These eight original questions follow the gas exchange section. Write your own answer first, then open the model answer.
Questions
1. State three features of a gas exchange surface and give the reason for each.
Answer
Large surface area: more gases can cross at the same time. Thin wall: a shorter diffusion distance. Moist lining: gases dissolve before they diffuse. A rich blood supply also keeps the concentration gradient steep.
2. A cube has a side of 3 cm. Calculate its surface area to volume ratio and compare it with a cube of side 1 cm.
Answer
Side 3 cm: surface area = 6 × 9 = 54 cm², volume = 27 cm³, ratio = 2. Side 1 cm: ratio = 6.
The larger cube has a smaller ratio, so larger organisms need specialised surfaces.
3. Explain how air enters the lungs during inhalation.
Answer
The intercostal muscles and diaphragm contract, so the ribs move up and out and the diaphragm flattens. The volume of the thorax increases.
The air pressure in the lungs falls below atmospheric pressure, so air flows in.
4. Explain why oxygen diffuses from an alveolus into the blood.
Answer
The concentration of oxygen in the air of the alveolus is higher than in the blood arriving at the lungs, so oxygen diffuses down the gradient.
Blood flow removes oxygen and breathing brings fresh air, so the gradient stays steep.
5. Why does an earthworm die if it dries out?
Answer
An earthworm exchanges gases through its skin. Gases must dissolve in a moist film before they diffuse, so a dry skin slows oxygen uptake until the cells cannot respire enough.
6. State one similarity and one difference between a fish gill and a human alveolus.
Answer
Similarity: both are thin, moist surfaces with a rich blood supply.
Difference: a gill takes dissolved oxygen from water flowing over it, whereas an alveolus takes oxygen from air moved in by breathing.
7. Exhaled air has 16.0% oxygen and 4.2% carbon dioxide. Inhaled air has 20.9% and 0.04%. Calculate the oxygen used and the carbon dioxide gained.
Answer
Oxygen used = 20.9 − 16.0 = 4.9 percentage points. Carbon dioxide gained = 4.2 − 0.04 = 4.16 percentage points.
8. A student says exhaled air contains no oxygen. Use the data in question 7 to correct this.
Answer
Exhaled air contains 16.0% oxygen, which is most of the amount in inhaled air. The body takes up only part of the oxygen in each breath.
If you got these wrong
For questions 1, 2, 4 and 5, revisit relating respiratory surfaces to diffusion. For question 3, see explaining ventilation in humans.
For question 6, see comparing gas exchange in selected organisms. For questions 7 and 8, see interpreting oxygen and carbon-dioxide data.
Log each slip in the mistake log and paper-error review, then build a timed set with the timed original practice session builder.
For a teacher to go through your answers, see online one-to-one Biology tuition.