IGCSE Biology Gas exchange & respiration notes

Gas exchange in humans happens in the alveoli, and the surfaces involved have four features the syllabus expects you to name: a large surface area (millions of alveoli), a thin surface (one flattened cell thick, so diffusion distances are tiny), a good blood supply (a dense capillary network keeps the concentration gradient steep) and good ventilation with air (breathing continually replaces the air, so gradients do not equalise).

A labelled diagram to picture: a bunch-of-grapes cluster of alveoli, each a thin-walled air sac wrapped in a net of capillaries; air arrives through bronchioles, oxygen diffuses into the blood while carbon dioxide diffuses into the alveolus, and the air flows back out — both gases moving down their own concentration gradients.

A labelled diagram to picture: air enters past the larynx into the trachea (windpipe), held open by rings of cartilage; the trachea splits into two bronchi, one to each lung; each bronchus branches into finer bronchioles, which end in clusters of alveoli wrapped in capillaries. Around the chest are the ribs, moved by the intercostal muscles, with the diaphragm — a sheet of muscle — forming the floor of the thorax below the lungs.

Supplement: the cartilage in the trachea keeps the airway open — it stops the trachea collapsing when pressure drops during inhalation. The intercostal muscles come in two layers: external intercostals between the ribs, used for breathing in, and internal intercostals, used for forced breathing out.

Supplement — airway defences: goblet cells in the lining of the trachea and bronchi secrete mucus, which traps pathogens and particles from the air; ciliated cells with hair-like cilia sweep the mucus — and its trapped cargo — upwards to the throat, where it is swallowed. This keeps the delicate alveoli almost sterile.

The test for the difference is limewater: bubble each air sample through limewater and compare. Expired air turns limewater milky (cloudy); inspired air leaves it clear — the milkiness is the carbon dioxide. Supplement — explaining the differences: the body’s cells use oxygen and release carbon dioxide in aerobic respiration, so exhaled air is poorer in oxygen and richer in carbon dioxide; the air picks up water by evaporation from the moist airway surfaces as it passes in and out.

Physical activity changes both the rate and the depth of breathing: a student breathing ~15 times per minute at rest may breathe faster and more deeply after running — you can investigate this by counting breaths and comparing chest expansion before and after exercise.

Supplement. Ventilation works on one rule: air flows from high pressure to low pressure. Breathing in (inhalation): the external intercostal muscles contract, pulling the ribs up and out, and the diaphragm contracts, flattening from its dome shape; the thorax volume increases, so the pressure inside the thorax falls below atmospheric pressure, and air is drawn into the lungs.

Breathing out (exhalation): the external intercostals relax and the internal intercostals contract, pulling the ribs down and in, while the diaphragm relaxes and domes upward; thorax volume decreases, pressure rises above atmospheric, and air is pushed out. Every ventilation answer should pair each movement with its volume change and its pressure change — naming muscles alone is half the story.

During physical activity the muscles respire faster, releasing more carbon dioxide into the blood. Supplement: the increased carbon dioxide concentration in the blood is detected by the brain, which responds by sending signals to breathe faster (increased rate) and more deeply (greater depth). This removes carbon dioxide quicker and supplies extra oxygen — matching gas exchange to demand. When activity stops, carbon dioxide levels fall and breathing returns to its resting rate and depth.

Respiration is not “breathing” — it is the release of energy from nutrient molecules inside cells. The energy released is used for muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and the maintenance of a constant body temperature. Learn the list as seven uses; “moving” alone is not precise enough for a mark.

Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Word equation: glucose + oxygen → carbon dioxide + water. Supplement balanced chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O — count the atoms to check the balance: 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.

Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen — and it releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partially broken down. In yeast: glucose → alcohol + carbon dioxide (fermentation; Supplement balanced: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂). In muscles during vigorous exercise: glucose → lactic acid.

Investigate and describe the effect of temperature on respiration in yeast. A labelled diagram to picture: a flask of yeast in glucose solution sealed with a bung and tube leading into a tube of limewater (or placed under water to count bubbles of gas). As the yeast respires the glucose, it releases carbon dioxide, which bubbles through and turns the limewater milky — the rate of bubbling measures the rate of respiration.

The pattern: respiration rises with temperature up to an optimum (warm temperatures give faster enzyme-catalysed reactions, so more bubbles per minute), then falls sharply above the optimum because the enzymes denature — near 0 °C respiration is very slow, around 35–40 °C it peaks, and by 60 °C it stops. Keep the yeast concentration, glucose concentration and volume the same at each temperature — only temperature changes.

Supplement. During vigorous exercise the muscles cannot get enough oxygen for full aerobic respiration, so they respire anaerobically and lactic acid builds up in the muscles and blood, causing an oxygen debt — the oxygen needed *after* exercise to clear the lactic acid. This is why anaerobic respiration cannot be sustained: lactic acid is harmful in quantity, and the debt must be repaid.

How the debt is removed after exercise — three linked steps: the heart rate stays fast, transporting lactic acid in the blood from the muscles to the liver; breathing stays deeper and faster, supplying the oxygen needed; the liver then aerobically respires the lactic acid, breaking it down (much of it can be converted back towards glucose). Only when the lactic acid is gone does pulse and breathing rate return to rest — the “catch-up” oxygen is the debt being repaid.