Diffusion and active transport — return questions key

1. DR1 · Return

Model answer. Diffusion is the net movement of particles from a higher concentration to a lower concentration, as a result of their random movement. It is faster at a higher temperature.

A full answer includes

• net; higher; lower; random; higher.

Also accept

• net; higher; lower; random; higher (warmer)

Watch for

• “Diffusion goes from low to high concentration.” — That is active transport. Diffusion goes down the gradient, from high to low.

2. DR2 · Return

Model answer. Surface area = 6 × 0.25 cm² = 1.5 cm². Volume = 0.125 cm³. Surface area : volume = 1.5 ÷ 0.125 = 12 cm⁻¹ (12 cm² of surface for each cm³).

Checked automatically

• Surface area : volume: 12 cm⁻¹ (within ±2%)
  • Common wrong answer 0.083 : That is volume ÷ surface area. Divide the surface area by the volume.
  • Common wrong answer 1.5 : That is the surface area. Now divide it by the volume, 0.125 cm³.

A full answer includes

• Surface area 1.5 cm², volume 0.125 cm³.
• Ratio 1.5 ÷ 0.125 = 12 cm⁻¹.

Also accept

• 12 cm⁻¹

Watch for

• “Working in mm and cm together.” — Use the same unit throughout: 0.5 cm for every side.

3. GR1 · Return

Model answer. Substances move into and out of cells by diffusion through the cell membrane. In a leaf, carbon dioxide diffuses in through pores called stomata. Many substances can diffuse in living things because they dissolve in water, which is a good solvent.

A full answer includes

• membrane; stomata; solvent.

Also accept

• membrane; stomata; solvent

Watch for

• “Carbon dioxide enters a leaf through its veins.” — Gases enter through the stomata and diffuse through the air spaces.

4. AR1 · Return

Model answer. Active transport moves particles against a concentration gradient. It uses energy released in respiration. The particles are carried through the membrane by protein carriers.

A full answer includes

• against; respiration; carriers.

Also accept

• against; respiration; carriers

Watch for

• “Active transport moves particles down a gradient.” — That is diffusion. Active transport moves them against the gradient.

5. AR2 · Return

Model answer. The concentration of nitrate ions is often higher inside root hair cells than in the soil water, so diffusion would carry them out. The cells use active transport to move them in against the gradient, using energy from respiration. The plant needs nitrate ions to make proteins.

A full answer includes

• Nitrate ions are often more concentrated inside the root hair cell than in the soil water.
• So they must be moved against the concentration gradient, which needs active transport (energy from respiration); the plant needs nitrate to make proteins.

Also accept

• higher inside than in the soil, so must move against the gradient by active transport; needed for proteins

Watch for

• “Nitrate ions diffuse into roots.” — Only when they are more concentrated in the soil. Usually they must be moved in against the gradient.