1.16 Osmosis
7 exam questions worth 18 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A red blood cell is placed in pure water.
What happens to the cell?
Choose one answer.
Correct Not quite: the answer is It swells and may burst
Mark scheme
B (It swells and may burst)
Model answer
B: It swells and may burst. Water enters the cell by osmosis. Animal cells have no cell wall, so the cell can burst.
Define osmosis.
Mark scheme
- the movement / diffusion of water (1)
- from a dilute solution to a concentrated solution through a partially permeable membrane (1)
Model answer
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution, through a partially permeable membrane.
A potato cylinder has a mass of 4.8 g at the start. After 30 minutes in a sugar solution its mass is 4.2 g.
Calculate the percentage change in mass.
Mark scheme
- change in mass = 4.2 − 4.8 = −0.6 (g) (1)
- −0.6 ÷ 4.8 × 100 (1)
- = −12.5 (%) (1)
- allow a negative sign shown in the working, or the answer as 12.5% decrease
Model answer
change = 4.2 − 4.8 = −0.6 g. percentage change = −0.6 ÷ 4.8 × 100 = −12.5%, so the mass decreased by 12.5%.
Plant cells in a dilute solution become turgid but do not burst.
Explain why.
Mark scheme
- water enters the cell / vacuole by osmosis (1)
- the cell swells and pushes against the cell wall (1)
- the strong cell wall stops the cell bursting (1)
Model answer
Water enters the vacuole by osmosis and the cell swells, pushing against the cell wall. The cell wall is strong, so it stops the cell bursting and the cell becomes firm (turgid).
A plant that has not been watered for several days starts to wilt.
Explain why.
Mark scheme
- there is little water in the soil, so the cells lose water by osmosis / water leaves the cells (1)
- the cells become flaccid / lose turgor (1)
- so they can no longer support the leaves and stem (1)
Model answer
With little water available, the plant cells lose water by osmosis and become flaccid. They can no longer push against each other to support the leaves and stem, so the plant wilts.
Describe, using the term water potential, how water moves into a root hair cell from the soil.
Mark scheme
- water moves from a region of higher water potential in the soil (1)
- to a region of lower water potential in the cell, through a partially permeable membrane (1)
Model answer
Water moves by osmosis from the soil, where the water potential is higher, into the root hair cell, where the water potential is lower, through the partially permeable cell membrane.
A red blood cell is placed in a concentrated salt solution.
Describe and explain what happens to the cell.
Mark scheme
- the solution is more concentrated than the cytoplasm / the water concentration is lower outside the cell (1)
- water leaves the cell by osmosis (1)
- through the partially permeable cell membrane (1)
- so the cell shrinks / crinkles (1)
Model answer
The salt solution is more concentrated than the cytoplasm of the red blood cell. Water leaves the cell by osmosis, through the partially permeable cell membrane, so the cell shrinks and crinkles.
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