3.1.3Movement into and out of Cells

Osmosis — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 8

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Hub / Topical Revision

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Part 1 of 7Defining osmosis

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.

Defining osmosis

A dilute solution has a high concentration of water molecules (a high water potential),

and a concentrated solution has a lower concentration of water molecules.

Water moves down its own concentration gradient, so it travels from the dilute solution into the concentrated one.

The membrane lets small water molecules pass but blocks larger solute molecules, such as sugar.

Like diffusion, osmosis is passive and needs no energy from respiration.

Part 2 of 7Direction of water movement

The simplest way to think about direction is that water moves toward the side with more dissolved solute.

If a cell with a 5% sugar solution inside is placed in pure water, water moves in; if it is placed in a 20% sugar solution, water moves out.

Isotonic

If the solution outside has the same concentration as the cytoplasm there is no net movement: the solutions are isotonic.

Outside solution compared with cell	Description	Net water movement
More dilute	Hypotonic	Into the cell
Same concentration	Isotonic	None
More concentrated	Hypertonic	Out of the cell

Part 3 of 7Osmosis in animal cells

Animal cells have no cell wall, so changes in the surrounding solution affect them directly.

Burst

If an animal cell,

such as a red blood cell, is placed in pure water, water enters by osmosis,
and since the membrane is not supported, the cell swells and may burst (lysis).

In a concentrated solution water leaves and the cell shrinks and becomes shrivelled.

This is why the body must regulate the concentration of its blood plasma, and why saline drips used in hospital are isotonic with blood.

Part 4 of 7Osmosis in plant cells

Plant cells have a rigid cell wall.

In a dilute solution, water enters the cell and the vacuole swells, pushing the cytoplasm against the wall.

The cell does not burst, because the wall resists, and becomes firm and rigid.

Turgid

This state is called turgid, and it is how soft plant tissue such as leaves and young stems is supported.

Flaccid

In a concentrated solution, water leaves the cell, the cell loses turgor and becomes flaccid, which causes wilting.

Plasmolysis

If much water is lost, the cell membrane pulls away from the cell wall; this is plasmolysis.

Water uptake by root hair cells also takes place by osmosis, because the soil water is more dilute than the cell sap.

Part 5 of 7Interpreting osmosis evidence

Questions may give mass changes of potato in sugar solutions.

Interpreting osmosis evidence

If the mass increases, water has entered by osmosis,

so the solution was more dilute than the cell contents; if the mass decreases, water has left,
so the solution was more concentrated.

The point where there is no change in mass is where the solution is isotonic with the potato cells,

and you can find this on a graph at the place where the line crosses the x-axis.

This is the focus of the next lesson.

Part 6 of 7Worked example and key phrases

Imagine a thin partially permeable bag containing 10% sugar solution, placed in a beaker of pure water.

Water molecules can pass through the membrane but sugar molecules cannot.

The water outside is the more dilute solution, so there is a net movement of water into the bag.

The bag swells, and if it were attached to a narrow tube the liquid level in the tube would rise.

This model, sometimes called a visking tubing experiment, shows osmosis clearly.

If the bag contained pure water and the beaker 10% sugar, the bag would lose water and shrink.

Examination answers earn marks for three key phrases: 'partially permeable membrane', 'from a dilute solution to a more concentrated solution' and 'net movement'. A good answer to 'why did the cell shrink?' is: 'The solution outside was more concentrated than the cytoplasm, so there was a net movement of water out of the cell by osmosis through the partially permeable membrane.'

Finally, remember that the term water potential is not required at GCSE, so use concentration of water or dilute and concentrated solutions instead.

Part 7 of 7Exam-style questions

Define osmosis. [2]2 marks
A red blood cell is placed in pure water. Explain what happens. [3]3 marks
Explain why a plant cell placed in pure water becomes turgid but does not burst. [2]2 marks

Key terms

Term	Meaning
Osmosis	Diffusion of water through a partially permeable membrane from dilute to concentrated solution.
Isotonic	Having the same solute concentration.
Turgid	Firm, full of water.
Flaccid	Limp, having lost water.
Plasmolysis	The membrane pulling away from the cell wall after water loss.
Partially permeable membrane	Membrane letting through water but not larger solutes.

Avoid these mistakes

Saying water moves from high to low solute concentration; it moves from low solute (dilute) to high solute.

Using 'water concentration' and 'solute concentration' in the same sentence without care.

Forgetting that osmosis needs a partially permeable membrane.

Saying plant cells burst in water; the cell wall prevents it.

Last updated: 20 October 2026