Active transport — answer key

1. AQ1 · Practice

Model answer. d. Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy released during respiration.

A full answer includes

• I chose d.

Also accept

• d

Watch for

• “Active transport is just fast diffusion.” — It moves particles the other way, against the gradient, and needs energy from respiration.

2. AQ2 · Practice

Options as numbered on the pupil sheet: 1 — Diffusion; 2 — Active transport; 3 — Both.

Model answer. Needs energy from respiration: active transport. Moves particles down a concentration gradient: diffusion. Moves particles through the cell membrane: both. Needs carrier proteins: active transport.

A full answer includes

• Needs energy from respiration → active transport.
• Moves particles down a concentration gradient → diffusion.
• Moves particles through the cell membrane → both.
• Needs carrier proteins → active transport.

Also accept

• energy–active transport; down the gradient–diffusion; through the membrane–both; carrier proteins–active transport

Watch for

• “Diffusion needs energy too.” — Diffusion uses only the kinetic energy of the particles; the cell supplies no energy.

3. AQ3 · Practice

Model answer. By active transport: protein carriers in the membrane move nitrate ions into the cell against the concentration gradient, using energy released in respiration. The plant needs the nitrate ions to make proteins, and they are scarce in the soil water.

A full answer includes

• I said they take in nitrate ions by active transport, against the concentration gradient.
• I said this uses energy from respiration.
• I said the plant needs nitrate ions to make proteins.

Also accept

• active transport against the gradient; energy from respiration; nitrate needed to make proteins

Watch for

• “Nitrate ions diffuse into the root.” — Diffusion would move them out, from the higher concentration inside the cell to the lower concentration in the soil.

4. AQ4 · Practice

Options as numbered on the pupil sheet: 1 — The ion is released inside the cell; 2 — The ion binds to the carrier on the outside; 3 — The carrier returns to its first shape; 4 — The carrier changes shape using energy from respiration.

Model answer. First the ion binds to the carrier protein on the outside of the membrane. Then the carrier changes shape, using energy released in respiration. Then it releases the ion inside the cell. Finally the carrier returns to its first shape, ready for the next ion.

A full answer includes

• The ion binds to the carrier on the outside → the carrier changes shape using energy from respiration → the ion is released inside the cell → the carrier returns to its first shape.

Also accept

• binds; changes shape using energy; releases inside; returns

Watch for

• “The carrier changes shape first, then the ion binds.” — The ion binds first; the change of shape moves it across.

5. AT1 · Apply

Model answer. (a) The uptake of nitrate ions stops, because they are taken in by active transport, which needs energy released in respiration to change the shape of the carrier proteins. (b) Oxygen still diffuses in while there is a concentration gradient: diffusion uses the kinetic energy of the particles, not energy from respiration. (Because the poisoned cells use less oxygen, the gradient becomes less steep, so less oxygen enters.)

A full answer includes

• (a) Nitrate uptake stops (or almost stops): active transport needs energy from respiration, and the carrier proteins cannot change shape without it.
• (b) Oxygen still diffuses in: diffusion needs no energy from respiration, only a concentration gradient (the gradient may become less steep as the cells use less oxygen).

Also accept

• (a) stops — active transport needs energy from respiration (carriers); (b) continues — diffusion needs no energy from respiration

Watch for

• “Everything entering the cell stops.” — Only active transport needs energy from respiration. Diffusion carries on while there is a concentration gradient.