Water for whole organisms — questions

1. explain

Why does water enter a root hair cell from the soil?

Circle one answer.

• The soil water has a higher water potential than the cell sap, so water enters by osmosis
• The root hair cell uses energy to pump water in by active transport
• The cell sap has a higher water potential than the soil water
• The cell wall sucks water in

Nudge (only if you are stuck): Compare the water potential of the dilute soil water with the more concentrated cell sap.

2. describe

Red blood cells are put into three liquids. Match each liquid to what happens to the cells.

Write the number of the matching option.

1. The cells swell and many burst
2. The cells keep their normal shape
3. The cells shrink and their edges crinkle

• Pure water: ______
• Salt solution as concentrated as blood plasma: ______
• Strong salt solution: ______

Nudge (only if you are stuck): Pure water has the highest water potential; strong salt solution the lowest. Animal cells have no cell wall.

3. explain

A gardener puts far too much fertiliser on the soil around a plant. The soil is wet, but the plant wilts. Explain why.

Nudge (only if you are stuck): Fertiliser dissolves in the soil water. What does that do to the water potential of the soil water compared with the root cells?

4. explain

(a) Explain why a pot plant wilts on a hot, windy afternoon. (b) Explain how watering the soil makes it upright again.

(a) Why it wilts

(b) How watering helps

Nudge (only if you are stuck): Compare how fast water is lost from the leaves with how fast it is taken in at the roots. What happens to the turgor of the cells?

5. explain

A plant cell and a red blood cell are both put into pure water. Why does the red blood cell burst but the plant cell does not?

Circle one answer.

• Only the red blood cell takes in water
• The plant cell has no cell membrane
• The plant cell has a strong cell wall that pushes back; the red blood cell has no cell wall
• Red blood cells have a lower water potential than water only when they are old

Nudge (only if you are stuck): Which layer does a plant cell have that an animal cell does not?

6. explain

A raisin is a dried grape. Left in a glass of water overnight, a raisin swells. A fresh grape left in a very strong sugar syrup shrivels. Explain both changes in terms of water potential.

Raisin in water

Grape in strong syrup

Nudge (only if you are stuck): Compare the water potential inside each fruit’s cells with the liquid around it.

7. explain / predict

Amoeba is a single-celled organism with no cell wall that lives in fresh water. It has a contractile vacuole that collects water and pumps it out of the cell. (a) Explain why water keeps entering the amoeba. (b) Predict what would happen if the contractile vacuole stopped working.

(a) Why water enters

(b) Prediction

Nudge (only if you are stuck): Fresh water is very dilute. Is the water potential inside the amoeba higher or lower than outside? Remember it has no cell wall.