notice that exercise has stopped — the brain is not watching what she is doing, it is monitoring a chemical. And do not explain any part of this with a shortage of oxygen: the syllabus mechanism is carbon dioxide throughout, and an oxygen-based answer is refused even where it sounds sensible.

Full Mark-Scheme Answer(a) Steady at 14 breaths per minute for the first 2 minutes [1]; rises steeply from 14 to 45 between 2 and 7 minutes [1]; falls gradually after exercise stops, from 45 to 23 by 11 minutes and to 15 by 14 minutes, so recovery is slower than the rise [1]. (b) Muscles contract more so muscle cells respire faster [1]; more carbon dioxide is produced [1]; the concentration of carbon dioxide in the blood rises and is detected by the brain [1]; the brain increases the rate and depth of breathing by sending impulses to the diaphragm and intercostal muscles [1]. (c) The carbon dioxide concentration in her blood is still above normal [1] and breathing stays raised until it has been removed and returns to its resting level [1].

Walkthrough 5 — A Damaged Lung, Told Only in NumbersA healthy pair of lungs contains about 400 million alveoli with a total surface area of 70 m². In a patient with a lung disease, the walls between neighbouring alveoli have broken down, leaving 100 million larger air spaces with a total surface area of 25 m². The volume of air the lungs can hold is unchanged. (a) Calculate the percentage decrease in surface area. [2] (b) Explain why the patient becomes breathless walking upstairs. [3] (c) Predict how the composition of the patient’s expired air will differ from a healthy person’s, and explain. [3]

45 out of 70, not 45 out of 25

The decrease is 70 − 25 = 45 m². As a percentage of the original: 45 ÷ 70 × 100 = 64.3 %. Nearly two thirds of the gas exchange surface has gone, even though the lungs still hold the same volume of air — which is the fact the question is built on.

The same air, spread over far less surface

The rate at which oxygen enters the blood depends on the surface area available for diffusion, not on how much air is in the lungs. With 64 % less area, far less oxygen can diffuse into the blood per second. Walking upstairs makes the muscles respire faster and demand more oxygen, and the damaged lungs cannot supply it — so the patient becomes breathless.

Less exchange means expired air is more like inspired air

If less oxygen diffuses out of the alveolar air, more of it is breathed back out, so the expired oxygen figure is higher than 16 % — closer to 21. If less carbon dioxide diffuses in from the blood, the expired carbon dioxide figure is lower than 4 % — closer to 0.04. The general rule is worth remembering: anything that reduces gas exchange moves expired air back towards the composition of inspired air.

Do not say the patient breathes in less air

The stem says the volume the lungs hold is unchanged, so ventilation is not the problem — gas exchange is. Candidates who blame a smaller lung volume have ignored a sentence that was put there deliberately. It is also the reason such patients often breathe faster, not less: the raised carbon dioxide in the blood drives ventilation up even though the lungs cannot make good use of it.

Full Mark-Scheme Answer(a) 70 − 25 = 45 m² [1]; 45 ÷ 70 × 100 = 64.3 % decrease [1]. (b) The surface area available for diffusion is greatly reduced [1], so less oxygen can diffuse into the blood per second [1]; climbing stairs makes the muscles respire faster and demand more oxygen, which the lungs cannot supply [1]. (c) Expired air will contain more oxygen than 16 % [1] and less carbon dioxide than 4 % [1], because less gas is exchanged so the air breathed out is closer in composition to the air breathed in [1].

Walkthrough 6 — An Experiment Written Up BadlyA student writes: “I put 20 cm³ of limewater in tube A and 8 cm³ in tube B. I breathed out through tube B five times and it went white. I sucked room air through tube A twenty times and nothing happened. This proves that we breathe out 100 times more carbon dioxide than we breathe in.” (a) Identify three faults in the investigation or the write-up. [3] (b) Describe how you would improve it. [3] (c) Write a conclusion that the corrected experiment could support. [2]

20 cm³ against 8 cm³ is not a fair test

The volumes of limewater are different, so a difference in the time taken to turn cloudy could be caused by the volume rather than by the air. It is worth noticing which way the error cuts: the tube with less limewater turned cloudy sooner, so part of the difference she observed might be nothing to do with her breath at all.

Unequal breaths, wrong word, and a number that was never measured

The number of breaths is different — five through one tube and twenty through the other — which is a second uncontrolled variable. “It went white” is the wrong observation: limewater turns cloudy or milky. And tube A never turned cloudy at all, so “twenty” is a minimum rather than a measurement, and no ratio of any kind can be calculated from it.

Fix one thing per fault, and say why

Use the same volume and the same concentration of limewater in both tubes. Use a single mouthpiece with two one-way valves so that the same breath draws room air through A and pushes expired air through B, which controls the number of breaths automatically. Continue until both tubes turn cloudy so that both numbers are real measurements. Repeat with several people and take a mean.

Qualitative apparatus supports a qualitative conclusion

The honest conclusion is that expired air contains more carbon dioxide than inspired air. That is all limewater can tell you. If you want the hundredfold figure you need a carbon dioxide sensor or a gas analyser, and you should say so — naming the better instrument is often the last mark on an improvement question.

Full Mark-Scheme Answer(a) Any three of: different volumes of limewater in the two tubes [1]; different numbers of breaths through each tube [1]; “went white” is the wrong observation, limewater turns cloudy or milky [1]; tube A never turned cloudy so no ratio can be calculated, and the conclusion of “100 times” is not supported [1]. (b) Same volume and concentration of limewater in both tubes [1]; use one mouthpiece with two one-way valves so the same breaths pass through both, and continue until both turn cloudy [1]; repeat with several people and take a mean [1]. (c) Expired air contains more carbon dioxide than inspired air [1]; the experiment cannot say how much more, so a quantitative claim would need a carbon dioxide sensor [1].

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. The distinction is where the marks live.

Question A

Where does gas exchange take place, and by what process?

Across the walls of the alveoli only, by diffusion down a concentration gradient. No energy is used and no carrier proteins are involved.

Question B

Where does respiration take place, and what is it?

In every living cell in the body. It is the chemical reaction that breaks down nutrient molecules to release energy, and it is not a gas movement at all.

Key DifferenceOne is a physical movement in one place; the other is a chemical reaction everywhere. Add ventilation — the movement of air in and out of the lungs — and you have the three words this whole topic is built on. A question offering all three as options is testing nothing else.

Question A

What are the external intercostal muscles doing during inhalation?

Contracting, which raises the ribs up and out and increases the volume of the thorax. Think external for expanding.

Question B

What are the internal intercostal muscles doing during inhalation?

Relaxing. They contract only during a forced exhalation, pulling the ribs down and in.

Key DifferenceThey are an antagonistic pair: one contracts while the other relaxes, because a muscle can only pull and never push. Writing “the intercostal muscles contract” without saying which is a Core-level answer, and Cambridge names both layers in the Supplement.

Question A

What does the diaphragm do during inhalation?

Its muscle contracts, so the dome flattens and the floor of the thorax moves down, increasing the volume.

Question B

What does the diaphragm do during exhalation?

Its muscle relaxes, so it returns to its domed shape and moves up, decreasing the volume.

Key DifferenceContract goes with flatten and down; relax goes with dome and up. Never write the direction on its own — “moves down” is usually half a mark, and “contracts and flattens” is the whole one.

Question A

What is the effect of exercise on the rate of breathing, and what units does it have?

It increases, from about 12–16 to as much as 40–50 breaths per minute. On a trace, the peaks get closer together.

Question B

What is the effect of exercise on the depth of breathing, and what units does it have?

It increases, from about 500 cm³ to as much as 2500–3000 cm³ per breath. On a trace, the peaks get taller.

Key DifferenceTwo separate measurements with different units, and multiplying them gives ventilation per minute. Answering “you breathe faster” gives one of the two available marks, and on a graph question it can give none, because a challenge paper will change one and leave the other alone.

Question A

Why does a good blood supply increase the rate of gas exchange?

It carries oxygen away as fast as it arrives, so the oxygen concentration in the blood stays low and the concentration gradient is maintained.

Question B

Why does good ventilation increase the rate of gas exchange?

It replaces the air in the alveolus, keeping the oxygen concentration in the air space high and the carbon dioxide concentration low, so the gradient is maintained.

Key DifferenceSame argument, opposite sides of the wall. Blood keeps the concentration low on the inside; ventilation keeps it high on the outside. Both answers should contain the phrase maintains the concentration gradient, and a question asking for both is asking you to notice that they are two halves of one idea.

Question A

What does the trachea contain that a bronchiole does not, and why?

C-shaped rings of cartilage, which hold it open so it cannot collapse when the pressure inside falls below atmospheric during inhalation.

Question B

What holds a bronchiole open, and what can it do that the trachea cannot?

The surrounding lung tissue. Its wall contains muscle, so it can change its diameter and alter where air goes in the lung.

Key DifferenceCartilage stops at the bronchi. This is also the mechanism behind an asthma attack: the muscle in the bronchiole walls contracts and there is no cartilage to keep the tube open, so the airway narrows sharply — something that could not happen in the trachea.

🔗 Gas Exchange Concept Map

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Click each node to see how the sub-topics connect into one story: a surface built for diffusion, a pump of muscle and bone that keeps it supplied, and a chemical in the blood that decides how hard the pump works.

⭐ CORE FRAMEWORK 1

Why a lung exists → what makes a good exchange surface → how the alveolus delivers it

Diffusion Is Fast Over a Micrometre and Useless Over a Metre ▶

A single-celled organism needs no lung: every part of it is a fraction of a millimetre from the surrounding water. As an organism grows, its volume rises faster than its surface area, so there is too little surface for too much inside, and the distance to the centre becomes far too long for diffusion.

The answer is a specialised exchange surface folded up inside the body. The same argument produced the villus in the small intestine and the root hair cell in a plant — three organs, one problem, one solution.

A transport system finishes the job. Blood carries oxygen most of the way at speed so that diffusion only has to cross the last micrometre from a capillary into a cell.

Four Features, and Two of Them Are the Same Idea ▶

Large surface area — 300–500 million alveoli, about 70 m², so more diffusion can happen at the same time. Thin surface — two walls each one cell thick, so a diffusion distance of about 1 µm.

Good blood supply keeps the oxygen concentration low on the blood side; good ventilation keeps it high on the air side. Both exist to maintain the concentration gradient, and saying so earns the mark twice.

The alveolar lining must be moist because gases dissolve before they diffuse — which is precisely why expired air is saturated with water vapour, and why a layer of mucus over an alveolus would be a disaster rather than a protection.

The Tour, and Which Structure Does What ▶

Air travels nose or mouth → larynx → trachea → bronchi → bronchioles → alveoli. The tubes get narrower and more numerous as you go down.

Cartilage in the trachea and bronchi holds them open against the falling pressure of inhalation. Bronchioles have none, but they do have muscle in their walls.

Everything above the alveolus is plumbing. Gas exchange happens at the alveolus and nowhere else, which is why a fixed volume of every breath — the air left in the tubes — is wasted.

⭐ CORE FRAMEWORK 2

Muscles move → volume changes → pressure changes → air flows

The Lungs Are Passive, the Thorax Is Not ▶

The lungs contain no muscle. They are elastic bags stretched by the thorax enlarging around them and they recoil when it shrinks. Nothing in the chest touches the air directly.

Inhalation: external intercostals contract, internal relax, ribs up and out; diaphragm contracts and flattens. Volume up, pressure down below atmospheric, air pushed in.

Exhalation: external relax, internal contract in forced breathing, ribs down and in; diaphragm relaxes and domes. Volume down, pressure up above atmospheric, air pushed out. Quiet exhalation is largely passive elastic recoil.

Rate and Depth Are Two Dials, Not One ▶

Ventilation = rate × depth. At rest, roughly 14 breaths per minute at 500 cm³ gives about 7 dm³ per minute; in hard exercise 45 breaths at 2500 cm³ gives over 110 dm³ per minute.

A fixed volume of every breath stays in the airways, so fewer, deeper breaths waste less. That is why the body increases depth as well as rate, and it is the point of every “two people, same ventilation” question.

On a trace, closer peaks = rate and taller peaks = depth. Read which one has actually changed before you write anything.

The Bell Jar: What It Shows and What It Hides ▶

Jar = thorax, glass tube = trachea, side tubes = bronchi, balloons = lungs, rubber sheet = diaphragm. Pull the sheet down and the volume rises, the pressure falls and air is pushed into the balloons.

What it gets right: the lungs are inflated from outside, and the cause is a volume and pressure change, not the lungs pulling.

What it hides: the jar is rigid, so there are no ribs and no intercostal muscles; the balloons are thick and hollow, so there is no gas exchange at all; and the sheet is pulled by hand rather than contracting and flattening by itself.

⭐ CORE FRAMEWORK 3

Carbon dioxide is the signal → the brain is the detector → the airways keep themselves clean

Why Exercise Changes Your Breathing ▶

Muscles contract more → muscle cells respire faster → more carbon dioxide is produced → the concentration of carbon dioxide in the blood rises → this is detected by the brain → the brain increases the rate and depth of breathing → carbon dioxide is removed and the concentration falls back.

It is not a fall in oxygen. Carbon dioxide is produced in proportion to the work being done and its concentration changes sharply, whereas the oxygen content of blood barely moves during ordinary exercise.

Because the brain is monitoring a concentration and not watching what you are doing, breathing stays raised after exercise stops, until the extra carbon dioxide has been cleared. That is the slow tail on every recovery graph.

Two Cells, One Partnership ▶

Goblet cells secrete mucus. The mucus traps pathogens and dust particles before they can reach the alveoli.

Ciliated cells carry cilia that beat, sweeping the mucus up the trachea to the back of the throat, where it is swallowed and destroyed by stomach acid.

Tobacco smoke destroys the cilia. The goblet cells carry on producing mucus — more of it, because the smoke irritates the lining — so mucus collects: hence the cough and the repeated chest infections. Half a partnership is worse than none.

Everything Loops Back to the Gradient ▶

Ventilation exists to keep alveolar oxygen high and carbon dioxide low. Blood flow exists to keep blood oxygen low and carbon dioxide high at the alveolus. Both are gradient maintenance.

The composition of expired air is the evidence that the gradient did its work: oxygen down from 21 to 16 %, carbon dioxide up from 0.04 to 4 %. Anything that damages the surface moves those figures back towards the inspired values.

And that is the single most useful prediction in Topic 11: less gas exchange means expired air more like inspired air. It works for emphysema, for a blocked bronchus, for asthma and for a paralysed diaphragm.

❌ “Why Is This Wrong?” Exercises

▼

Six real student answers. Find the fault before you reveal it.

Exercise 1: “Explain how air is made to enter the lungs during inhalation. [4]”

Student’s Answer“The intercostal muscles and the diaphragm contract, which makes the chest bigger. The lungs then expand and suck the air in through the trachea.”

The FlawTwo faults, both expensive. “The intercostal muscles” is too vague at Extended level, because there are two sets doing opposite things. And “the lungs expand and suck” skips the mechanism entirely — the lungs have no muscle, and nothing can pull on a gas. The answer names the cause and then jumps straight to the effect, missing the two marks in the middle.

Correct Answer“The external intercostal muscles contract while the internal intercostal muscles relax, raising the ribs up and out [1]. The diaphragm muscle contracts and flattens, moving downwards [1]. The volume of the thorax therefore increases [1], so the pressure inside falls below atmospheric pressure and air is pushed in down the pressure gradient [1].”

Key RuleMuscles, then volume, then pressure, then air — four steps, and usually four marks. If your answer goes straight from muscles to air, you have written half of it.

Exercise 2: “Explain why the alveoli are well adapted for gas exchange. [4]”

Student’s Answer“There are millions of alveoli, so there is a big surface area which means more oxygen is available. They have a good blood supply so the oxygen can be carried around the body, and they are moist and thin.”

The FlawAll four features are in there somewhere, and it will score about two. “More oxygen is available” is not what a large area does — nothing makes oxygen available. The blood supply reason belongs to a different topic: here it matters because of the gradient, not because of delivery. And “moist and thin” is a list without a consequence, which is the commonest way to lose the last mark.

Correct Answer“A very large surface area, so more diffusion can occur at the same time [1]. Walls one cell thick, giving a short diffusion distance of about 1 µm so diffusion is rapid [1]. A dense network of capillaries carries oxygen away as fast as it arrives, keeping the concentration in the blood low and maintaining the concentration gradient [1]. Good ventilation replaces the air in the alveolus, keeping alveolar oxygen high and carbon dioxide low, maintaining the gradient from the other side [1].”

Key RuleEvery feature needs a consequence bolted onto it. Feature plus “so that…” is a mark; feature alone is a list.

Exercise 3: “Explain why breathing becomes faster and deeper during exercise. [4]”

Student’s Answer“When you exercise your muscles need more oxygen. The oxygen level in your blood drops, and your lungs detect this and make you breathe faster so that more oxygen gets in.”

The FlawThree errors, and the answer will score close to nothing despite sounding entirely sensible. The trigger is not a fall in oxygen — it is a rise in carbon dioxide. The detector is not the lungs — it is the brain. And it says only “faster”, when the question asked about faster and deeper.

Correct Answer“The muscles are contracting more, so the muscle cells respire faster and produce more carbon dioxide [1]. The concentration of carbon dioxide in the blood rises [1]. This is detected by the brain [1], which sends impulses to the diaphragm and intercostal muscles so that the rate and the depth of breathing both increase [1], removing the extra carbon dioxide.”

Key RuleCarbon dioxide, brain, rate and depth. If any of those three is missing, the answer is incomplete no matter how fluent it reads.

Exercise 4: “A student bubbles expired air through limewater. Describe the result and state what it shows. [3]”

Student’s Answer“The limewater turns white. This shows that respiration is happening in the lungs and that all the oxygen has been used up.”

The FlawThree separate faults in twenty words. Limewater turns cloudy, not white. The carbon dioxide was made by respiring cells all over the body and carried to the lungs in the blood, so the experiment says nothing about where respiration happens. And limewater tests for carbon dioxide only — it can tell you nothing at all about oxygen, which in any case is still 16 % in expired air.

Correct Answer“The limewater turns cloudy (milky) [1], and it does so after only a few breaths, whereas limewater bubbled with room air stays clear for far longer [1]. This shows that expired air contains more carbon dioxide than inspired air [1].”

Key RuleA test tells you about the one thing it detects, and a comparison tells you about a difference, not a quantity. Claim more than that and you lose the mark you had already earned.

Exercise 5: “Explain the roles of goblet cells, mucus and ciliated cells in protecting the breathing system. [3]”

Student’s Answer“The cilia produce mucus which traps the bacteria and dust, and then it is moved down into the lungs where white blood cells destroy it.”

The FlawThe middle idea is right and everything round it is wrong. Goblet cells produce the mucus, not the cilia. And the direction is reversed: mucus is swept up and out, not down into the lungs. Sending it down would defeat the entire purpose, since the alveoli are a dead end with no way of clearing themselves.

Correct Answer“Goblet cells secrete mucus onto the lining of the airways [1]. The mucus traps pathogens and dust particles so they cannot reach the alveoli [1]. Ciliated cells have cilia which beat and sweep the mucus up the trachea to the back of the throat, where it is swallowed [1].”

Key RuleThree structures, three separate jobs, three marks. Merging any two of them into one sentence collapses the answer to a single mark.

Exercise 6: “A patient has lost about 60 % of the surface area of their alveoli. Suggest how this affects the composition of their expired air. [3]”

Student’s Answer“Their expired air will have less oxygen in it because they cannot get enough oxygen into their body, and more carbon dioxide because it is building up in their blood.”

The FlawBoth predictions are exactly backwards, and the reasoning shows why: the student has thought about the blood instead of the air. It is true that less oxygen reaches their blood and that carbon dioxide builds up in it — but the question asked about the air that comes back out.

Correct Answer“Expired air will contain more oxygen than the usual 16 % [1] and less carbon dioxide than the usual 4 % [1], because with a smaller surface area less gas is exchanged, so the air breathed out is closer in composition to the air breathed in [1].”

Key RuleLess gas exchange means expired air becomes more like inspired air. Hold that sentence and you can answer this question for emphysema, asthma, a blocked bronchus or a paralysed diaphragm without thinking twice.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[6 marks]

(a) State the four features of a gas exchange surface. [2] (b) Explain how a good blood supply and good ventilation both increase the rate of gas exchange. [2] (c) Name one other structure in the human body, and one in a plant, that shows the same set of features. [2]

Model Answer(a) A large surface area, a thin surface [1], a good blood supply and good ventilation with air [1].
(b) The blood supply carries oxygen away as fast as it arrives, so the concentration of oxygen in the blood stays low and the concentration gradient is maintained [1]. Ventilation replaces the air in the alveolus, keeping the oxygen concentration in the air space high and the carbon dioxide concentration low, maintaining the gradient from the other side [1].
(c) In the human body, the villus of the small intestine [1]. In a plant, the root hair cell [1].

Examiner’s NotesPart (b) is where the marks separate candidates. Both answers must contain the word gradient; “so more oxygen can be carried round the body” is an answer about transport in animals and scores nothing here. In (c), naming the structure alone is usually enough, but adding the shared feature (huge folded surface, wall one cell thick) costs one line and protects you if the mark scheme wants a justification.

Question 2

[7 marks]

(a) Name, in order, the structures through which inhaled air passes from the larynx to the gas exchange surface. [2] (b) State the function of the cartilage in the trachea, and explain why it is needed during inhalation. [2] (c) Bronchioles contain no cartilage. Suggest one advantage and one disadvantage of this. [3]

Model Answer(a) Larynx → trachea → bronchi [1] → bronchioles → alveoli [1].
(b) The cartilage rings hold the trachea open and prevent it collapsing [1]; during inhalation the pressure inside the airway falls below atmospheric pressure, so without a stiff wall the tube would be squashed shut by the higher pressure outside [1].
(c) Advantage: the bronchiole walls contain muscle and can change diameter [1], so airflow can be directed to different parts of the lung. Disadvantage: they can be narrowed or closed much more easily [1], for example when the muscle contracts during an asthma attack, and there is no cartilage to hold them open [1].

Examiner’s NotesIn (b), “holds it open” alone will usually get the first mark; the second is for connecting it to the pressure drop, which is what makes this a Supplement answer rather than a Core one. In (c) the word suggest means you are applying knowledge, so a sensible reasoned answer will be credited even if it is not the one printed here — but the reason is the mark, never the bare statement.

Question 3

[8 marks]

(a) Describe fully what happens in the thorax during inhalation, naming both sets of intercostal muscles. [5] (b) Explain why quiet breathing out requires almost no muscular effort. [2] (c) State one situation in which the internal intercostal muscles are important. [1]

Model Answer(a) The external intercostal muscles contract and the internal intercostal muscles relax [1]. The ribs move up and out [1]. The diaphragm muscle contracts and flattens, moving downwards [1]. The volume of the thorax increases [1]. The pressure inside falls below atmospheric, so air is pushed in down the pressure gradient [1].
(b) The external intercostal muscles and the diaphragm simply relax [1], and the stretched, elastic lungs and rib cage recoil back to their resting size on their own, reducing the volume of the thorax [1].
(c) Any one of: forced or deep breathing out, coughing, sneezing, blowing, or breathing out hard during vigorous exercise [1].

Examiner’s NotesFive marks means five distinct events, and one of them must be the pressure. The most common way to score 3 out of 5 here is to describe the muscles and the ribs beautifully and then finish with “so air goes in”. Part (b) is a good discriminator: candidates who think exhalation is an active push cannot answer it at all.

Question 4

[8 marks]

Inspired air contains 21 % oxygen and 0.04 % carbon dioxide. Expired air contains 16 % oxygen and 4 % carbon dioxide. (a) Explain the difference in the oxygen figures. [2] (b) Explain the difference in the carbon dioxide figures. [2] (c) State two other ways in which expired air differs from inspired air, and explain one of them. [3] (d) Explain why the nitrogen figure is unchanged. [1]

Model Answer(a) Oxygen diffuses from the alveolar air into the blood, down its concentration gradient [1], because the blood arriving is deoxygenated and the oxygen is carried away and used by respiring cells [1].
(b) Carbon dioxide produced by respiring cells is carried in the blood to the lungs [1]; its concentration in the blood is higher than in the alveolar air, so it diffuses out into the alveolus and is breathed away [1].
(c) Expired air contains more water vapour and is warmer (about 37 °C) [1 for both]. Explanation of either: water evaporates from the moist lining of the airways and alveoli into the air [1]; or the air is warmed to body temperature by contact with the warm surfaces of the airways [1].
(d) Nitrogen is neither used nor produced by the body [1].

Examiner’s NotesEvery explanation in this question must contain the word diffuses or gradient. Note how (a) and (b) are mirror images: in each case the gas moves from where it is more concentrated to where it is less. In (c), do not waste a line explaining both differences if only one explanation is asked for — write one properly instead.

Question 5

[7 marks]

A student sets up two boiling tubes of limewater and a mouthpiece with two one-way valves, so that breathing in draws room air through tube A and breathing out pushes expired air through tube B. (a) State the expected result in each tube. [2] (b) State three variables that must be controlled. [3] (c) Explain why a conclusion that “respiration takes place in the lungs” would not be supported by this experiment. [2]

Model Answer(a) Tube B turns cloudy (milky) after only a few breaths [1]; tube A stays clear for far longer [1].
(b) Any three of: the same volume of limewater in each tube; the same concentration of limewater; the same number of breaths through each; the same person breathing at the same rate; the same temperature [3].
(c) The carbon dioxide was produced by respiring cells throughout the body, not in the lungs [1]; it is carried to the lungs in the blood and diffuses out at the alveoli, so the lungs are where it leaves the body rather than where it is made [1].

Examiner’s Notes“Cloudy” or “milky” is the only accepted wording; “white” is refused. In (b), “the same limewater” is too vague to earn a mark — name the variable (volume, concentration) each time. Part (c) is the sort of question that separates a candidate who has learned a practical from one who has understood it.

Question 6

[8 marks]

At rest a man breathes 13 times per minute, each breath 520 cm³. During exercise he breathes 38 times per minute, each breath 2400 cm³. (a) Calculate his ventilation in dm³ per minute in each condition. [3] (b) Calculate the factor by which his ventilation has increased. [1] (c) Explain the mechanism that produced this change. [4]

Model Answer(a) At rest: 13 × 520 = 6760 cm³ [1] = 6.76 dm³ per minute [1]. During exercise: 38 × 2400 = 91 200 cm³ = 91.2 dm³ per minute [1].
(b) 91.2 ÷ 6.76 = 13.5 times [1].
(c) The muscles contract more, so muscle cells respire faster [1] and produce more carbon dioxide, raising its concentration in the blood [1]. This is detected by the brain [1], which sends impulses to the diaphragm and intercostal muscles so that both the rate and the depth of breathing increase [1].

Examiner’s NotesDo the multiplication in cm³ and convert once at the end; converting first is where units go wrong. A factor is a division, so 13.5 times — not 13.5 % and not 84.4 dm³, which would be the increase. In (c), an answer built on falling oxygen scores zero however well written.

Question 7

[7 marks]

A model of the thorax is made from a bell jar, a glass tube ending in two balloons, and a rubber sheet across the open base. (a) State what the bell jar, the balloons and the rubber sheet represent. [3] (b) Describe and explain what happens to the balloons when the rubber sheet is pulled downwards. [2] (c) Give two ways in which the model is a poor representation of a real thorax. [2]

Model Answer(a) Bell jar = the thorax or rib cage [1]; balloons = the lungs [1]; rubber sheet = the diaphragm [1].
(b) The balloons inflate [1], because pulling the sheet down increases the volume inside the jar, so the pressure falls and air is pushed in through the glass tube [1].
(c) Any two of: the jar is rigid, so it cannot show the ribs moving up and out and there are no intercostal muscles [1]; the balloons are thick and hollow with no alveoli, so no gas exchange can occur [1]; the sheet is pulled by hand rather than being a muscle that contracts and flattens by itself [1].

Examiner’s NotesIn (b) the words volume and pressure are the mark; “the balloons inflate because you pulled the sheet” describes without explaining. In (c), aim for criticisms about the biology the model omits rather than about the materials it is made of — “the balloons are the wrong colour” earns nothing.

Question 8

[7 marks]

(a) Explain the roles of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles. [3] (b) Tobacco smoke destroys the cilia. Explain two consequences of this for a smoker. [2] (c) Suggest why the alveoli themselves are not protected by a layer of mucus. [2]

Model Answer(a) Goblet cells secrete mucus onto the surface of the airway lining [1]. The mucus traps pathogens and dust particles so they do not reach the alveoli [1]. Ciliated cells have cilia that beat and sweep the mucus up the trachea to the back of the throat, where it is swallowed [1].
(b) Mucus is still produced but is no longer moved, so it collects in the airways and the smoker coughs to clear it [1]. Bacteria trapped in the stagnant mucus are not removed, so the smoker suffers more chest infections [1].
(c) A layer of mucus over an alveolus would increase the diffusion distance [1], slowing or preventing gas exchange across a surface whose whole design depends on being one cell thick [1].

Examiner’s NotesPart (c) is a “suggest” question and there is no sentence in the syllabus to recall — you are being asked to notice a trade-off. That is exactly the kind of thinking a challenge paper rewards, and the phrase diffusion distance is what turns a plausible guess into a marked answer.

Question 9

[8 marks]

A patient has a condition in which the nerve supplying the diaphragm no longer works, although the intercostal muscles are unaffected. (a) Explain the effect on the volume of air taken in with each breath. [3] (b) Predict and explain what happens to the patient’s breathing rate. [3] (c) Predict how the composition of the patient’s expired air compares with that of a healthy person, and explain. [2]

Model Answer(a) The diaphragm can no longer contract and flatten [1], so one of the two mechanisms that enlarge the thorax is lost. The volume of the thorax increases less than normal on each breath [1], so the pressure falls less far below atmospheric and a smaller volume of air is pushed in — the breaths are shallower [1].
(b) The breathing rate increases [1]. Less air reaches the alveoli each breath, so less carbon dioxide is removed and its concentration in the blood rises [1]; this is detected by the brain, which increases the rate of breathing to compensate [1].
(c) Expired air will contain slightly more oxygen and less carbon dioxide than usual [1], because less gas is exchanged per breath so the air breathed out is closer in composition to the air breathed in [1].

Examiner’s NotesNotice that the answer to (a) is shallower breaths, not no breathing — there are two mechanisms and only one has failed. Part (b) is the control chain used in reverse, and it is worth practising in that direction, because challenge papers rarely ask it the easy way round.

Question 10

[9 marks]

(a) Distinguish between ventilation, gas exchange and respiration, stating where each takes place. [3] (b) Trace the journey of one oxygen molecule from the air outside the body to a muscle cell, naming every structure it passes through. [4] (c) Explain why an athlete’s expired air contains a lower percentage of oxygen during exercise than at rest. [2]

Model Answer(a) Ventilation is the movement of air into and out of the lungs, and takes place in the thorax [1]. Gas exchange is the diffusion of oxygen and carbon dioxide across the alveolus wall, and takes place at the alveoli [1]. Respiration is the chemical reaction that breaks down nutrient molecules to release energy, and takes place inside every living cell [1].
(b) Nose or mouth → larynx → trachea → bronchus → bronchiole → alveolus [1]; it then diffuses across the alveolus wall and the capillary wall, a distance of about 1 µm [1]; it combines with haemoglobin in a red blood cell and is carried in the blood [1]; at the muscle it diffuses out of the capillary into the muscle cell, where it is used in respiration [1].
(c) The muscle cells are respiring faster, so more oxygen is removed from the blood and the blood arriving at the lungs contains even less oxygen [1]; the concentration gradient at the alveolus is therefore steeper, more oxygen diffuses out of the alveolar air, and the air breathed out contains a smaller percentage of oxygen [1].

Examiner’s NotesPart (a) is the single most valuable three marks in Topic 11, and it is pure vocabulary — learn it as three sentences with three locations. In (b) the mark for diffuses is easy to lose by writing “goes into the blood”; name the process. Part (c) asks you to run the gradient argument forwards from a change in the body, which is exactly what a challenge paper means by application.