This mini-lesson walks you through Cambridge IGCSE Gas exchange & respiration: gas exchange surfaces, the lungs and breathing, inspired vs expired air, aerobic & anaerobic respiration and their equations, oxygen debt, and the effect of exercise.
Make a guess: why does the sprinter keep panting after the race?
Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Watch for the Key term flags. Press Start when you're ready.
We breathe air down the trachea, which branches into bronchi and then tiny bronchioles, ending in millions of air sacs called alveoli. This is where gas exchange happens:
Adaptations to learn: large surface area, thin (one-cell-thick) walls, good blood supply and good ventilation with air — all speed up diffusion of gases.
Know these parts: larynx, trachea, bronchi, bronchioles, alveoli (with their capillaries), lungs, ribs, intercostal muscles and the diaphragm.
Supplement — ventilation: breathing in — the external intercostal muscles contract, so the ribs move up and out; the diaphragm contracts and flattens → thorax volume rises, pressure falls → air flows in. Breathing out — they relax (the internal intercostals contract in forced breathing out) → volume falls, pressure rises → air is pushed out.
Supplement — protecting the airways: cartilage keeps the trachea open. Goblet cells make mucus that traps pathogens and particles; ciliated cells beat to sweep the mucus up and away from the lungs.
Test: bubble the air through limewater — it turns cloudy (milky) much faster with breathed-out air, because it has more carbon dioxide.
Exercise increases the rate and depth of breathing.
Supplement — explaining it: cells use oxygen and make carbon dioxide in respiration; water evaporates from the moist lining of the alveoli. During exercise, muscles respire faster, so the carbon dioxide concentration in the blood rises; the brain detects this and makes breathing faster and deeper.
Respiration is the release of energy from food (glucose) inside every living cell. Aerobic respiration uses oxygen and releases a large amount of energy. It mostly happens in the mitochondria.
The energy released is used for muscle contraction, protein synthesis, cell division, growth, active transport, nerve impulses and keeping body temperature constant.
Careful wording: respiration releases (transfers) energy — it does not "make" or "produce" energy. Energy cannot be created; it comes from the glucose.
Anaerobic respiration releases energy from glucose without oxygen. It releases much less energy per glucose than aerobic respiration, because the glucose is only partly broken down.
In yeast and plants, anaerobic respiration is different — it forms alcohol (ethanol) and carbon dioxide (this is fermentation, used in brewing and bread-making):
Tap a statement, then tap the type of respiration it describes. (Some are true of both.)
During hard exercise, muscles can't get enough oxygen, so they respire anaerobically, building up lactic acid in the muscles and blood. This causes muscle fatigue.
The extra oxygen needed afterwards to break down this lactic acid (by aerobic respiration in the liver) is called the oxygen debt. This is why you keep breathing hard and fast after you stop exercising — to "repay" the debt.
Tap a description on the left, then its matching term on the right.
Lungs: trachea → bronchi → bronchioles → alveoli (gas exchange site)
Alveoli: large surface area, thin walls, good blood supply, good ventilation
Gas exchange: O₂ into blood, CO₂ out of blood, by diffusion
Breathing: intercostals & diaphragm change thorax volume and pressure; breathed-out air has less O₂, more CO₂ and water vapour
Aerobic: glucose + oxygen → carbon dioxide + water (lots of energy)
Anaerobic (muscle): glucose → lactic acid (little energy)
Anaerobic (yeast): glucose → alcohol + carbon dioxide (fermentation)
Oxygen debt: extra O₂ needed after exercise to break down lactic acid in the liver
You've covered the key ideas in Cambridge IGCSE Gas exchange & respiration. Press Finish to see your score.
You've worked through Gas exchange & respiration for Cambridge IGCSE Biology. 🎉
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Next: test yourself in the Verify stage.