Diffusion and active transport — final check key

1. DK1 · Final check

Model answer. (a) Surface area of Q = 6 × 16 cm² = 96 cm²; volume = 64 cm³; ratio = 96 ÷ 64 = 1.5 per cm. (b) Cube P clears first: its centre is only 0.5 cm from the surface (Q’s is 2 cm), and it has more surface for each cm³ (6 per cm against 1.5 per cm).

Checked automatically

• (a) Surface area : volume of Q: 1.5 cm⁻¹ (within ±2%)
  • Common wrong answer 0.67 : That is volume ÷ surface area. Divide the surface area by the volume.
  • Common wrong answer 96 : That is the surface area. Now divide it by the volume, 64 cm³.

A full answer includes

• (a) 96 cm² ÷ 64 cm³ = 1.5 per cm.
• (b) P clears first: the distance from its surface to its centre is shorter, and it has a larger surface area to volume ratio (6 per cm, against 1.5 per cm).

Also accept

• (a) 1.5 per cm; (b) P: shorter distance and larger SA:V

Watch for

• “Q clears first because it has more surface.” — Q has more surface in total, but less for each cm³, and its centre is four times further away.

2. DK2 · Final check

Model answer. (a) As the temperature rises, the cubes clear in less time (18, 12, then 8 minutes). (b) At a higher temperature the acid particles have more kinetic energy, so they move faster and diffusion into the agar is faster. (c) Any two of: the size of the cubes, the concentration of the acid, the volume of acid, the same batch of agar.

A full answer includes

• (a) The higher the temperature, the shorter the time to clear.
• (b) At a higher temperature the particles have more kinetic energy and move faster, so diffusion is faster.
• (c) Any two: cube size; acid concentration; volume of acid; the same agar (same indicator and alkali).

Also accept

• (a) higher temperature, shorter time; (b) more kinetic energy, faster particles, faster diffusion; (c) two of size, concentration, volume, agar

Watch for

• “Heat makes the agar melt, so the acid gets in.” — The agar stays solid at these temperatures; the particles simply move faster.

3. GK1 · Final check

Model answer. (a) Into the leaf: the cells use oxygen in respiration, so its concentration inside falls below that in the air. (b) Out of the leaf. (c) Respiration makes carbon dioxide and, in the dark, photosynthesis does not use it, so its concentration inside the leaf becomes higher than outside; it diffuses out through the stomata down its concentration gradient.

A full answer includes

• (a) Oxygen diffuses into the leaf.
• (b) Carbon dioxide diffuses out of the leaf.
• (c) Respiring cells make carbon dioxide, so its concentration inside the leaf is higher than in the air; it diffuses out down its concentration gradient.

Also accept

• (a) in; (b) out; (c) made in respiration, not used in the dark: higher inside, diffuses out down the gradient

Watch for

• “Leaves always take in carbon dioxide.” — Only when photosynthesis uses it up faster than respiration makes it. In the dark, carbon dioxide builds up inside and diffuses out.
• “The leaf pushes carbon dioxide out.” — No energy from respiration is needed: carbon dioxide diffuses down its concentration gradient.

4. AK1 · Final check

Model answer. (a) Active transport: the iodide ions move into the cells against their concentration gradient. (b) Carrier proteins in the membrane bind iodide ions on the outside, change shape using energy released in respiration, and release the ions inside the cell. (c) Aerobic respiration needs oxygen; with too little oxygen the cells release less energy, so less energy is available for active transport and fewer ions are moved in.

A full answer includes

• (a) Active transport.
• (b) A carrier protein binds an iodide ion on the outside, changes shape using energy and releases the ion inside the cell.
• (c) With less oxygen, aerobic respiration slows, so less energy is released for active transport and fewer ions are taken in.

Also accept

• (a) active transport; (b) carriers bind, change shape using energy, release inside; (c) less aerobic respiration, less energy

Watch for

• “Iodide ions diffuse into the seaweed.” — Diffusion would move them out, from the higher concentration in the cells to the sea water.
• “Mitochondria make energy.” — Energy cannot be made: respiration releases energy stored in glucose.