D2.3 Water potential

Water potential explains osmosis, solute effects, plant tissue changes, cell swelling, plasmolysis and isotonic medical conditions in living systems and cells.

Learning objectives

D2.3.1Solvation with water• Water forms hydration shells around ions and polar solutes• Hydrogen bonding and charge attraction reduce free water movement D2.3.2Water movement• Water moves by osmosis across partially permeable membranes• It moves from hypotonic/lower solute solutions toward hypertonic/higher solute solutions D2.3.3Osmosis into/out of cells• Osmosis direction depends on internal and external solute concentration• Isotonic conditions have dynamic water movement but no net osmosis D2.3.4Changes in plant tissue• Plant tissue changes mass or length when placed in sucrose solutions• Percentage change graphs estimate isotonic/osmotic concentration D2.3.5Effects on cells without wall• Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions• Freshwater protists use contractile vacuoles to expel excess water D2.3.6Effects on cells with wall• Plant cells become turgid in hypotonic solutions as vacuoles swell• Hypertonic solutions cause flaccidity and plasmolysis from water loss D2.3.7Medical applications• Isotonic saline prevents harmful water gain or loss in body cells• IV fluids and transplant organ baths must match tissue osmotic concentration D2.3.8(HL)—Water potential• Water potential is potential energy of water per unit volume, measured in kPa• Pure water at standard conditions has water potential of 0 kPa D2.3.9(HL)—Movement from higher to lower potential• Water moves from higher water potential to lower water potential• Solutes lower water potential by restricting water molecule movement D2.3.10(HL)—Solute and pressure potential• Water potential equals solute potential plus pressure potential• Solute potential is zero or negative; pressure potential is often positive in walled cells D2.3.11(HL)—Water potential in plant tissue• Water entering plant cells increases pressure potential and dilutes solutes• Water leaving plant cells lowers pressure potential and makes solute potential more negative

Solvation is the interaction of a solvent with dissolved particles. In water, it produces an organized shell of water molecules around an ion or polar molecule.

Around a positive ion, the partially negative oxygen ends point inward.
Around a negative ion, the partially positive hydrogen ends point inward.
Around a polar molecule, water forms hydrogen bonds with exposed partial charges.

These attractions separate and stabilize solute particles, allowing them to remain dispersed in solution.

Polar water molecules orient around an ion and a polar solute to form hydration shells.

Hydrogen bonding and ion–dipole attraction hold some water molecules in hydration shells, reducing their freedom to move compared with water in a more dilute solution.

Solution	Solute particles	Water associated with solute	Tendency of water to leave
more dilute	fewer	smaller fraction	greater
more concentrated	more	larger fraction	lower

This molecular difference explains why water tends to move from a dilute solution toward a more concentrated solution when the two are connected through a membrane permeable to water.

The solute does not pull every water molecule across. Net movement results from unequal random movement in the two directions.

Osmosis is the net movement of water across a selectively permeable membrane from the side with lower solute concentration to the side with higher solute concentration.

The definition requires all three ideas:

the moving substance is water
the two sides differ in solute concentration
the membrane allows water to cross but restricts at least some solute

Say that water moves by osmosis. Osmosis is the process, not a verb performed by water.

Water crosses a partially permeable membrane from a hypotonic solution toward a hypertonic solution.

Tonicity is relative: a solution is described as hypotonic, isotonic or hypertonic to a particular cell or reference solution.

External solution relative to cytoplasm	External solute concentration	Net water movement
hypotonic	lower	into the cell
isotonic	equal	none overall
hypertonic	higher	out of the cell

To predict direction, compare solute concentration on the two sides first; then point the net water movement toward the side with the higher solute concentration.

Hypotonic does not mean that the cell contains little water. It describes the external solution's solute concentration relative to the cell.

In an isotonic condition, water crosses the membrane in both directions at equal rates, so there is no net osmosis and no sustained change in cell volume.

Molecular level	Whole-cell result
water molecules continue moving randomly both ways	inflow equals outflow
individual crossings still occur	average cell volume remains stable

“No net movement” does not mean “no movement.” It means that the two opposing rates balance.

more dissolved particles → more water associated in hydration shells → lower freedom and movement tendency of water → net water movement across a selective membrane toward that side

For any cell diagram:

identify inside and outside
compare solute concentration
confirm that water can cross the membrane
draw net water movement toward the higher solute concentration
if concentrations are equal, state dynamic equilibrium

Use	Avoid
water moves by osmosis	water osmosises
no net osmosis	no water movement
external solution is hypertonic to the cell	the cell is hypertonic, without a reference

A plant-tissue osmosis investigation asks how bathing-solution concentration changes tissue mass or length while other influential variables are controlled.

Prepare: make a range of accurately measured sucrose concentrations; cut tissue from the same plant source to equal starting dimensions; record each sample's initial mass or length.

Expose: immerse replicate samples at every concentration for the same time and at the same temperature.

Measure: remove each sample, blot it with the same pressure and duration, then record final mass or length.

Control or design choice	Why it matters
same tissue source and starting dimensions	limits biological and surface-area variation
same solution volume, time and temperature	keeps exposure and movement rate comparable
uniform blotting	prevents surface liquid from being counted as tissue mass
replicates at every concentration	reveals variation and supports a reliable mean

percentage change = ((final value − initial value) ÷ initial value) × 100

A potato cylinder changes from 4.00 g to 4.36 g.

change = 4.36 − 4.00 = 0.36 g

percentage change = (0.36 ÷ 4.00) × 100 = +9.0%

Result	Interpretation
positive percentage	tissue gained water overall
zero percentage	no detected net change
negative percentage	tissue lost water overall

Plot mean percentage change on the y-axis against sucrose concentration on the x-axis. Percentage change normalizes samples that did not begin with exactly the same mass or length.

The sucrose concentration where the fitted relationship crosses 0% change estimates the concentration isotonic with the tissue: water still moves, but there is no net osmosis.

Judge the estimate using the data, not only the drawn line:

use replicate means rather than one sample
inspect spread or error bars
interpolate only within the measured concentration range
report appropriate precision rather than extra decimal places

A zero mean can hide variation among samples. It estimates the tissue's isotonic point under these conditions; it is not proof that every cell had identical concentration.

Mean percentage change in plant-tissue mass falls as sucrose concentration increases and crosses zero at the estimated isotonic concentration.

External condition	Net water movement	Animal-cell outcome
hypotonic	into cell	swelling; possible lysis if membrane tension becomes too high
isotonic	balanced	stable average volume
hypertonic	out of cell	shrinking and crenation

The plasma membrane can stretch only a limited amount and provides little resistance to internal hydrostatic pressure. Without a rigid wall, continued water entry can rupture it.

A wall-less cell swells and may lyse in a hypotonic solution, crenates in a hypertonic solution, while a freshwater protist expels excess water.

Freshwater is usually hypotonic to a protist's cytoplasm, so water enters continuously by osmosis even when the organism is functioning normally.

Entry: water moves into the cell by osmosis because freshwater has higher water potential than the cytoplasm.

Collection: excess water enters the contractile-vacuole system and the vacuole fills.

Expulsion and reset: the filled vacuole connects to the cell surface and contracts, releasing water outside; the collection cycle then begins again.

Osmotic entry is passive, but vacuole filling and expulsion require cellular work. The vacuole does not stop osmosis; it balances its consequence by active water removal.

External condition	Water and pressure change	Cell state
hypotonic	water enters; vacuole expands; wall pressure rises	turgid
isotonic or mildly water-losing	little turgor pressure	flaccid
strongly hypertonic	water leaves; protoplast shrinks from wall	plasmolysed

The cellulose wall limits expansion. Its resistance creates turgor pressure, protects the membrane from bursting and helps support non-woody plant tissue.

During plasmolysis the plasma membrane and cytoplasm withdraw from the wall; the cell wall itself does not shrink away.

Plant cells are shown as turgid, flaccid and plasmolysed, with changes in vacuole volume and membrane position.

External condition	Cell without wall	Cell with wall
hypotonic	swells; may lyse	becomes turgid as wall resists expansion
isotonic	stable average volume	commonly flaccid because little pressure develops
hypertonic	shrinks or crenates	loses turgor; may plasmolyse

The direction of net water movement follows the same concentration difference in both cell types. The difference is the mechanical response after water moves.

Use lysis and crenation for wall-less animal cells; use turgid, flaccid and plasmolysed for plant cells.

Application	Why isotonicity matters
intravenous replacement fluid	avoids widespread red-cell swelling, lysis or crenation
solution carrying an injected medicine	prevents the solvent from imposing a harmful osmotic gradient
transplant-organ bath	limits cell-volume damage while the organ is outside the body

The solution should match the tissue's effective osmotic concentration, not reproduce every molecule in cytoplasm. Sterile 0.9% sodium chloride is commonly used because it is approximately isotonic with human extracellular fluid.

Isotonic does not mean chemically identical. It means that the solution does not cause harmful sustained net water gain or loss by the cells.

Red blood cells swell, remain stable or crenate in hypotonic, isotonic or hypertonic fluids, and a transplant organ is protected in an isotonic bath.

solution concentration → net water movement → change in tissue mass, length, cell volume or pressure → observable cell or tissue outcome

Evidence	What it supports
positive plant-tissue percentage change	bathing solution was hypotonic to the tissue
zero-change graph intercept	estimated isotonic concentration
animal-cell lysis or crenation	large water gain or loss without wall resistance
plant-cell turgor or plasmolysis	water gain or loss modified by wall mechanics
stable cells in a medical bath	solution is effectively isotonic with the tissue

First determine where water moves; then use cell structure to predict the consequence. The same osmotic rule explains a graph intercept, a red blood cell, a plant cell and the choice of a medical fluid.

Water potential (Ψw) is the potential energy of water per unit volume, expressed in pressure units, usually kilopascals (kPa). It indicates the tendency of water to move.

Pure water at standard atmospheric pressure and 20°C is assigned Ψw = 0 kPa. Dissolved solute lowers water potential, so an unpressurized solution normally has a negative value.

A value closer to zero is higher. For example, −100 kPa is higher water potential than −500 kPa.

Pure water is placed at zero kilopascals and increasingly concentrated solutions at progressively lower water potentials.

When water can cross between two regions, net movement is from the region with higher water potential to the region with lower water potential.

Case	Side A	Side B	Higher Ψw	Net water movement
1	0 kPa	−300 kPa	A	A → B
2	−100 kPa	−500 kPa	A	A → B
3	−700 kPa	−250 kPa	B	B → A
4	−400 kPa	−400 kPa	equal	no net movement

Compare the signed numbers on a number line. The value farther to the right—closer to zero—is higher, even when both values are negative.

Solutes lower Ψw because water–solute interactions reduce the potential energy and movement freedom of water relative to pure water.

Ψw = Ψs + Ψp

water potential = solute potential + pressure potential

Component	Physical cause	Usual sign	Effect when magnitude increases
solute potential, Ψs	dissolved particles interacting with water	0 or negative	more solute makes Ψs more negative
pressure potential, Ψp	hydrostatic pressure on water	often positive in a turgid walled cell	more pressure raises Ψw

Pressure potential is not always positive: tension in xylem can create negative Ψp. Use the stated biological context rather than memorizing one sign for every system.

The water-potential equation is paired with solute lowering potential and wall pressure raising potential in a plant cell.

A plant cell has Ψs = −700 kPa and Ψp = +300 kPa. It is placed in a solution with Ψs = −250 kPa and Ψp = 0 kPa. Predict the initial net movement of water.

Cell: Ψw = −700 + 300 = −400 kPa.
Solution: Ψw = −250 + 0 = −250 kPa.
−250 kPa is higher than −400 kPa.
Water therefore moves from the solution into the cell.

A complete answer shows both totals with units, compares the signed values and states the direction. Comparing Ψs alone would ignore the cell's pressure potential and can give the wrong conclusion.

In a hypotonic external solution:

water enters → vacuole expands → cell contents press harder on the wall → Ψp rises; solutes are diluted → Ψs becomes less negative; total Ψw rises

In a hypertonic external solution:

water leaves → cell pressure falls → Ψp decreases; remaining solutes become more concentrated → Ψs becomes more negative; total Ψw falls

Water movement changes solute concentration, not necessarily the number of solute particles. The amount of water in the cell has changed.

Water entry expands a plant-cell vacuole and raises pressure, while water exit lowers pressure and concentrates solutes.

Net water movement changes the receiving or losing cell in ways that reduce the original water-potential difference.

Initial condition	Changes inside plant cell	How the gradient is reduced
external Ψw is higher	water enters; Ψp rises and Ψs becomes less negative	cell Ψw rises toward external Ψw
external Ψw is lower	water leaves; Ψp falls and Ψs becomes more negative	cell Ψw falls toward external Ψw

A turgid cell can reach equilibrium because positive wall pressure raises its Ψw.
In a strongly hypertonic solution, pressure can fall to approximately zero and the protoplast may plasmolyse.
At equal Ψw, water still crosses both ways but net movement is zero.

This is why total Ψw—not solute concentration alone—is needed for walled cells: pressure develops as part of the response and changes the final equilibrium.

solute interactions and pressure set Ψw → water crosses a selective membrane from higher Ψw to lower Ψw → water movement changes cell volume, Ψs and Ψp → the gradient decreases toward equilibrium

For an HL prediction:

calculate or identify total Ψw on both sides
compare signed values correctly
move water from higher Ψw to lower Ψw
predict how cell Ψs and Ψp will change
state the structural outcome and the equilibrium condition

Qualitative language	Water-potential language
hypotonic external solution	external Ψw is higher than cell Ψw
hypertonic external solution	external Ψw is lower than cell Ψw
isotonic / no net osmosis	total Ψw is equal across the membrane

Solute concentration predicts many simple cases, but total water potential is the general rule. In walled cells, pressure is part of both the response and the final balance.

2 marks

An experiment was carried out to determine the change in mass due to osmosis. Potato cubes were soaked in different concentrations of sodium chloride for 12 hours. The table shows the mean data for the mass of potato before and after the 12-hour period.

	Mean mass / g ( $\boldsymbol{\pm} \mathbf{0 . 0 1 ~ g} \boldsymbol{)}$
Sodium chloride concentration / mol dm ${ }^{-3}$	Original	Final
0.00	1.02	1.37
0.10	1.00	1.25
0.20	1.01	1.08
0.30	1.04	1.05
0.40	0.98	0.80
0.50	1.03	0.60
0.60	1.00	0.34

Outline the conditions necessary for osmosis to occur.

3 marks

Explain the reason that animal cells and tissues under investigation must be maintained in solutions with the same osmolarity.

2 marks

Student osmosis experiments often involve putting plant tissue such as potato cylinders in several salt solutions of different concentrations and measuring the mass before and after immersion. Outline how data collected from such an experiment could be used to estimate the osmolarity of the plant tissues.

3 marks

The table shows the percentage of red blood cells that have ruptured (hemolysis) when placed in sodium chloride solutions of different molar concentrations.

NaCl molarity / M	Hemolysis / \%
0.000	100
0.075	67
0.150	2
0.460	2

Explain the effect of placing red blood cells in distilled water (0.000 M NaCl).

7 marks

Explain the process of osmosis with reference to its effects on plant cells.

4 marks

Explain the need for isotonic conditions in human blood plasma and tissue fluid.

1 mark

The water potential of a plant cell is -0.24 kPa . If the pressure potential of the cell is 0.46 kPa , what is the solute potential of that cell?

\Psi_{\mathrm{w}}=\Psi_{\mathrm{s}}+\Psi_{\mathrm{p}}

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—D3.1 ReproductionRead

D2.3 Water potential

Learning objectives

Water Polarity Builds Hydration Shells around Solutes

Dissolved Solutes Reduce the Freedom of Water Molecules

Osmosis Requires Water, a Gradient and a Selective Membrane

Tonicity Compares an External Solution with a Cell

Isotonic Solutions Are in Dynamic Equilibrium

Predict Osmosis from Solute Interaction to Net Movement

A Fair Plant-Tissue Test Is Built before It Is Measured

Percentage Change Makes Different Samples Comparable

The Zero-Change Intercept Estimates the Isotonic Concentration

Cells without Walls Cannot Resist Large Volume Changes

Contractile Vacuoles Remove a Continuous Osmotic Inflow

A Plant Cell Wall Converts Water Entry into Turgor

The Same Osmosis Produces Different Outcomes with and without a Wall

Medical Fluids Protect Cells by Matching Effective Osmotic Concentration

Osmosis Links Measurements, Cell Structure and Safe Fluid Choice

Water Potential Places Water on a Relative Energy Scale

Water Moves from Higher Ψw to Lower Ψw

Solute and Pressure Make Independent Contributions to Ψw

Calculate Both Total Potentials before Predicting Movement

Water Entry and Exit Change Both Plant-Cell Potentials

Changing Cell Potential Reduces the Gradient toward Equilibrium

Use One Water-Potential Model from Molecules to Tissues

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