D2.3 Water potentialIB Biology SL: Revision notes

Water is polar, so it dissolves many substances by solvation: water molecules cluster around each solute particle. Positive ions (e.g. Na⁺) attract the δ− oxygen of water; negative ions (e.g. Cl⁻) attract the δ+ hydrogens. Polar molecules such as glucose dissolve because their –OH groups form hydrogen bonds with water. Non-polar molecules such as oils cannot form these attractions, so they do not dissolve.

Osmosis is the net movement of water across a partially permeable membrane from a less concentrated solution to a more concentrated solution (lower to higher solute concentration). Compare solutions with these terms:

• hypotonic — lower solute concentration than the cell: net water movement into the cell;
• hypertonic — higher solute concentration than the cell: net water movement out of the cell;
• isotonic — same solute concentration: no net movement.

In an isotonic solution water still moves both ways at equal rates — a dynamic equilibrium, not an absence of movement.

Plant tissue gains mass and length in hypotonic solutions and loses them in hypertonic ones. Cylinders of tissue (e.g. potato) are placed in a range of sucrose concentrations and percentage change in mass or length is calculated, which allows for different starting sizes. The concentration giving zero change is isotonic with the tissue; find it by interpolating between the two concentrations either side of zero.

With repeats at each concentration, the standard deviation shows the spread of results and the standard error (SD/√n) shows how precisely the mean is known. SE can be drawn as error bars; means whose error bars overlap zero change cannot be confidently called hypo- or hypertonic.

In a hypotonic medium, animal cells gain water, swell and burst (red blood cells: haemolysis). In a hypertonic medium they lose water and shrink, and red blood cells show crenation (a spiky outline). Freshwater unicellular organisms such as Paramecium and Amoeba constantly gain water, and remove it using contractile vacuoles. Multicellular organisms keep their tissue fluid isotonic with their cells to prevent harmful changes.

In a hypotonic medium, a plant cell takes in water, but the cellulose wall resists expansion, so turgor pressure builds up until no further net entry occurs; the cell is turgid and does not burst. Turgor supports non-woody plants. In a hypertonic medium, water leaves; the cytoplasm and membrane shrink and pull away from the wall — plasmolysis — and the plant wilts.

Intravenous (IV) fluids, such as saline, are made isotonic with blood plasma so that blood cells neither swell nor shrink. Organs for transplantation are bathed in cold isotonic solution so their cells do not gain or lose water and remain viable until transplanted.