IGCSE Biology | Answers | 25 Questions
State the three features of a gas exchange surface in humans.
Name the gas that makes up approximately 78% of atmospheric air.
State the approximate percentage of oxygen in atmospheric air.
Name the tiny air sacs found at the ends of the bronchioles, where gas exchange takes place.
Name the dome-shaped muscle found below the lungs that plays a role in breathing.
State what happens to the shape of the diaphragm when it contracts during inhalation.
State one difference between inspired air and expired air in terms of carbon dioxide.
Name the specialised cells in the lining of the airways that produce mucus.
State the function of cilia on ciliated cells in the trachea and bronchi.
State two changes in breathing that occur during vigorous physical activity.
Explain why the alveoli are efficient gas exchange surfaces. Refer to two structural features in your answer.
Accept any two of the following, each with a clear feature → explanation link:
- Very large total surface area (millions of alveoli) — more gas can cross the surface at the same time, increasing the overall rate of gas exchange.
- Walls are only one cell thick — the diffusion distance is very short, so gases cross quickly.
- Moist inner surface — gases dissolve in the moisture, which allows them to diffuse through the wall.
- Rich blood supply — blood constantly removes oxygen and delivers carbon dioxide, maintaining a large concentration difference that keeps diffusion fast.
A student blows expired air through a straw into limewater. The limewater turns milky within a few seconds. Explain why expired air causes this reaction, but atmospheric air would take much longer to produce the same result.
Expired air contains approximately 4% carbon dioxide, whereas atmospheric (inspired) air contains only about 0.04% carbon dioxide. Limewater turns milky in the presence of CO₂. Because expired air has about 100 times more CO₂ than atmospheric air, it reacts with the limewater much more quickly and produces a visible result much sooner.
Explain why expired air contains more water vapour than inspired air.
The inner surfaces of the alveoli and airways are moist. As air moves through the lungs, water evaporates from these moist surfaces into the air. By the time the air is breathed out, it has picked up this water vapour and is nearly saturated. Inspired air from the atmosphere typically has a much lower water vapour content.
Describe what happens to the diaphragm and external intercostal muscles during inhalation, and explain how these changes cause air to move into the lungs.
- The diaphragm contracts and flattens downward.
- The external intercostal muscles contract, pulling the ribs upward and outward.
- Both changes increase the volume of the thorax.
- The increased volume causes the air pressure inside the lungs to fall below atmospheric pressure.
- Air is pushed in from outside (down the pressure gradient) until the pressure equalises — air enters the lungs.
During a normal resting exhalation, the diaphragm and external intercostal muscles relax. Explain how this causes air to leave the lungs. Refer to volume and pressure in your answer.
- The diaphragm relaxes and returns to its dome shape. The external intercostal muscles relax and the ribs fall downward and inward.
- The volume of the thorax decreases.
- The decreased volume causes the air pressure inside the lungs to rise above atmospheric pressure.
- Air is pushed out of the lungs (down the pressure gradient) until the pressure equalises.
A student runs a 400 m race. After finishing, her breathing has changed compared to when she was at rest.
(a) State two changes in her breathing. (b) Explain why each change is needed.
Increased rate: During exercise, muscles respire faster and use more oxygen and produce more carbon dioxide. More frequent breaths replace the oxygen used and remove the extra CO₂ more quickly.
Increased depth: Each breath moves a larger volume of air in and out of the lungs. This means more oxygen enters and more CO₂ is removed with every breath, increasing the total amount of gas exchanged per minute.
Explain why the alveolus wall being only one cell thick is important for efficient gas exchange.
A wall that is only one cell thick creates a very short diffusion distance. Oxygen and carbon dioxide molecules only need to travel across a tiny gap to move between the air in the alveolus and the blood in the capillary. The shorter the diffusion distance, the faster molecules can cross, so the rate of gas exchange is higher. A thicker wall would slow diffusion significantly.
Explain why a good blood supply around the alveoli is important for maintaining a high rate of gas exchange.
Blood flowing continuously through the capillaries around the alveoli constantly carries away the oxygen that has just diffused in, and constantly brings fresh carbon dioxide to the alveolus surface. This prevents oxygen building up in the blood (which would reduce the concentration difference) and prevents CO₂ levels falling too low in the blood.
By maintaining a large concentration difference on both sides of the alveolus wall, the blood supply keeps diffusion fast. Without this constant flow, the concentration differences would equalise and diffusion would slow to a stop.
A patient has a condition where their goblet cells do not produce enough mucus. Explain the consequences of this for the gas exchange system.
Mucus normally coats the lining of the airways and traps pathogens (bacteria, viruses) and particles (dust, debris) that enter with the air. If goblet cells produce too little mucus, the airway lining is less able to trap these particles.
Pathogens and particles are therefore not captured effectively and can travel deeper into the airways. They may reach the alveoli, increasing the risk of infection and damage to the gas exchange surfaces. The lungs are less protected against inhaled threats.
The table below shows the approximate composition of inspired and expired air.
| Gas | Inspired air | Expired air |
|---|---|---|
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
| Nitrogen | 78% | 78% |
During gas exchange in the alveoli, oxygen diffuses from the alveolus into the blood (from high concentration to low concentration). The blood absorbs this oxygen and carries it to the body’s cells. Because oxygen is removed from the air inside the alveoli, the air that is eventually breathed out contains less oxygen than the air that was breathed in.
The alveoli are the site of gas exchange in the lungs.
(a) State three structural characteristics. (b) Explain the direction of oxygen diffusion. (c) Explain why blood flow is important.
Any three of:
- Enormous total surface area (millions of alveoli)
- Walls are only one cell thick (very short diffusion distance)
- Moist inner surface (gases dissolve, enabling diffusion through the wall)
- Surrounded by a dense network of capillaries (good blood supply)
Diffusion always moves molecules from an area of higher concentration to an area of lower concentration. The concentration of oxygen is higher in the air inside the alveolus than in the blood arriving at the capillaries (because the blood has already delivered oxygen to the body and returned with little left). Therefore, oxygen diffuses from the alveolus into the blood — not in the opposite direction.
As blood flows through the capillaries, it continuously carries away the oxygen that has just diffused in. This prevents oxygen from building up in the blood. By keeping the oxygen concentration in the blood lower than in the alveolus, the blood flow maintains a large concentration difference across the alveolus wall. A large concentration difference means a fast rate of diffusion. Without constant blood flow, oxygen would accumulate in the blood, the concentration difference would shrink, and diffusion would slow down.
The breathing mechanism involves changes to the thorax.
(a) Describe inhalation. (b) Describe normal resting exhalation. (c) Explain forceful exhalation during exercise.
- The diaphragm contracts → it flattens and moves downward.
- The external intercostal muscles contract → the ribs move upward and outward.
- Both movements increase the volume of the thorax.
- The increased volume causes the air pressure inside the lungs to fall below atmospheric pressure.
- Air is pushed in from outside down the pressure gradient → air enters the lungs.
- The diaphragm relaxes → returns to its dome shape.
- The external intercostal muscles relax → ribs fall downward and inward under their own weight.
- The volume of the thorax decreases.
- The air pressure inside the lungs rises above atmospheric pressure.
- Air is pushed out of the lungs down the pressure gradient.
The internal intercostal muscles contract. This pulls the ribs further downward and inward beyond their resting position, causing a greater decrease in the volume of the thorax. The pressure inside the lungs rises higher than during normal exhalation, forcing air out more powerfully and more completely.
A student investigates the differences between inspired and expired air using three tests.
(a) Condensation on cold glass. (b) Which air sample is warmer? (c) Why does nitrogen stay the same?
Expired air contains much more water vapour than atmospheric air. As air passes through the warm, moist airways and alveoli, water evaporates from the lining into the air. By the time it is breathed out, the air is nearly saturated with water vapour. Atmospheric air has a much lower water vapour content. When the water-vapour-rich expired air meets the cold glass, more water condenses out of it, producing more visible condensation.
Air inside the lungs is warmed to approximately body temperature (37°C) as it passes through the warm, moist airways. Atmospheric (inspired) air is at room temperature, which is lower. As a result, expired air is warmer than the air that was breathed in.
The body does not use nitrogen and does not produce it. Nitrogen is not involved in any gas exchange process in the lungs. The same amount of nitrogen that enters the lungs during inhalation leaves during exhalation, so its percentage remains unchanged at approximately 78% in both inspired and expired air.
The trachea and bronchi contain specialised cells that protect the gas exchange system.
(a) Name the two cell types. (b) Describe how they work together. (c) Consequences of ciliated cells permanently stopping.
- Goblet cells produce and secrete mucus, which forms a sticky layer coating the inside of the airway.
- The mucus traps pathogens (bacteria, viruses) and particles (dust, pollen) as they enter with the air.
- Ciliated cells have tiny hair-like cilia on their surface that beat continuously in a coordinated wave.
- The cilia sweep the mucus — with everything trapped in it — upward towards the throat, where it is swallowed or expelled.
- Together, they remove pathogens and particles before they reach the alveoli, protecting the delicate gas exchange surface from infection and damage.
Goblet cells continue to produce mucus as normal. Mucus still forms in the airways and continues to trap pathogens and particles. However, without working ciliated cells, there is nothing to sweep the mucus upward.
The mucus builds up and accumulates in the airways. The trapped pathogens are not removed and remain in the airways for longer. They can travel deeper into the lungs and reach the alveoli, greatly increasing the risk of infection. The blocked airways also make it harder for air to flow to the alveoli, reducing the efficiency of gas exchange.
Explain how the structure of the human gas exchange system makes it efficient at exchanging gases between the air and the blood. Refer to: the structure of the alveoli; the role of breathing movements; and one other structural feature.
A strong answer will include all three required areas with clear structure → function links. Below is a model answer:
The alveoli are tiny air sacs at the ends of the bronchioles. There are millions of them, giving a very large total surface area across which gases can diffuse simultaneously. Their walls are only one cell thick, creating a very short diffusion distance so oxygen and carbon dioxide move across quickly. Each alveolus is surrounded by a dense network of capillaries that provides a good blood supply — blood constantly carries away oxygen and brings carbon dioxide, keeping the concentration differences large and diffusion fast.
Breathing movements continually renew the air inside the alveoli. During inhalation, the diaphragm contracts and flattens, and the external intercostal muscles contract moving the ribs up and out. This increases the volume of the thorax, reducing the pressure inside the lungs so fresh air (rich in oxygen) enters. During exhalation, these muscles relax, the thorax volume decreases, pressure rises, and stale air (rich in carbon dioxide) is expelled. This ventilation prevents oxygen from being used up and carbon dioxide from building up in the alveoli, keeping the concentration gradients steep and diffusion efficient.
- Airway structure (trachea, bronchi, bronchioles): The branching network of airways efficiently delivers fresh air from the mouth and nose all the way to the alveoli, ensuring that every alveolus is supplied with air during each breath.
- Large surface area as a general feature: The three key features of any gas exchange surface (large surface area, thin surface, good blood and air supply) are all present in the human system, together maximising the rate at which gases can cross.
