Osmosis — return questions key

1. WR1 · Return

Model answer. c. A partially permeable membrane lets small molecules such as water through easily but holds back some larger molecules, such as sucrose.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “A partially permeable membrane lets everything through slowly.” — It lets some molecules through easily (such as water) and holds others back (such as sucrose).

2. WR2 · Return

Model answer. Any two: digested food molecules such as glucose dissolve in water in the gut so they can be absorbed; urea is removed from the body dissolved in water as urine; blood plasma carries dissolved substances such as glucose, amino acids and carbon dioxide around the body.

A full answer includes

• I described one job, such as dissolving digested food (glucose, amino acids) so it can be absorbed.
• I described a second, different job, such as carrying urea out in urine or carrying dissolved substances in blood plasma.

Also accept

• two of: digestion (food dissolved and absorbed), excretion (urea in urine), transport (substances dissolved in plasma)

Watch for

• “Water carries oxygen dissolved in plasma.” — Most oxygen is carried by red blood cells; choose glucose, amino acids, carbon dioxide or urea as dissolved examples.

3. WR3 · Return

Model answer. The level in the capillary tube falls. The water in the bag has a higher water potential than the sugar solution outside, so there is a net movement of water out of the bag by osmosis through the partially permeable tubing.

A full answer includes

• I predicted that the level falls.
• I said water leaves the bag by osmosis into the sugar solution, which has a lower water potential.

Also accept

• the level falls: water leaves the bag by osmosis

Watch for

• “The level rises because sugar enters the bag.” — Very little sugar crosses in this time; water leaves the bag, so the level falls.

4. PR1 · Return

Model answer. A plant cell in water takes in water by osmosis and becomes turgid. The pressure of the contents on the wall, turgor pressure, helps support the plant. The cell does not burst because of the strong cell wall.

A full answer includes

• g1: osmosis.
• g2: turgid.
• g3: turgor pressure.
• g4: cell wall.

Also accept

• osmosis; turgid; turgor pressure; cell wall

Watch for

• “The membrane stops the cell bursting.” — The membrane is thin; it is the strong cell wall that stops a plant cell bursting.

5. PR2 · Return

Model answer. (1.68 − 1.50) ÷ 1.50 × 100 = 0.18 ÷ 1.50 × 100 = +12 %.

Checked automatically

• Percentage change in mass: 12 % (within ±0.2 %)
  • Common wrong answer 0.18 : Divide the change by 1.50 g and multiply by 100.
  • Common wrong answer 10.7 : Divide by the starting mass, 1.50 g, not the final mass.

A full answer includes

• (1.68 − 1.50) ÷ 1.50 × 100 = +12 %.

Also accept

• +12 %

Watch for

• “The change is 0.18 %.” — That is the change in grams: divide by 1.50 g and multiply by 100.

6. PR3 · Return

Model answer. In a flaccid cell the contents have lost some water and no longer press on the cell wall, but they still touch it. A plasmolysed cell has lost more water: its cytoplasm and cell membrane have pulled away from the cell wall.

A full answer includes

• I said a flaccid cell’s contents still touch the wall but no longer press on it.
• I said in a plasmolysed cell the cytoplasm and membrane have pulled away from the wall, after more water loss.

Also accept

• flaccid: contents touch but do not press on the wall; plasmolysed: contents pulled away from the wall

Watch for

• “Flaccid and plasmolysed mean the same.” — Flaccid comes first (no pressure on the wall); plasmolysis is later, when the contents pull away.

7. UR1 · Return

Options as numbered on the pupil sheet: 1 — Water in the soil around the root hairs; 2 — A root hair cell, by osmosis; 3 — Cells across the root, by osmosis from cell to cell; 4 — The tubes that carry water up the plant.

Model answer. Water in the soil enters a root hair cell by osmosis, passes by osmosis from cell to cell across the root, and reaches the tubes that carry it up the plant.

A full answer includes

• Soil water → root hair cell → cells across the root → tubes that carry water up the plant.

Also accept

• soil water, root hair cell, cells across the root, tubes up the plant

Watch for

• “Water enters the plant through the leaves.” — Almost all water enters through the root hair cells.

8. UR2 · Return

Model answer. Pure water has a higher water potential than the inside of a red blood cell, so water enters by osmosis. The cell swells and bursts, because it has no cell wall.

A full answer includes

• g1: higher.
• g2: osmosis.
• g3: burst.
• g4: cell wall.

Also accept

• higher; osmosis; burst; cell wall

Watch for

• “Red blood cells go turgid in water.” — Without a cell wall nothing pushes back, so they burst.