Diffusion and cell size — answer key

1. DQ1 · Practice

Model answer. b. Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement.

A full answer includes

• I chose b.

Also accept

• b

Watch for

• “Particles diffuse because they want to spread out.” — Particles have no wants. They move at random; more happen to move away from where they are crowded than move back.
• “Diffusion needs energy from the cell.” — The energy is the kinetic energy of the moving particles themselves; the cell supplies none.

2. DQ2 · Practice

Model answer. (a) Surface area = 6 × (3 cm × 3 cm) = 54 cm². (b) Volume = 3 × 3 × 3 = 27 cm³. (c) Surface area : volume = 54 ÷ 27 = 2 per cm (2 cm² of surface for each cm³).

Checked automatically

• (a) Surface area: 54 cm² (within ±2%)
  • Common wrong answer 9 : That is one face. A cube has six faces.
• (b) Volume: 27 cm³ (within ±2%)
  • Common wrong answer 9 : That is the area of one face. Volume is side × side × side.
• (c) Surface area : volume: 2 cm⁻¹ (within ±2%)
  • Common wrong answer 0.5 : That is volume ÷ surface area. Divide the surface area by the volume.

A full answer includes

• (a) 6 × 9 cm² = 54 cm².
• (b) 27 cm³.
• (c) 54 ÷ 27 = 2 per cm.

Also accept

• (a) 54 cm²; (b) 27 cm³; (c) 2 per cm

Watch for

• “The surface area is 9 cm².” — That is one face. A cube has 6 faces: multiply by 6.

3. DQ3 · Practice

Model answer. In the same time the acid diffused the same distance into every cube. The centre of the 0.5 cm cube is only 0.25 cm from the surface, so the colourless layers from opposite faces met and the cube cleared. The centre of the 2 cm cube is 1 cm from the surface, so most of it was still pink.

A full answer includes

• I said the acid travelled about the same distance into each cube in the same time.
• I said the centre of the small cube is only 0.25 cm from the surface, so the border reached it; the centre of the big cube is 1 cm in, so the border had not reached it.

Also accept

• the same distance in the same time; the small cube’s centre is close to the surface (0.25 cm), the big cube’s far (1 cm).

Watch for

• “The acid soaked into the small cube faster.” — The border was the same width on every cube: the acid moved at the same rate. The small cube cleared because there was less distance to go.

4. DQ4 · Practice

Model answer. Warmer acid: the particles have more kinetic energy and move faster, so diffusion is faster. More concentrated acid: a steeper concentration gradient, so more particles cross in each second. A smaller cube does not make the particles move faster: the acid has a shorter distance to go, and there is more surface for each cm³, so the cube clears sooner. Colder acid slows diffusion.

A full answer includes

• I chose warmer acid, more concentrated acid and a smaller cube.
• I can say why: warmer acid — particles move faster; stronger acid — steeper concentration gradient; smaller cube — less distance to the centre (the particles do not move faster).

Also accept

• warmer acid; more concentrated acid; smaller cube (shorter distance, not faster particles)

Watch for

• “A smaller cube makes the acid particles move faster.” — The particles move just as fast; in a small cube they have less distance to go to reach the centre.
• “A bigger cube clears sooner because it has more surface.” — It has more surface in total but much more volume, and its centre is further away.

5. DQ5 · Practice

Model answer. Yes. The particles keep moving at random because they have kinetic energy. Once the concentration is the same everywhere, as many particles move one way as the other, so there is no net movement and the colour stays even.

A full answer includes

• I said yes, they keep moving at random.
• I said there is no net movement now: as many move one way as the other.

Also accept

• yes; they still move at random but there is no net movement because the concentration is even.

Watch for

• “Diffusion has stopped, so the particles have stopped.” — The particles never stop moving; only the net movement stops.

6. DT1 · Apply

Model answer. An amoeba is so small that every part of it is close to the surface, and it has a large surface area for its volume, so oxygen diffusing through its surface reaches all of it fast enough. A mouse has a much smaller surface area to volume ratio and its inner cells are far from the skin, so diffusion alone would be far too slow; it needs lungs and blood.

A full answer includes

• I said the amoeba is tiny, so oxygen has only a short distance to diffuse to its centre.
• I said a small organism has a large surface area to volume ratio, so its surface can supply all of its volume.
• I said a mouse has a much smaller ratio and its centre is far from the surface, so diffusion from its skin would be too slow.

Also accept

• short distance and large SA:V for the amoeba; small SA:V and long distance for the mouse, so diffusion is too slow.

Watch for

• “A mouse has more surface, so it gets enough oxygen.” — It has more surface in total, but much less for each cm³ of body, and its inner cells are far from the skin.