D2.3 Water potentialIB Biology HL: 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.

Without a wall, cells swell and burst in hypotonic media and shrink and crenate in hypertonic media. Freshwater protists expel water with contractile vacuoles; multicellular organisms keep tissue fluid isotonic. With a wall, cells develop turgor pressure in hypotonic media and undergo plasmolysis in hypertonic media. IV fluids and solutions for organs awaiting transplant are isotonic so cells do not gain or lose water.

Water potential (ψw) is the potential energy of water per unit volume. Its absolute value cannot be measured, so it is expressed relative to pure water at atmospheric pressure and 20 °C, defined as 0. Units are usually kilopascals (kPa). Water moves from higher to lower water potential because it loses potential energy as it does so, in the same way that water flows downhill.

For cells with walls, ψw = ψs + ψp.

• Solute potential (ψs) ranges from zero downwards: solutes attract water molecules and lower their potential energy.
• Pressure potential (ψp) is generally positive inside cells, because the wall pushes back on the contents (turgor).
• In xylem vessels under tension, ψp is negative, because the sap is being pulled up.

Example: ψs = −700 kPa, ψp = +400 kPa gives ψw = −300 kPa.

In a hypotonic solution (higher ψw outside), water enters; ψp rises as the wall resists expansion until ψw inside equals ψw outside. In pure water, entry stops when ψp = −ψs. In a hypertonic solution (lower ψw outside), water leaves; ψp falls to zero and the cells plasmolyse, then losses continue until ψs inside equals ψw outside. The solution giving no change in mass is equal in water potential to the tissue.