D2.3 Water potential

Solvation occurs when polar water molecules surround and interact with dissolved ions or polar solute molecules.

Water's partially negative oxygen is attracted to positive ions, while its partially positive hydrogens are attracted to negative ions. Polar solutes can also form hydrogen bonds with water.

These attractions form hydration shells, separate solute particles and keep them dispersed. Water molecules engaged around solutes have less freedom of movement than in pure water.

When sodium chloride dissolves, oxygen ends of water face Na⁺ and hydrogen ends face Cl⁻, producing oriented hydration shells.

Water does not form hydrogen bonds with every solute: ion–dipole attraction hydrates ions, while hydrogen bonding requires suitable polar groups.

Across a partially permeable membrane, net water movement is from the less concentrated solution toward the more concentrated solution.

If solution A is 0.10 mol dm⁻³ sucrose and solution B is 0.40 mol dm⁻³, A is hypotonic to B and net water movement is from A to B if water can cross but sucrose cannot.

At SL, express the comparison using solute concentration—not 'high water concentration'. Tonicity is relative and depends on solutes that do not freely cross the membrane.

Osmosis is the net movement of water across a partially permeable membrane, and the cell's environment determines its direction.

In an isotonic environment, water molecules continue crossing in both directions at equal rates. This is dynamic equilibrium, not an absence of molecular movement.

A cell placed in hypertonic solution loses water and decreases in volume because more water leaves than enters.

Always state the solution relative to the cell. 'Hypotonic' or 'hypertonic' without a comparison has incomplete meaning.

Changes in plant-tissue mass or length across a series of sucrose concentrations can be used to estimate the isotonic concentration.

Prepare equal tissue pieces, record initial mass or length, incubate them for the same time in known sucrose solutions, blot consistently, record final values and calculate change.

Plot mean percentage change against sucrose concentration. The x-intercept, where change is 0%, estimates the isotonic concentration. Replicates allow standard deviation to compare spread and standard error/error bars to compare uncertainty in means.

Positive mass change at 0.20 mol dm⁻³ and negative change at 0.30 mol dm⁻³ place the isotonic estimate between those concentrations; interpolate from the fitted graph rather than choosing the nearest raw point.

Zero mean change estimates isotonic conditions; it does not mean water molecules stopped moving. Consistent blotting, initial size and incubation time are needed for a fair comparison.

Cells without a wall can burst in hypotonic solution or shrink in hypertonic solution because their plasma membrane cannot resist large volume changes.

Freshwater unicellular organisms continually gain water from their hypotonic environment, so contractile vacuoles collect and expel excess water. Multicellular animals instead maintain near-isotonic tissue fluid around cells.

A plasma membrane can deform but does not provide the rigid mechanical restraint of a cell wall; active water removal is an adaptation, not a reversal of osmosis.

A cell wall resists expansion when water enters, converting osmotic water uptake into turgor pressure.

The wall’s rigidity balances the inward tendency of water. If water leaves, pressure falls and the membrane can pull away from the wall, producing plasmolysis.

Distinguish wall restraint from membrane transport: water direction first, then pressure and shape.

A plant cell in dilute solution becomes turgid rather than bursting because the wall pushes back as the vacuole expands.

A wall prevents unlimited swelling but does not stop osmosis or guarantee that a severely dehydrated cell survives.

Medical fluids are made isotonic with body tissues to avoid harmful net water gain or loss by cells.

An isotonic saline infusion replaces extracellular fluid without causing appreciable net movement of water into or out of red blood cells.

Isotonic means matched effective osmotic concentration relative to the tissue; it does not mean the solution has the same chemical composition as cytoplasm.

Water potential (ψw) is the potential energy of water per unit volume and is usually measured in kilopascals (kPa).

Absolute potential energy cannot be measured, so water potentials are expressed relative to pure water at atmospheric pressure and 20°C, which is assigned ψw = 0 kPa.

Adding solute makes water potential negative relative to pure water. Applying positive pressure can raise water potential. The numerical value predicts which region's water can release more potential energy by moving.

A solution at −300 kPa has lower water potential than pure water at 0 kPa; water has a tendency to move from the pure water toward the solution if a pathway exists.

Water potential is not solute concentration alone and is not an absolute store of energy. State the reference conditions and units.

The direction of net water movement is set by total water potential, not by one component in isolation.

A higher Ψ means water has a greater tendency to leave. Comparing total values incorporates both solute and pressure effects and explains why a concentrated but pressurized cell may not lose water.

Write both total Ψ values, compare them numerically, then state the direction and the condition for equilibrium.

Water moves from −0.2 MPa to −0.5 MPa, but not from a region at −0.8 MPa to −0.5 MPa; the latter direction is reversed.

‘Higher’ means numerically less negative; movement stops when the relevant potentials are equal or other forces intervene.

In cells with walls, total water potential is the sum of solute potential and pressure potential.

If ψs = −600 kPa and ψp = +100 kPa, then ψw = −600 + 100 = −500 kPa. Water tends to enter from a neighbouring region at −400 kPa.

Keep signs and units through the calculation. Pressure potential is not always zero or positive: tension in functioning xylem gives a negative value.

When plant tissue is bathed in hypotonic or hypertonic solution, water movement changes both solute potential and pressure potential until equilibrium or plasmolysis is reached.

A cell initially at ψs = −600 kPa and ψp = +300 kPa has ψw = −300 kPa. In an external solution below −300 kPa, water leaves, turgor falls and the internal potentials change.

Do not describe only solute concentration: in a walled cell, increasing pressure potential can stop net entry even while the cell sap remains more concentrated than the outside solution.