Water potential — IB Diploma Biology SL
IB Biology D2.3: solvation, osmosis and tonicity, effects on cells with and without walls, isotonic solutions in medicine and (HL) water potential.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Osmosis is the net movement of water across a partially permeable membrane from a less concentrated solution to a more concentrated solution. Solutes dissolve because water forms hydrogen bonds with polar molecules and is attracted to positive and negative ions. In an isotonic environment there is dynamic equilibrium, not zero water movement.
Plant tissue gains mass and length in hypotonic solutions because water enters its cells by osmosis, and loses mass and length in hypertonic solutions because water leaves. Plotting percentage change against solute concentration lets you deduce the isotonic concentration where the line crosses zero. Standard deviation and standard error show how reliable repeat measurements are.
Cells without a cell wall swell and burst in a hypotonic medium and shrink and crenate in a hypertonic medium. Cells with a wall develop turgor pressure in hypotonic solutions and undergo plasmolysis in hypertonic ones. Isotonic solutions are used for intravenous fluids and for bathing organs before transplantation to prevent harmful water movement.
Exam questions
4 exam-style questions (14 marks), each with its mark scheme.
Answer the questionsFlashcards
10 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (continuity and change) on one page, so you can see where this subtopic fits.
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Quick check questions
Why are cations and anions both able to dissolve in water?
Show answer
Water is polar: cations attract the δ− oxygen ends and anions attract the δ+ hydrogen ends of water molecules.
In which direction does water move between a hypotonic and a hypertonic solution?
Show answer
From the hypotonic (less concentrated) solution to the hypertonic (more concentrated) solution.
What is plasmolysis?
Show answer
The cell membrane pulling away from the cell wall as water leaves a plant cell in a hypertonic solution.
Why are intravenous fluids isotonic?
Show answer
So that blood cells neither gain nor lose water, which would make them burst or crenate.
Frequently asked questions
What is the difference between hypertonic, hypotonic and isotonic?
These terms compare the solute concentrations of two solutions. A hypertonic solution has a higher solute concentration than the other, a hypotonic solution has a lower one, and isotonic solutions have the same solute concentration. Water moves by osmosis from the hypotonic solution to the hypertonic solution.
What happens to animal cells in a hypotonic solution?
Animal cells in a hypotonic solution gain water by osmosis, because the solution has a lower solute concentration than the cytoplasm. With no cell wall to resist, the cell swells and may burst. Red blood cells bursting is called haemolysis. In a hypertonic solution they shrink and crenate instead.
Why do plant cells not burst in pure water?
Plant cells do not burst because of their cell wall. As water enters by osmosis, the cytoplasm and vacuole push against the wall and turgor pressure develops. The strong wall resists further expansion, so the cell becomes turgid. Without a wall, an animal cell would swell and burst.
How do you find the isotonic point in an osmosis practical?
Measure the percentage change in mass or length of plant tissue in a range of solute concentrations, then plot percentage change against concentration with a line of best fit. The isotonic concentration is where the line crosses zero percentage change, because there the tissue neither gains nor loses water overall.
Written and checked against the IB Biology SL specification · Updated October 2026