Water for whole organisms — answer key

1. UQ1 · Practice

Model answer. a. The soil water has a higher water potential than the cell sap, so water moves into the root hair cell by osmosis through its partially permeable membrane.

A full answer includes

• I chose a.

Also accept

• a

Watch for

• “The root sucks water in.” — There is no suction at the root hair: water moves in by osmosis down a water potential gradient.

2. UQ2 · Practice

Options as numbered on the pupil sheet: 1 — The cells swell and many burst; 2 — The cells keep their normal shape; 3 — The cells shrink and their edges crinkle.

Model answer. Pure water: water enters by osmosis, so the cells swell and many burst. Salt solution as concentrated as plasma: as much water enters as leaves, so they keep their shape. Strong salt solution: water leaves by osmosis, so they shrink and crinkle.

A full answer includes

• Pure water: they swell and many burst.
• Salt solution as concentrated as plasma: no change in shape.
• Strong salt solution: they shrink and their edges crinkle.

Also accept

• water: burst; plasma-strength salt: no change; strong salt: shrink

Watch for

• “Red blood cells become turgid in water.” — They have no cell wall to push back, so they swell and burst.

3. UQ3 · Practice

Model answer. The dissolved fertiliser makes the soil water very concentrated, so its water potential is lower than that of the cell sap in the root hair cells. Water now moves out of the root hair cells by osmosis instead of in. The plant cannot take up water, its cells lose water and become flaccid, and the plant wilts even though the soil is wet.

A full answer includes

• I said the fertiliser makes the soil water concentrated, so its water potential is lower than in the root hair cells.
• I said water then leaves the root hair cells by osmosis (or cannot enter), so the plant cells lose water, become flaccid and the plant wilts.

Also accept

• soil water now has a lower water potential than the roots, so water leaves the roots by osmosis

Watch for

• “The fertiliser poisons the plant.” — The wilting is explained by water potential: water leaves the roots by osmosis.

4. UQ4 · Practice

Model answer. (a) On a hot, windy day the leaves lose water faster than the roots take it in. The cells lose water and become flaccid: they no longer press on their cell walls, so the leaves and stems are not supported and the plant wilts. (b) After watering, water enters the root hair cells from the soil by osmosis and passes to the other cells; they take in water, become turgid and press outwards on their walls, so the plant is supported again.

A full answer includes

• (a) The leaves lose water faster than the roots take it in, so the cells lose water, become flaccid and can no longer support the leaves and stems.
• (b) Water enters the root hair cells and then the other cells by osmosis; the cells become turgid again and press on their walls, holding the plant up.

Also accept

• (a) cells lose water and become flaccid; (b) cells take in water by osmosis and become turgid

Watch for

• “The stems go soft because they are tired.” — Support comes from turgid cells; a wilted plant has flaccid cells.

5. UQ5 · Practice

Model answer. c. Both take in water by osmosis, but the plant cell has a strong cell wall that pushes back, so it becomes turgid; the red blood cell has no wall and bursts.

A full answer includes

• I chose c.

Also accept

• c

Watch for

• “Plant cells do not take in water.” — Both cells take in water by osmosis; only the wall makes the difference.

6. UT1 · Apply

Model answer. The raisin’s cells contain very concentrated sugar and have a lower water potential than the water around them, so water enters them by osmosis and the raisin swells. The strong syrup has a lower water potential than the grape’s cells, so water leaves the grape cells by osmosis and the grape shrivels.

A full answer includes

• Raisin: its cells have a lower water potential than the water, so water enters by osmosis and it swells.
• Grape in syrup: the syrup has a lower water potential than the grape cells, so water leaves by osmosis and it shrivels.

Also accept

• raisin: water in by osmosis; grape: water out by osmosis to the syrup

Watch for

• “The grape shrivels because sugar is pulled out of it.” — Water leaves the grape by osmosis into the syrup, which has a lower water potential.

7. UT2 · Apply

Model answer. (a) Fresh water has a higher water potential than the cytoplasm of the amoeba, so there is a constant net movement of water into the cell by osmosis through its cell membrane. (b) Without the contractile vacuole the water would not be removed: the amoeba would swell and, having no cell wall, would burst (like a red blood cell in water).

A full answer includes

• (a) The fresh water has a higher water potential than the cytoplasm, so water enters by osmosis all the time.
• (b) Water would keep entering, the cell would swell and, with no cell wall, it would burst.

Also accept

• (a) higher water potential outside, water enters by osmosis; (b) it would swell and burst

Watch for

• “The amoeba drinks water on purpose.” — Water enters by osmosis whether the amoeba needs it or not; the contractile vacuole removes the extra.