Water goes where it is wanted least. Learn the rule once, in cells, and it explains wilting, drips in hospitals and how a tree lifts a tonne.
Water dissolves things because it is lopsided. That one fact — polarity — is why solutions exist and why osmosis happens.
Oxygen pulls the shared electrons closer, so it carries a slight negative charge and each hydrogen a slight positive. Opposite ends attract: hydrogen bonds.
Ions and polar molecules get surrounded by oriented water: oxygens toward Na⁺, hydrogens toward Cl⁻. The shell pulls particles apart and keeps them apart — that is dissolving.
Hydrophobic molecules — lipids, hydrocarbons — have nothing for water to grip. They cluster together instead. Cells use that for membranes (B2.1).
Solute = anything water grabs. Salts, sugars, amino acids, proteins with charged groups: all dissolve, all lower the concentration of free water. That lowered free-water concentration is what osmosis reads.
Slide the bath from pure water to strong salt. Water moves from the less concentrated solution to the more concentrated one, across the membrane. What happens next depends on whether the cell has a wall.
Water enters. No wall: the cell swells and bursts (lysis). Wall: the cell fills until the wall pushes back — turgid, and that is the plant's normal, healthy state.
Water still crosses, but equally in both directions: no net movement. Animal cells keep their shape; plant cells are flaccid — the plant wilts.
Water leaves. No wall: the cell shrinks and crinkles (crenation). Wall: the protoplast shrinks away from the wall — plasmolysis. The wall itself barely moves.
Equal potato cylinders in a range of sucrose solutions gain or lose mass. Enter your class results; the bench plots them and reads off the concentration where nothing happens.
| Sucrose / mol dm⁻³ | Initial mass / g | Final mass / g | % change |
|---|
Why percentage, and why blot. Cylinders start at slightly different masses, so only % change is comparable. Surface water on a cylinder is not water that entered the cells — blot the same way every time, and say so in your method. The x-intercept is the solution that matches the cell sap.
The membrane is the same. The wall decides whether swelling is fatal or useful.
What the practical shows. Mount onion skin in distilled water and the cells are turgid; in strong salt the protoplast peels away from the wall. Rinse in water and it recovers — proof that the membrane, not the wall, is the selectively permeable layer.
Water entering pushes the membrane against the wall: turgor pressure. It holds leaves flat and stems up without wood. Lose it and the plant wilts.
Nothing stops a red cell bursting in fresh water. So blood is kept isotonic, and freshwater protists pump water out with contractile vacuoles — homeostasis, every second.
A plasmolysed cell recovers in water. A lysed cell is gone — the membrane has torn. Crenated cells can recover if the bath is corrected quickly.
Any fluid that touches living cells has to match them.
About 0.15 mol dm⁻³ — isotonic with blood plasma. Used for intravenous rehydration, diluting drugs, and washing wounds. Pure water in a vein would lyse red cells; strong salt would crenate them.
A kidney or heart waiting for transplant sits in a cold isotonic solution so its cells neither swell nor shrink during the hours in transit.
Saline eye drops, contact-lens solution, nasal rinses: all isotonic, because the cells they touch have no wall.
"More concentrated" gets vague once pressure joins in. Water potential (Ψ, psi) turns the rule into arithmetic: water moves from higher Ψ to lower Ψ.
The potential energy of water per unit volume, measured in kPa or MPa. Pure water at atmospheric pressure is set at Ψ = 0 — the highest it gets. Everything else is negative.
Water moves from higher (less negative) to lower (more negative) Ψ. Same as "dilute to concentrated", but it also handles pressure and tension.
Solute potential Ψs: always ≤ 0; more solute, more negative. Pressure potential Ψp: positive in a turgid cell (the wall pushing back), zero in an open beaker, negative in xylem under tension.
Why a plant cell in pure water doesn't burst. As it swells, Ψp rises until it exactly cancels Ψs: Ψ = 0, equal to the water outside. Equilibrium — with the cell full and stiff. An animal cell has no Ψp to offer, so it keeps taking water until it splits.
R = 0.0831 L bar mol⁻¹ K⁻¹, T in kelvin. In an open beaker Ψp = 0, so Ψ = Ψs. 10 bar = 1 MPa. Compare with the tissue's Ψ to predict direction.
Tap a node to light up its links. The pink dashed link is the whole topic in one line.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
Single best answer. You get the reasoning as soon as you commit. HL items are marked.