Osmosis — Cambridge IGCSE Biology
Osmosis is the diffusion of water through partially permeable membranes; water potential, turgor and plasmolysis explain what happens to plant cells.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Osmosis is the diffusion of water through a partially permeable membrane, which lets small molecules such as water through but not larger ones such as sucrose. Water is also a solvent, which matters for digestion, excretion and transport. Plant cells take in water by osmosis, and water pressing outwards on the cell wall makes the cells firm, supporting the plant.
Investigating osmosis
PracticalOsmosis can be investigated using dialysis tubing or potato cylinders in sugar solutions. A tubing bag of concentrated sucrose solution placed in water gains mass as water enters by osmosis. Potato cylinders gain mass in dilute solutions and lose mass in concentrated ones. Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.
Water potential
SupplementOsmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Pure water has the highest water potential. Root hair cells have a lower water potential than the soil water, so water enters them by osmosis.
Turgor, flaccid cells and plasmolysis
SupplementA turgid plant cell has gained water by osmosis, so its vacuole presses the cell contents against the cell wall; this pressure is turgor pressure. In a concentrated solution, water leaves and the cell becomes flaccid. If more water leaves, the cytoplasm shrinks and the cell membrane pulls away from the cell wall, which is plasmolysis.
Exam questions
7 exam-style questions (23 marks), each with its mark scheme.
Answer the questionsFlashcards
14 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of movement into and out of cells on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheets
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
Give three ways in which water's role as a solvent is important in organisms.
Show answer
Digestion, excretion and transport (substances dissolve in it).
What is moved by osmosis?
Show answer
Water, through a partially permeable membrane.
How does the pressure of water in plant cells support a plant?
Show answer
Water inside the cells presses outwards on the cell walls, making the cells firm.
Supplement Define osmosis using water potential.
Show answer
The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
Supplement What is plasmolysis?
Show answer
The cell membrane and cytoplasm pulling away from the cell wall because water has left the cell by osmosis.
Frequently asked questions
How does osmosis differ from diffusion?
Osmosis is a type of diffusion that involves only water moving through a partially permeable membrane. Diffusion is the net movement of any particles, such as oxygen or carbon dioxide, down a concentration gradient. Both are passive, so neither needs energy from respiration. Never write that sugar moves by osmosis.
How does osmosis affect plant cells?
Plant cells in a dilute solution take in water by osmosis, and the water presses outwards on the cell wall, making the cells firm. Firm cells support the stems and leaves of non-woody plants. In a concentrated solution, plant cells lose water, the pressure falls, the cells become soft and the plant wilts.
Why don't plant cells burst in pure water?
Supplement Plant cells do not burst in pure water because the strong cell wall resists the pressure. Water enters by osmosis, the vacuole swells and presses the cell contents against the wall, and the cell becomes turgid. Animal cells have no cell wall, so if they take in too much water they can burst.
What happens to potato cylinders in concentrated sugar solution?
Potato cylinders lose mass in a concentrated sugar solution because water leaves their cells by osmosis. In dilute solutions or distilled water they gain mass because water enters. Where a graph of percentage change in mass crosses the x-axis, the solution has the same concentration as the inside of the potato cells.
How does water move in osmosis?
Supplement Water moves by osmosis from a region of higher water potential, a dilute solution, to a region of lower water potential, a concentrated solution, through a partially permeable membrane. The membrane lets water molecules through but not larger molecules such as sucrose. It is a net movement, so some water molecules move both ways.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026