Water moves down a gradient that has nothing to do with concentration alone. Pressure, solutes, gravity — all of it matters.
Every sub-topic below feeds at least one of these questions.
What factors affect the movement of water into or out of cells?
How do plant and animal cells differ in their regulation of water movement?
The required syllabus content for D2.3, in order. Each card is one lesson-sized checkpoint.
Solvation with water as the solvent
Water movement from less concentrated to more concentrated solutions
Water movement by osmosis into or out of cells
Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions
Effects of water movement on cells that lack a cell wall
Effects of water movement on cells with a cell wall
Medical applications of isotonic solutions
Water can surround and disperse solute particles because of its polarity. Hydrogen bonds form with polar solutes; hydration shells form around ions.
Water is polar — δ⁻ on oxygen, δ⁺ on hydrogens. When polar solutes (like glucose) dissolve, water forms hydrogen bonds with their polar groups, pulling them into solution. When ionic compounds (like NaCl) dissolve, water's δ⁻ oxygens cluster around cations (Na⁺) and δ⁺ hydrogens cluster around anions (Cl⁻) — hydration shells separate and disperse the ions.
Water moves from where solute is low to where solute is high. Always think about solute concentration, not water concentration.
Cell loses water → shrinks.
Dynamic equilibrium — water moves both ways equally; no net change.
Cell gains water → swells.
Bathing plant tissue in a series of solutions of different solute concentrations and measuring mass change reveals the concentration that's isotonic with the cell sap.
Classic IB experiment: cut equal cylinders of potato; weigh; immerse in sucrose solutions of varying concentrations (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 M); after a fixed time, blot and reweigh; calculate % change in mass; plot vs concentration.
Reliability: at least 5 concentrations, at least 5 repeats each. Use standard deviation or standard error for error bars.
Without a cell wall, an animal cell has nothing to push back against incoming water — and nothing to maintain shape if water leaves.
Water moves out; cell shrivels. Red blood cells in concentrated salt: shrivelled spiky shape.
No net movement; cell maintains normal volume. The physiological state for most animal cells.
Water rushes in; cell swells and bursts. Red blood cells in pure water lyse within seconds.
A freshwater pond is hypotonic to a Paramecium's cytoplasm. Water continuously enters by osmosis. To prevent cytolysis, Paramecium uses a contractile vacuole — fills with the excess water and periodically expels it. ATP-powered active transport against the osmotic gradient. Essential homeostasis for freshwater life without a cell wall.
Plant cells in hypotonic conditions become turgid; in hypertonic, plasmolysed. The cell wall prevents bursting and allows turgor pressure.
Water enters → cytoplasm/vacuole pushes outward against the cell wall → cell wall pushes back. The resulting turgor pressure keeps the cell firm. Healthy plants are turgid.
No turgor pressure. Cell is soft but intact. Plants may droop.
Water leaves; cytoplasm shrinks; plasma membrane pulls away from the cell wall. Wilting plant cells are plasmolysed.
Intravenous fluids interact directly with red blood cells. Wrong tonicity causes either crenation or lysis. So clinical IV solutions are specifically formulated to be isotonic with blood plasma.
Examples of isotonic IV fluids: 0.9% saline solution (normal saline); 5% glucose; Ringer's lactate. Each carefully tuned to roughly the osmolarity of plasma (~300 mOsm/L) so that red blood cells experience no net water movement during infusion. Distilled water given IV would be catastrophic — RBCs would lyse en masse.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Water potential as the potential energy of water per unit volume
Movement of water from higher to lower water potential
Contributions of solute potential and pressure potential to the water potential of cells with walls
Water potential and water movements in plant tissue
If you can't define one of these in a sentence, that's where to revise next.
“What variables influence the direction of movement of materials in tissues?”
“What are the implications of solubility differences between chemical substances for living organisms?”