IGCSE and O Level Biology osmosis
Learn what osmosis is, how it changes plant and animal cells, and how to answer practical questions, with 12 original questions and answers.
Watch water cross a membrane
The small blue dots are water molecules. The large amber circles are solute molecules, and they are too big to pass through the white membrane. Choose a solution for each side and watch the water levels.
Left side
Right side
- Water moving right
- 0
- Water moving left
- 0
- Net movement
- 0
- Time
- 0 s
This is a simplified model, because real molecules are far too small to see. Sizes and speeds are exaggerated, and the counts change slightly each run.
Osmosis key words
| Term | Meaning |
|---|---|
| Solvent | The liquid that dissolves a substance. In cells and organisms, the solvent is usually water. |
| Solute | The substance that dissolves, such as sugar or salt. |
| Solution | A solute dissolved in a solvent. |
| Dilute solution | Has less solute and more water. It has a higher water potential. |
| Concentrated solution | Has more solute and less water. It has a lower water potential. |
| Partially permeable membrane | Lets small molecules such as water pass, but stops larger molecules such as sugar. |
| Water potential | A measure of how readily water molecules move. Pure water has the highest water potential. |
| Osmosis | The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. |
| Turgid | A plant cell that is swollen and firm because water has entered. |
| Turgor pressure | The pressure of the cell contents pushing outwards on the cell wall. |
| Flaccid | A plant cell that is soft because it has lost water, so it no longer presses on the wall. |
| Plasmolysis | The cell membrane and cytoplasm pull away from the cell wall after a plant cell loses a lot of water. |
How to use this page
Osmosis questions often turn on one or two words. Water moves, not sugar. A plant cell becomes plasmolysed, while an animal cell does not. This page teaches the topic in the words you need to write, for Cambridge IGCSE Biology 0610 and O Level Biology 5090.
Start with the animation and the key words. Then learn what happens to cells, try the two main experiments, and try the percentage change calculation. After that, answer the practice questions without looking. Every question and answer here is original, and the same notes and questions are in the free PDF. Once you can explain water movement using water potential, describe a turgid and a plasmolysed cell, and plan a fair potato experiment, you are ready for most osmosis questions on a practical paper.
Why water moves
Water molecules move randomly and cross the membrane in both directions. More cross from the dilute side to the concentrated side, so the overall movement is called net movement. The energy comes from the kinetic energy of the moving molecules, so osmosis does not need energy from respiration.
Water is also the solvent in living things. Digestion, excretion and transport all depend on substances being dissolved in it.
What osmosis does to cells
Dilute solution or pure water
The solution has a higher water potential than the cell, so water enters by osmosis.
Plant cell
Turgid. The vacuole swells and the contents press on the wall. The wall stops the cell from bursting.
Animal cell
Swells and may burst, because it has no cell wall.
Same water potential as the cell
Water crosses both ways at the same rate, so there is no net movement.
Plant cell
Stays the same. No overall change in size.
Animal cell
Stays the same. No overall change in size.
Concentrated solution
The solution has a lower water potential than the cell, so water leaves by osmosis.
Plant cell
Flaccid, then plasmolysed if it loses a lot of water. The membrane pulls away from the wall.
Animal cell
Shrinks. It cannot plasmolyse, because it has no cell wall.
Why this matters to plants
- Turgid cells press against each other and give soft parts of a plant their support.
- When a plant loses more water than it takes in, cells become flaccid and the plant wilts.
- Root hair cells take in water from soil by osmosis when the soil water has a higher water potential than the cell.
Osmosis experiments to know
Potato cylinders in sugar solutions
- Cut equal potato cylinders with a cork borer, and trim them to the same length.
- Blot each cylinder dry and measure its mass.
- Put one cylinder in each boiling tube of a different sugar solution, plus one in distilled water.
- Leave them for the same time, such as 30 minutes.
- Remove each cylinder, blot it dry and measure its mass again.
- Calculate the percentage change in mass. Repeat at each concentration and find the mean.
Fair test. Distilled water shows the largest gain. Keep the volume of solution, the temperature and the time the same.
Remember. Blot in the same way each time, since water left on the surface adds to the mass.
Dialysis tubing and sugar solution
- Tie one end of a length of dialysis tubing and fill it with concentrated sugar solution.
- Fix a narrow glass tube into the open end and mark the starting liquid level.
- Stand the tubing in a beaker of distilled water.
- Measure the liquid level in the narrow tube at regular intervals.
Fair test. Set up the same apparatus with distilled water inside the tubing. The level should not rise.
Remember. Rinse the outside of the tubing first, so that no sugar is in the beaker at the start.
Looking at plasmolysis
- Peel a thin layer of red onion epidermis and put it on a slide.
- Add a drop of concentrated sugar solution and a cover slip.
- Look at the cells with a microscope, and compare them with cells in distilled water.
Fair test. Cells in distilled water stay turgid, so they can be compared with the cells in the sugar solution.
Remember. In a plasmolysed cell, the coloured contents shrink away from the cell wall.
Percentage change in mass
Percentage change = (final mass - starting mass) ÷ starting mass × 100
Example with made-up values: a cylinder goes from 20.0 g to 22.5 g. The change is 2.5 g, and 2.5 ÷ 20.0 × 100 = 12.5, so it gained 12.5%. A loss gives a negative answer.
Percentage change is used because the starting masses may not be exactly the same, so it allows a fair comparison.
On a graph of percentage change against concentration, the line crosses zero where there is no net movement. At that point the solution and the potato cells have about the same water potential.
How to write your answers
| Avoid | Write |
|---|---|
| Water moves from high to low concentration. | Water moves from higher to lower water potential, or from a dilute to a concentrated solution. |
| Sugar moved into the potato. | Water moved out of the potato by osmosis. Sugar molecules are too large to pass through the membrane. |
| Water stops moving at equilibrium. | Water still crosses both ways, but there is no net movement. |
| The plant cell burst. | The plant cell became turgid. The cell wall stops it from bursting. |
| The red blood cell was plasmolysed. | The red blood cell shrank. Plasmolysis happens only in cells with a cell wall. |
| The cell wall is partially permeable. | The cell membrane is partially permeable. The cell wall is fully permeable. |
| The cell is flaccid, so its membrane has pulled away. | A flaccid cell is soft. A plasmolysed cell has its membrane pulled away from the wall. |
Fair tests and evaluation
- Keep other variables the same. Use cylinders from the same potato, and keep their size, the solution volume, the temperature and the time the same.
- Blot in the same way. Water left on the outside changes the mass, so dry each cylinder in the same way.
- Use a range of concentrations. Include distilled water, and use more concentrations near the point where the mass does not change.
- Repeat and find a mean. Repeats help you spot an anomalous result and make the results more reliable.
- Use a precise balance. A small change in mass needs a balance that reads to at least 0.1 g.
- Plot a graph. A line of best fit lets you estimate the concentration at which there is no net movement.
Hazards and precautions
| What | Hazard | Precaution |
|---|---|---|
| Cork borer or scalpel | Can cut skin | Cut away from your hand, on a white tile or cutting surface. |
| Glass tubes and slides | Can break and cut | Handle with care and report any breakage. |
| Spilt solutions | Floors can become slippery | Wipe up spills straight away. |
Osmosis questions and answers
12 original questions, 42 marks. Write your answer first, then open the answer and mark it.
Defining osmosis [3 marks]
Describe what is meant by osmosis.
Show the answer
- One mark for the net movement of water molecules.
- One mark for moving from higher to lower water potential, or from a dilute to a concentrated solution.
- One mark for moving through a partially permeable membrane.
Solute and solvent [2 marks]
Salt is dissolved in water. Name the solute and the solvent.
Show the answer
- One mark for salt as the solute.
- One mark for water as the solvent.
Percentage change [4 marks]
A potato cylinder of 16.0 g has a final mass of 14.0 g. (a) Calculate the percentage change in mass. [2] (b) Explain why it lost mass. [2]
Show the answer
- (a) One mark for working: -2.0 ÷ 16.0 × 100.
- (a) One mark for -12.5%.
- (b) One mark for the solution having a lower water potential than the potato cells.
- (b) One mark for water leaving the cells by osmosis.
Reading a graph from results [5 marks]
Made-up results for potato cylinders. Sugar concentration in mol/dm3 and percentage change in mass: 0.0, +18. 0.2, +8. 0.4, -2. 0.6, -12. 0.8, -20. (a) Estimate the concentration with no change in mass. [2] (b) State what this shows about the potato cells. [1] (c) Explain the result at 0.8 mol/dm3. [2]
Show the answer
- (a) One mark for choosing a value between 0.2 and 0.4.
- (a) One mark for about 0.36 mol/dm3.
- (b) One mark for the solution and the cells having about the same water potential.
- (c) One mark for the solution having a lower water potential than the cells.
- (c) One mark for water leaving by osmosis.
Why use percentage change? [2 marks]
Explain why a student calculates percentage change in mass and not just the change in mass.
Show the answer
- One mark because the cylinders may have different starting masses.
- One mark because percentage change allows a fair comparison.
Plant and animal cells [4 marks]
A plant cell and a red blood cell are both put in distilled water. Describe and explain what happens to each cell.
Show the answer
- One mark for water entering both cells by osmosis.
- One mark for the plant cell becoming turgid.
- One mark for the red blood cell swelling and possibly bursting.
- One mark because the plant cell wall stops bursting, and a red blood cell has no wall.
Plasmolysis [3 marks]
Onion cells are placed in a concentrated sugar solution. Explain how they become plasmolysed.
Show the answer
- One mark for the solution having a lower water potential than the cell.
- One mark for water leaving the cell by osmosis.
- One mark for the cell membrane and cytoplasm pulling away from the cell wall.
Why plants wilt [3 marks]
A pot plant has not been watered for a week and its leaves droop. Explain why.
Show the answer
- One mark for cells losing water.
- One mark for cells becoming flaccid, or losing turgor pressure.
- One mark for cells no longer giving the leaves support.
Dialysis tubing [4 marks]
Dialysis tubing containing sugar solution is placed in a beaker of water. (a) Predict what happens to the liquid level in a narrow tube fixed to the tubing. [1] (b) Explain your prediction. [2] (c) Describe a suitable control. [1]
Show the answer
- (a) One mark for the level rising.
- (b) One mark for the water in the beaker having a higher water potential.
- (b) One mark for water entering the tubing by osmosis.
- (c) One mark for using distilled water inside the tubing.
Spot the mistakes [3 marks]
Read this student's account: "The sugar moved out of the solution into the potato because of diffusion, so the potato gained mass and its cell walls stretched until they burst." Give three corrections.
Show the answer
- One mark for water moving, not sugar.
- One mark for osmosis through a partially permeable membrane.
- One mark because a plant cell does not burst, since the wall stops it.
Evaluating a method [3 marks]
A student dries some cylinders with paper, but not others, before finding their final mass. Explain how this affects the results and suggest an improvement.
Show the answer
- One mark for water on the surface adding to the mass.
- One mark for the results not being a fair comparison, or the gain appearing too high.
- One mark for blotting every cylinder in the same way.
Planning [6 marks]
Plan an experiment to find the concentration of sugar solution with the same water potential as potato tissue.
Show the answer
- Any six of these, one mark each:
- Prepare a range of sugar solutions, including distilled water.
- Cut potato cylinders of the same size.
- Blot and measure the starting mass of each.
- Put a cylinder in each solution, using the same volume.
- Leave for the same time at the same temperature.
- Blot and measure the final mass of each.
- Calculate the percentage change in mass.
- Repeat and calculate a mean.
- Plot a graph and read the concentration where the change is zero.
Osmosis PDF
Download or print the notes, diagrams, experiments, questions and answers.
0610 and 5090: what to check
Cambridge IGCSE Biology 0610
In the syllabus for 2026 to 2028, Core students need water as a solvent, water moving through partially permeable membranes, and the investigations. Extended students must also use water potential and the terms turgid, turgor pressure, plasmolysis and flaccid.
Cambridge O Level Biology 5090
The syllabus for 2026 to 2028 has no Core and Supplement split, so all of this page applies, including water potential and effects on animal tissues.
Try real questions from Cambridge IGCSE Biology 0610 and Cambridge O Level Biology 5090, where Cambridge publishes its own papers free of charge.
Frequently asked questions
What is osmosis in simple words?
Water moves through a partially permeable membrane from a dilute solution to a more concentrated solution. The correct term for the direction is from higher to lower water potential.
What is the difference between a solute and a solvent?
A solute is the substance that dissolves, such as sugar. A solvent is the liquid that dissolves it, such as water. Together they make a solution.
What is the difference between flaccid and plasmolysed?
A flaccid cell has lost water and is soft, but its membrane is still against the wall. A plasmolysed cell has lost more water, so the membrane and cytoplasm have pulled away from the wall.
Does osmosis need energy?
It does not need energy from respiration. The molecules move because of their own kinetic energy. Active transport is the process that uses energy from respiration.
Why do plant cells not burst?
The strong cell wall resists the pressure of the swelling contents. Animal cells have no cell wall, so they can burst.
Does water stop moving at equilibrium?
No. Water molecules still cross in both directions, but at the same rate, so there is no net movement.
Is osmosis in the practical papers?
Yes. Both syllabuses list osmosis among the contexts for their practical papers: Papers 5 and 6 for 0610, and Papers 3 and 4 for 5090.
Keep revising
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