Diffusion in living things — answer key

1. GQ1 · Practice

Model answer. c. Oxygen diffuses through the cell membrane, from the higher concentration outside to the lower concentration inside, where it is used up in respiration.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “The cell membrane pumps oxygen in.” — No energy from respiration is needed: oxygen moves by diffusion, using the kinetic energy of its own particles, down its concentration gradient.

2. GQ2 · Practice

Options as numbered on the pupil sheet: 1 — From the air in the lungs into the blood; 2 — From the air into a leaf through the stomata; 3 — From the small intestine into the blood; 4 — From cells into the blood.

Model answer. Oxygen diffuses from the air in the air sacs of the lungs into the blood. Carbon dioxide diffuses from the air into a leaf through its stomata. Digested glucose diffuses from the small intestine into the blood. Urea diffuses out of cells into the blood.

A full answer includes

• Oxygen → from the air in the lungs into the blood.
• Carbon dioxide → from the air into a leaf through the stomata.
• Glucose → from the small intestine into the blood.
• Urea → from cells into the blood.

Also accept

• oxygen–lungs to blood; carbon dioxide–air to leaf; glucose–gut to blood; urea–cells to blood

Watch for

• “Carbon dioxide diffuses out of a leaf in sunlight.” — In sunlight the leaf uses carbon dioxide for photosynthesis, so its concentration inside is lower and it diffuses in.

3. GQ3 · Practice

Model answer. A thin wall means oxygen and carbon dioxide have only a short distance to diffuse between the air and the blood, so diffusion is fast.

A full answer includes

• I said the gases have only a short distance to diffuse.
• I said this makes diffusion faster.

Also accept

• short diffusion distance, so faster diffusion

Watch for

• “A thin wall lets more air in.” — The wall does not let air flow through; it shortens the distance gases diffuse across.

4. GQ4 · Practice

Model answer. The flowing blood carries oxygenated blood away and brings blood with little oxygen. So the oxygen concentration in the blood next to the air sac stays lower than in the air: the concentration gradient stays steep and oxygen keeps diffusing in.

A full answer includes

• I said the blood arriving has a low concentration of oxygen.
• I said this keeps the concentration gradient steep, so oxygen keeps diffusing in.

Also accept

• keeps the concentration gradient steep (low oxygen in the blood arriving)

Watch for

• “Blood flow pushes oxygen in.” — Blood flow does not push the oxygen; it keeps the gradient steep so diffusion continues.

5. GQ5 · Practice

Model answer. Glucose diffuses from the small intestine into the blood, from its higher concentration to its lower concentration. It can do this because it is dissolved in water: water is the solvent, glucose is a solute, and dissolved particles can move about and diffuse. The blood plasma then carries the dissolved glucose round the body.

A full answer includes

• I said glucose diffuses from the small intestine into the blood, down its concentration gradient.
• I said glucose is dissolved in water (water is the solvent), so its particles can move and diffuse; the blood then carries it in solution.

Also accept

• diffusion down the gradient into the blood; dissolved in water (solvent), so it can diffuse and be carried in the blood

Watch for

• “Glucose can only enter the blood by active transport.” — While glucose is more concentrated in the gut, it also moves down its concentration gradient by diffusion; active transport can add to the uptake.

6. GT1 · Apply

Model answer. Gills need a large surface area (many thin filaments), thin walls so the diffusion distance is short, and a steep concentration gradient, kept by water bringing fresh oxygen and blood carrying oxygen away.

A full answer includes

• Any two: a large surface area (many thin filaments); thin walls (short distance); a steep concentration gradient kept by the flow of water and blood.

Also accept

• two of: large surface area; thin walls / short distance; steep gradient kept by flow

Watch for

• “Gills need a thick wall to protect the blood.” — A thick wall would make the diffusion distance longer and slow the uptake of oxygen.