Answers · Gas exchange and breathing

Gas exchange and breathing — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. AK1 · Final check

Model answer. Put the same volume of limewater in two tubes joined to one mouthpiece, with one-way valves. Breathing in draws room air through one tube; breathing out pushes breath through the other. Count the breaths until a cross behind each tube can no longer be seen. The tube with expired air turns cloudy much sooner, so expired air contains more carbon dioxide.

A full answer includes

I wrote that both tubes hold the same volume of limewater.
I wrote that breathing in draws room air through one tube.
I wrote that breathing out pushes breath through the other tube.
I wrote that the breaths are counted until the cross behind each tube disappears.
I wrote that the tube with expired air turns cloudy sooner.
I did not write that both airs pass through the same tube.

Also accept

same limewater; inspired through one, expired through other; count breaths to the same end point; expired clouds sooner

Watch for

“Breathe in and out through the same tube.” — Each air needs its own tube, so the two can be compared.

2. AK2 · Final check

Model answer. C. Expired air has less oxygen than inspired air (about 16% instead of 21%), more carbon dioxide (about 4% instead of 0.04%) and more water vapour.

A full answer includes

I chose row C.
I did not choose row A: expired air still has much more oxygen than carbon dioxide.

Also accept

Watch for

“Row A: 3.8% oxygen and 16.9% carbon dioxide.” — The body takes only part of the oxygen. Expired air still has much more oxygen than carbon dioxide.
“Row B: 20.9% oxygen and 0.04% carbon dioxide.” — Those are the figures for inspired air. The body takes in oxygen and adds carbon dioxide.
“Row D: less water vapour than room air.” — Water evaporates into the air in the lungs, so expired air has more water vapour, not less.

3. AK3 · Final check

Model answer. The air in an alveolus has a higher concentration of oxygen than the blood arriving in the capillaries, so oxygen diffuses into the blood. The blood carries it to the body's cells, which use it in aerobic respiration, so less oxygen is breathed out.

A full answer includes

I wrote that oxygen diffuses from the air in the alveoli into the blood.
I wrote that the blood carries it to cells.
I wrote that cells use it in aerobic respiration.
I did not write that oxygen is used up in the lungs.

Also accept

diffuses into the blood in the alveoli; used in respiration by cells

Watch for

“Oxygen is turned into carbon dioxide in the lungs.” — Oxygen diffuses into the blood in the lungs and is used by cells in respiration.

4. LK1 · Final check

Options as numbered on the pupil sheet: 1 — oesophagus; 2 — alveolus; 3 — larynx; 4 — intercostal muscle; 5 — bronchus; 6 — heart; 7 — capillary; 8 — trachea; 9 — rib; 10 — bronchiole; 11 — lung; 12 — diaphragm.

Model answer. 3, the box-shaped top of the airway in the neck, is the larynx. 6, the tube with rings below it, is the trachea. 10, the wide branch that leads into the lung, is a bronchus, and 2, a narrow branch inside the lung, is a bronchiole. 7 is the lung. 4 is a rib, and 8, between two ribs, is an intercostal muscle. 1, the curved band under the lung, is the diaphragm. In the circle, 9 is the air space of an alveolus and 5 is a capillary round it. The oesophagus and the heart are not in the diagram.

A full answer includes

I matched 1 to the diaphragm.
I matched 2 to a bronchiole.
I matched 3 to the larynx.
I matched 4 to a rib.
I matched 5 to a capillary.
I matched 6 to the trachea.
I matched 7 to a lung.
I matched 8 to an intercostal muscle.
I matched 9 to an alveolus.
I matched 10 to a bronchus.
I did not match 10 to a bronchiole.
I did not use the names oesophagus and heart.

Also accept

1 diaphragm; 2 bronchiole; 3 larynx; 4 rib; 5 capillary; 6 trachea; 7 lung; 8 intercostal muscle; 9 alveolus; 10 bronchus

Watch for

“10 is the trachea.” — The trachea is the tube with rings, 6. 10 is one of its branches, leading into the lung: a bronchus.
“2 is a bronchus.” — 2 is a narrow branch inside the lung: a bronchiole. The bronchus, 10, is the wide branch before the lung.
“8 is a rib.” — The ribs, like 4, are the oval sections of bone. 8 is the muscle between two of them.
“5 is an alveolus.” — 5 is the fine vessel round the sac: a capillary. The alveolus is the sac itself, 9.
“6 is the oesophagus.” — The oesophagus carries food and is not drawn here. The tube with rings carries air.

5. LK2 · Final check

Model answer. For example: the alveoli have a large surface area, so more gas can diffuse at the same time; and their walls and the capillary walls are one cell thick, so the gases diffuse only a short distance. (A good blood supply and good ventilation keep the concentration difference.)

A full answer includes

(Any two of the next four lines.) I wrote a large surface area, so more gas diffuses at once.
(Any two of the four.) I wrote a thin surface, one cell thick, so a short diffusion distance.
(Any two of the four.) I wrote a good blood supply, which keeps a concentration difference.
(Any two of the four.) I wrote good ventilation, which keeps fresh air in the alveoli.
I did not write that the alveoli pump air into the blood.

Also accept

two features, each with how it helps diffusion

Watch for

“The alveoli pump air into the blood.” — Gases cross by diffusion; the features only make diffusion faster.

6. LK3 · Final check

Model answer. (a) G, the layer nearer the lung: the internal intercostal muscles. (b) The cartilage rings keep the trachea open. (c) Mucus made by goblet cells traps pathogens and particles; the cilia of ciliated cells sweep the mucus up the trachea to the throat, away from the lungs.

A full answer includes

(a) I identified G.
(a) I stated the internal intercostal muscles.
(b) I stated that it keeps the trachea open.
(c) I wrote that mucus, made by goblet cells, traps pathogens and particles.
(c) I wrote that the cilia sweep the mucus up to the throat.
I did not write that the cilia trap the pathogens.

Also accept

(a) G, internal intercostals; (b) keeps trachea open; (c) mucus traps, cilia move it

Watch for

“The cilia trap the pathogens.” — Mucus traps them; the cilia move the mucus.

7. VK1 · Final check

Model answer. The diaphragm contracts and flattens, and the external intercostal muscles contract and pull the ribs up and out (the internal ones relax). The volume of the thorax increases, so the pressure inside falls below atmospheric pressure, and air flows in through the trachea into the lungs.

A full answer includes

I wrote that the diaphragm contracts and flattens.
I wrote that the external intercostal muscles contract.
I wrote that the ribs move up and out.
I wrote that the volume of the thorax increases.
I wrote that the pressure inside falls below atmospheric pressure.
I wrote that air flows in.
I did not write that the lungs expand first and suck the air in.

Also accept

muscles contract; volume increases; pressure falls; air flows in

Watch for

“The lungs expand and suck the air in.” — The chest gets larger first; the pressure falls; then air flows in.

8. VK2 · Final check

Model answer. (a) Depth = volume per minute ÷ rate = 28.8 ÷ 18 = 1.6 dm³. (b) 1.6 dm³ ÷ 0.6 dm³ = 2.67…, so her breaths are about 2.7 times as deep (to 2 significant figures).

Checked automatically

(a) Depth of each breath: 1.6 dm³ (exact value)
- Common wrong answer 518 dm³: You multiplied. Depth = volume per minute ÷ rate.
(b) How many times deeper: 2.7 (2 significant figures)
- Common wrong answer 0.38 : You divided the resting depth by the depth after exercise. Divide 1.6 by 0.6.
- Common wrong answer 1.0 : That is the difference in dm³. "How many times" needs a division.

A full answer includes

(a) I divided 28.8 dm³ by 18.
(a) I wrote 1.6 dm³.
(b) I divided 1.6 dm³ by 0.6 dm³.
(b) I wrote 2.7 times, to 2 significant figures.
I did not multiply 28.8 by 18.

Also accept

(a) 1.6 dm³; (b) 2.7

Watch for

“The depth is 518 dm³.” — That multiplies the volume by the rate. Depth = volume per minute ÷ rate.
“Her breaths are 1.0 times deeper.” — 1.6 − 0.6 = 1.0 dm³ is how much deeper. "How many times" needs a division.

9. VK3 · Final check

Model answer. Count a person's breaths at rest for 30 s and multiply by 2 for breaths per minute. Then do a set exercise, such as three minutes of step-ups, and count again for 30 s straight afterwards. Keep the person, the exercise and the counting time the same, and repeat or use several people to find averages. The rate of breathing is higher after exercise.

A full answer includes

I wrote that I would count the breaths at rest for a set time, such as 30 s.
I wrote that I would convert the count to breaths per minute.
I wrote that the person does a set exercise for a set time.
I wrote that I would count again in the same way straight after the exercise.
I wrote that the same person is used for both counts.
I wrote that the test is repeated, or several people are used, and a mean is found.
I wrote that I expect the rate to increase after exercise.
I did not count the breaths several minutes after the exercise stopped.

Also accept

count at rest; same count after set exercise; control variables and repeats; rate increases

Watch for

“Count at rest only once and after exercise for a whole hour.” — Both counts must be done the same way, straight after the exercise.

10. VK4 · Final check

Model answer. When she stops, her leg muscles contract less and respire more slowly, so they make less carbon dioxide. Her fast, deep breathing keeps removing carbon dioxide, so its concentration in the blood falls. The brain detects the fall and sends fewer nerve impulses to the diaphragm and intercostal muscles, so they contract less often and less strongly, and her breathing becomes slower and shallower.

A full answer includes

I wrote that her leg muscles respire more slowly and make less carbon dioxide.
I wrote that the carbon dioxide concentration in her blood falls.
I wrote that the brain detects the fall.
I wrote that the brain sends fewer impulses to the diaphragm and intercostal muscles.
I did not write that the brain detects the oxygen in her blood.

Also accept

less carbon dioxide made; its concentration in the blood falls; brain detects; fewer impulses to breathing muscles

Watch for

“Her breathing slows because the brain detects that she has enough oxygen again.” — The brain detects the carbon dioxide concentration in the blood. As it falls, breathing slows.