Water and membranes — answer key

1. WQ1 · Practice

Model answer. b. Osmosis is the net movement of water molecules through a partially permeable membrane, from a dilute solution (higher water potential) to a more concentrated solution (lower water potential).

A full answer includes

• I chose b.

Also accept

• b

Watch for

• “Osmosis is the movement of sugar through a membrane.” — Osmosis is the net movement of water. In these examples the membrane lets water through but holds the sugar back.
• “Water moves towards where there is more water.” — Water moves from the dilute solution, where there is more water, into the concentrated solution.

2. WQ2 · Practice

Options as numbered on the pupil sheet: 1 — Digestion; 2 — Excretion; 3 — Transport.

Model answer. Glucose dissolving in the gut before absorption: digestion. Urea dissolved in urine: excretion. Carbon dioxide in plasma and mineral ions carried up a plant: transport.

A full answer includes

• Glucose dissolving in the gut before it is absorbed: digestion.
• Urea dissolved in urine: excretion.
• Carbon dioxide carried in blood plasma: transport.
• Mineral ions carried up from the roots: transport.

Also accept

• glucose: digestion; urea: excretion; carbon dioxide: transport; mineral ions: transport

Watch for

• “Water is only a filler in cells.” — Water dissolves the substances that are digested, excreted and transported; without it none of these could happen.

3. WQ3 · Practice

Model answer. Osmosis is the net movement of water molecules from a dilute solution (higher water potential) to a concentrated solution (lower water potential) through a partially permeable membrane. The sugar molecules cannot cross this membrane.

A full answer includes

• g0: net.
• g1: dilute.
• g2: concentrated.
• g3: partially permeable.
• g4: cannot cross (are too large to pass through this membrane).

Also accept

• net; dilute; concentrated; partially permeable; cannot cross

Watch for

• “Water only moves one way in osmosis.” — Water molecules cross both ways; more cross from the dilute side, so the net movement is towards the concentrated solution.
• “The sugar molecules move across to even out the concentrations.” — This membrane holds the sugar back; only water crosses.

4. WQ4 · Practice

Model answer. (a) The water outside has a higher water potential than the sugar solution inside, so there was a net movement of water into bag A by osmosis through the partially permeable tubing; the bag gained mass. (b) The sugar solution outside has a lower water potential than the water inside, so water left bag B by osmosis and it lost mass. (c) A control: with no difference in concentration there is no net movement of water, so any change in A and B is due to osmosis, not the tubing or handling.

A full answer includes

• (a) Water moved into bag A by osmosis, from the water outside (higher water potential, dilute) to the sugar solution inside (lower water potential).
• (b) Water moved out of bag B by osmosis into the more concentrated sugar solution outside.
• (c) It is a control: it shows that the change is caused by the difference in concentration, not by the tubing or by handling.

Also accept

• (a) water in by osmosis from higher to lower water potential; (b) water out by osmosis; (c) a control

Watch for

• “Sugar moved out of bag A.” — The bag gained mass: water moved in. Sugar molecules cross dialysis tubing only very slowly.
• “Bag B lost mass because water evaporated.” — Bag A in the same conditions gained mass; the difference is the direction of the water potential gradient.

5. WQ5 · Practice

Model answer. Bag B: (9.1 − 10.0) ÷ 10.0 × 100 = −9 % (a loss). Bag C: (13.5 − 12.0) ÷ 12.0 × 100 = 1.5 ÷ 12.0 × 100 = +12.5 %.

Checked automatically

• Bag B: -9 % (within ±0.5 %)
  • Common wrong answer 9 : Bag B lost mass: the change is negative.
  • Common wrong answer -0.9 : Multiply by 100 to get a percentage.
• Bag C: 12.5 % (within ±0.3 %)
  • Common wrong answer 1.5 : Divide by the starting mass, 12.0 g, and multiply by 100.
  • Common wrong answer 11.1 : Divide by the starting mass (12.0 g), not the final mass.

A full answer includes

• Bag B: (9.1 − 10.0) ÷ 10.0 × 100 = −9 %.
• Bag C: (13.5 − 12.0) ÷ 12.0 × 100 = +12.5 %.

Also accept

• B −9 %; C +12.5 %

Watch for

• “The change for bag C is 1.5 %.” — That is the change in grams. Divide it by the starting mass (12.0 g), then multiply by 100.
• “A loss can be written as a positive percentage.” — Show a loss with a minus sign: −9 %.

6. WQ6 · Practice

Model answer. Nitrate ions dissolve in the water in the soil. The plant takes in this water with the ions dissolved in it, and water carries the dissolved ions up from the roots to the leaves. Without water as a solvent the ions could not be carried.

A full answer includes

• I said the nitrate ions are dissolved in the water in the soil.
• I said the water, with the ions dissolved in it, is taken into the roots and carried up the plant to the leaves.

Also accept

• ions dissolved in soil water; the water carries them up to the leaves

Watch for

• “The ions are carried up on their own, like a gas.” — The ions travel dissolved in water; water is the solvent that transports them.

7. WT1 · Apply

Model answer. The salt dissolves in the moisture on the cut surface, making a very concentrated solution with a low water potential. The cucumber cells have a higher water potential, so water moves out of them by osmosis through their partially permeable cell membranes. That water collects on the plate, and the cells that lost water make the slices limp.

A full answer includes

• I said the salt makes a concentrated solution on the surface, with a lower water potential than the cells.
• I said water left the cucumber cells by osmosis through their partially permeable membranes, forming the liquid.

Also accept

• water left the cells by osmosis into the concentrated salt solution (lower water potential)

Watch for

• “The salt pulled the juice out by suction.” — There is no suction: water moves by osmosis from the higher water potential inside the cells to the lower water potential outside.

8. WT2 · Apply

Model answer. Urea is excreted dissolved in water: water is the solvent that carries it out of the body in urine. However concentrated the urine is, some water must leave with the urea, so the kangaroo rat always loses some water when it excretes.

A full answer includes

• I said urea is removed from the body dissolved in water (urine).
• I said water is the solvent, so some water must leave with the urea, however concentrated the urine.

Also accept

• urea is carried out dissolved in water, so water must leave with it

Watch for

• “The kangaroo rat loses water because it is hot.” — Heat explains evaporation; the question is about excretion: urea has to be dissolved in water to be removed.