Osmosis — final check key

1. WK1 · Final check

Model answer. d. Y is more concentrated (lower water potential), so there is a net movement of water from X into Y by osmosis; the sugar cannot cross.

A full answer includes

• I chose d.

Also accept

• d

Watch for

• “Sugar moves from Y to X.” — The sugar molecules cannot cross the partially permeable membrane.
• “Nothing happens because both are sugar solutions.” — Their concentrations differ, so their water potentials differ and water moves.

2. WK2 · Final check

Model answer. (a) The bag loses mass. (b) The 30 % solution outside has a lower water potential than the 10 % solution inside, so there is a net movement of water out of the bag by osmosis through the partially permeable tubing. (c) Any one: the starting volume in the bag, the volume of solution in the beaker, the temperature, the time, the size of the bag.

A full answer includes

• (a) The bag loses mass.
• (b) The solution outside is more concentrated (lower water potential), so water leaves the bag by osmosis through the partially permeable tubing.
• (c) For example: the volume of solution, the temperature, the time, the size of the bag.

Also accept

• (a) loses mass; (b) water leaves by osmosis to the lower water potential outside; (c) one controlled variable

Watch for

• “The bag gains mass because both contain sugar.” — Compare the concentrations: water moves towards the more concentrated solution, which is outside.

3. WK3 · Final check

Model answer. (a) Digestion: small food molecules such as glucose or amino acids dissolve in the water in the gut, then are absorbed into the blood. (b) Excretion: urea dissolves in water and leaves the body in urine. (c) Transport: substances such as glucose, amino acids, mineral ions and carbon dioxide are carried around the body dissolved in blood plasma.

A full answer includes

• (a) For example glucose or amino acids dissolve in the gut and are absorbed into the blood.
• (b) Urea dissolves in water and is removed from the body in urine.
• (c) For example glucose, amino acids, mineral ions or carbon dioxide are carried around the body dissolved in plasma.

Also accept

• (a) dissolved food absorbed; (b) urea removed in urine; (c) substances carried in plasma

Watch for

• “Water carries red blood cells, so they are dissolved.” — Cells are carried in plasma but are not dissolved; name a dissolved substance such as glucose.

4. PK1 · Final check

Model answer. The change goes from +2.1 % to −4.9 % between 0.2 and 0.4 mol/dm³, so 0 % lies 2.1 ÷ 7.0 of the way from 0.2: 0.2 + 0.2 × 0.3 = 0.26 mol/dm³.

Checked automatically

• Concentration with no net movement of water: 0.26 mol/dm³ (within ±0.03 mol/dm³)
  • Common wrong answer 0.4 : At 0.4 mol/dm³ the carrot lost mass; look between 0.2 and 0.4.
  • Common wrong answer 0.3 : Closer: the gain at 0.2 (+2.1 %) is smaller than the loss at 0.4 (−4.9 %), so zero is nearer 0.2.

A full answer includes

• Between 0.2 and 0.4 mol/dm³, nearer 0.2: about 0.26 mol/dm³ (accept 0.23–0.29).

Also accept

• about 0.26 mol/dm³

Watch for

• “No net movement happens at 0.4 mol/dm³.” — At 0.4 the carrot lost mass. The zero point is between the last gain and the first loss.

5. PK2 · Final check

Model answer. (a) A: turgid; B: flaccid; C: plasmolysed. (b) Most dilute A, then B, then C most concentrated. (c) Solution C was more concentrated, with a lower water potential than the cell sap, so there was a net movement of water out of the cells by osmosis. The contents shrank, stopped pressing on the walls and then pulled away from them (plasmolysis); the walls kept their shape.

A full answer includes

• (a) A turgid, B flaccid, C plasmolysed.
• (b) A, B, C.
• (c) Solution C had a lower water potential than the cell sap, so the cells lost water by osmosis until the contents pulled away from the walls.

Also accept

• (a) turgid, flaccid, plasmolysed; (b) A, B, C; (c) water left by osmosis until the contents pulled away

Watch for

• “B is plasmolysed because the cells have lost water.” — In B the contents still touch the walls: the cells are flaccid. Plasmolysis is when they pull away, as in C.

6. UK1 · Final check

Model answer. Soil X: the soil water is more dilute than the cell sap, so it has a higher water potential; water moves into the root hair cells by osmosis and the plant can take up water (its cells stay turgid). Soil Y: the soil water is more concentrated than the cell sap, so it has a lower water potential; water moves out of the root hair cells by osmosis, the plant cannot take up water, its cells become flaccid and it wilts.

A full answer includes

• Soil X: net movement of water from the soil into the root hair cells (the soil water has the higher water potential); the plant takes up water and its cells stay turgid.
• Soil Y: net movement of water out of the root hair cells into the soil (the soil water has the lower water potential); the plant loses water, its cells become flaccid and it wilts.

Also accept

• X: water into the roots, plant takes up water; Y: water out of the roots, plant wilts

Watch for

• “In soil Y the plant takes up more water because there is more dissolved in it.” — More solute means a lower water potential: water leaves the roots.

7. UK2 · Final check

Model answer. (a) Tube 1: the solution with the same water potential as plasma (no net movement of water). Tube 2: pure water. Tube 3: strong salt solution (water left the cells, so they shrank and crinkled). (b) Pure water has a higher water potential than the inside of the red blood cells, so water entered them by osmosis. Red blood cells have no cell wall, so they swelled until they burst, leaving only torn membranes.

A full answer includes

• (a) Tube 2: pure water; tube 3: strong salt solution; tube 1: same water potential as plasma.
• (b) Water entered the cells by osmosis because the water had a higher water potential; with no cell wall they swelled and burst.

Also accept

• (a) 1 same as plasma, 2 pure water, 3 strong salt; (b) water entered by osmosis, no wall, so they burst

Watch for

• “The cells in tube 2 dissolved in the water.” — Cells do not dissolve; they took in water by osmosis and burst.