Water potential

📚The biology

Solvation

Solvation is the process by which solute particles become surrounded by solvent molecules and dissolve. With water as the solvent:

• polar solutes such as glucose form hydrogen bonds with water molecules;
• positively charged ions (such as Na⁺) attract the partially negative oxygen ends of water molecules;
• negatively charged ions (such as Cl⁻) attract the partially positive hydrogen ends.

Each dissolved particle is surrounded by a shell of water molecules. Those water molecules are held by the solute and are less free to move (A1.1).

Comparing solutions

Water moves by osmosis from a solution with a lower solute concentration to one with a higher solute concentration, across a partially permeable membrane. Describe the direction in terms of solute concentration, not “water concentration”. Three terms compare two solutions:

Osmosis into and out of cells

• If a cell is placed in a hypotonic solution, water moves by osmosis into the cell.
• If a cell is placed in a hypertonic solution, water moves out of the cell.
• In an isotonic solution, water molecules still cross the membrane constantly in both directions, but at equal rates. There is a dynamic equilibrium with no net movement — not an absence of movement.

Measuring osmosis in plant tissue

The standard experiment uses cylinders or strips of plant tissue such as potato:

1. Cut cylinders of equal size from the same tissue, blot dry, and record the initial mass (and/or length) of each.
2. Place them in a range of sucrose or salt solutions of known concentration, for example 0.0 to 0.5 mol dm⁻³, with repeats at each concentration.
3. After a fixed time, remove, blot dry in the same way, and record the final mass.
4. Calculate the percentage change in mass for each cylinder, so that cylinders of slightly different starting mass can be compared.
5. Plot mean percentage change against concentration and draw a line of best fit. The concentration at which the line crosses zero change is the solution that is isotonic to the tissue.

With repeats at each concentration, the standard deviation (SD) shows how much the repeats vary, and the standard error of the mean (SE = SD ÷ \( \sqrt{n} \)) shows how precisely the mean is known. SE can be shown as error bars on the graph. You do not need to memorize the formulae; calculators and spreadsheets do the work. Comparing SD for mass and for length measurements shows which measurement is more reliable.

Cells without a cell wall

Cells without walls therefore need protection from osmotic changes:

• Freshwater unicellular organisms such as Paramecium and Amoeba live in a hypotonic environment, so water constantly enters. Contractile vacuoles collect the excess water and expel it from the cell, using energy from ATP.
• Multicellular animals must keep their tissue fluid isotonic with their cells, so cells neither swell nor shrink. This is one function of the kidneys (D3.3).

Cells with a cell wall

Isotonic solutions in medicine

• Intravenous fluids given to patients who are dehydrated or have lost blood are isotonic with blood plasma (for example 0.9% sodium chloride, “normal saline”). Pure water would enter red blood cells and burst them; a hypertonic solution would shrink them.
• Organs for transplantation are bathed and stored in cold isotonic solutions, so that their cells neither swell nor shrink by osmosis while they are outside the body.
• Isotonic solutions are also used as eye drops and to rinse wounds.

Water potential AHL

Water potential (\( \psi_{\mathrm{w}} \), psi) is the potential energy of water per unit volume. It is a measure of the tendency of water to move from one place to another. The absolute potential energy of water cannot be measured, so values are given relative to pure water at atmospheric pressure and 20 °C, which is defined as zero. The units are usually kilopascals (kPa).

Water moves down a water potential gradient AHL

Water moves from a region of higher water potential to a region of lower water potential. The reason is energy: like a ball rolling downhill, water tends to move to where its potential energy is lower. Dissolving solutes lowers water potential, because water molecules bound in solvation shells are less free to move; applying pressure raises it, because pressure gives the water more energy.

Solute potential and pressure potential AHL

In a plant cell with a wall, water potential has two components:

Because both terms are usually quoted with signs, water potential in cells is almost always zero or negative. The less negative value is the higher water potential: −300 kPa is higher than −700 kPa.

Explaining changes in plant tissue AHL

In a hypotonic solution (for example pure water, \( \psi_{\mathrm{w}} = 0 \)): the cell’s water potential is lower (more negative), because its \( \psi_{\mathrm{s}} \) is negative. Water moves into the cell. As it enters, the contents push against the wall and pressure potential rises, while solute potential becomes slightly less negative as the contents are diluted. Water keeps entering until the cell’s \( \psi_{\mathrm{w}} \) rises to equal that of the solution, at which point the cell is turgid and net movement stops. The tissue gains mass and length.

In a hypertonic solution (solution \( \psi_{\mathrm{w}} \) more negative than the cell): water moves out of the cell. Pressure potential falls towards zero as the cell loses turgor, and solute potential becomes more negative as the contents are concentrated. If water loss continues after \( \psi_{\mathrm{p}} \) reaches zero, the membrane pulls away from the wall (plasmolysis). The tissue loses mass and length.