D2.3 Water potential

Solvation is the attraction of water molecules to ions or polar molecules, forming a hydrated environment around the solute.

Water is polar: its partial charges orient toward charged or polar regions. These attractions separate particles and keep them dispersed, which helps cells transport dissolved substances.

Check: solute charge or polarity; water orientation; particle separation; resulting solution.

When sodium chloride dissolves, oxygen ends of water face Na+ while hydrogen ends face Cl−, forming hydration shells.

Non-polar solutes are not automatically soluble in water; polarity and the relevant intermolecular attractions matter.

Water moves from higher water potential to lower water potential through a selectively permeable pathway.

The gradient is a difference in the tendency of water to leave a region. A membrane may let water cross while restricting solute, so net movement is osmosis rather than solute diffusion.

Predict direction by comparing the two water potentials before deciding which particles cross.

If a cell interior is −0.4 MPa and the surrounding solution is −0.1 MPa, water enters the cell because −0.1 MPa is higher.

Net movement can stop at equilibrium; individual water molecules still move in both directions.

Osmosis is the net movement of water across a partially permeable membrane in response to a water-potential difference.

Water crosses more readily than many solutes. As water enters or leaves, cell volume and internal pressure change until the gradient is reduced or opposing forces balance it.

Classify a case by membrane selectivity, initial water potentials, and whether volume rises or falls.

A cell placed in a solution with lower water potential loses water, so its mass decreases even though solute particles do not cross the membrane.

Do not say water moves toward ‘more solute’ without checking the full water potential and membrane conditions.

Water potential differences change plant-cell turgor and therefore tissue firmness and growth.

Water entering a vacuolated cell raises turgor pressure against the wall; water leaving reduces turgor and can pull the membrane from the wall. Tissue-level effects add across many cells.

Link condition to tissue outcome: external potential; water movement; turgor; visible change.

A leafy shoot in a concentrated solution loses water, cells become flaccid, and the leaves droop because turgor falls.

Wilting is not proof that every cell has died; restoring water can recover turgor if damage is limited.

A cell without a rigid wall changes volume when osmosis alters its water content.

The plasma membrane can deform, but it cannot provide the same mechanical restraint as a cell wall. Water entry raises internal pressure; excessive entry can rupture the membrane.

Use the membrane boundary and water direction to predict whether volume rises, falls, or risks lysis.

A red blood cell in a hypotonic solution takes in water and may haemolyse because no cell wall limits expansion.

The outcome also depends on solute concentrations, membrane permeability, and whether active regulation is available.

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 chosen so their effective water potential does not cause harmful net water movement across cell membranes.

If an infused solution is too dilute, cells gain water; if too concentrated, cells lose water. Matching osmotic conditions protects cell volume while solutes are delivered.

Evaluate a fluid by predicting water movement into or out of cells before considering its clinical use.

An isotonic saline infusion can replace extracellular fluid without making red blood cells swell or shrink appreciably.

‘Isotonic’ is relative to the cells and relevant solutes; filtration, metabolism, and patient condition can change the outcome.