IGCSE · Practice questions · Osmosis

Higher to lower water potential.

Six original Cambridge-style questions on osmosis: water as a solvent, dialysis tubing experiments, plant tissue in solutions, percentage change in mass and the effects of osmosis on plant and animal cells.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Water moves, solute stays.

Osmosis (Extended): net movement of water from higher water potential (dilute) to lower water potential (concentrated) through a partially permeable membrane.
Percentage change = (final − initial) ÷ initial × 100; divide by the initial mass.
No change in mass means the solution matches the concentration of the cell sap.
Plant cells: turgid, flaccid, plasmolysed; the wall stops bursting. Animal cells can burst.

Recall

[3 marks]

Describe the role of water as a solvent in organisms, with reference to digestion, excretion and transport.

Digestion: enzymes act on food in solution, and the small soluble products dissolve in water so they can be absorbed. ✓
Excretion: urea and excess salts are removed dissolved in water, as urine. ✓
Transport: substances such as glucose, ions and hormones are carried dissolved in blood plasma (and in xylem and phloem in plants). ✓

Explain

[4 marks]

A length of dialysis tubing is filled with concentrated sucrose solution, attached to a capillary tube and placed in a beaker of water. Predict and explain what happens to the liquid level in the capillary tube over 30 minutes.

The level rises. ✓
Dialysis tubing is partially permeable: water molecules can pass through but sucrose molecules are too large. ✓
The water in the beaker is more dilute than the solution in the tubing (Extended: it has a higher water potential). ✓
So there is a net movement of water into the tubing by osmosis, increasing the volume inside and pushing liquid up the capillary. ✓

Calculation

[3 marks]

Potato cylinders were weighed, left in solutions for 40 minutes, then blotted and reweighed.

Sucrose / mol dm⁻³	Initial mass / g	Final mass / g
0.0	2.40	2.64
0.4	2.50	2.45
0.8	2.45	2.21

Calculate the percentage change in mass for each cylinder, to one decimal place.

0.0: (2.64 − 2.40) ÷ 2.40 × 100 = +10.0%. ✓
0.4: (2.45 − 2.50) ÷ 2.50 × 100 = −2.0%. ✓
0.8: (2.21 − 2.45) ÷ 2.45 × 100 = −9.8%. ✓

Data

[3 marks]

Using your answers to the previous question, estimate the concentration of sucrose that would cause no change in mass, and explain what this tells you about the potato cells. Give one reason the cylinders were blotted before reweighing.

The change crosses zero between 0.0 and 0.4 mol/dm³, close to 0.4: about 0.3 to 0.35 mol/dm³ is a reasonable estimate (best read from a graph). ✓
At this concentration there is no net movement of water, so the cell sap has about the same concentration (water potential) as the solution. ✓
Blotting removes surface solution, which would otherwise add mass that is not due to osmosis. ✓

Explain

[4 marks]

Extended. A plant has not been watered for several days and has wilted. Use the terms water potential, turgor pressure and flaccid to explain why it wilted and how it recovers after watering.

As the soil dries its water potential falls, so the root cells cannot take in enough water to replace what the leaves lose. ✓
With less water in the vacuoles, the cells no longer press outwards on their walls: turgor pressure falls and the cells become flaccid. ✓
Flaccid cells cannot support the stem and leaves, so the plant wilts. ✓
After watering, the soil water has a higher water potential than the root cells, water enters by osmosis, the cells become turgid again and the plant is supported. ✓

Analysis

[3 marks]

Extended. Red blood cells and onion epidermis cells are each placed in (a) pure water and (b) a concentrated salt solution. Describe and explain the appearance of each type of cell in each liquid.

(a) Pure water has a higher water potential than the cells, so both take in water by osmosis. The red blood cell swells and bursts because it has no cell wall; the onion cell becomes turgid because its cell wall resists the turgor pressure. ✓
(b) The salt solution has a lower water potential, so both lose water by osmosis. The red blood cell shrinks with a crinkled outline. ✓
The onion cell becomes plasmolysed: the vacuole and cytoplasm shrink and the cell membrane pulls away from the cell wall. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.