IB Biology · Theme D Continuity and change · Cells
D2.3 Water potential
Polar water molecules surround ions and polar solutes, which is why things dissolve. Water moves by osmosis from the solution with less solute to the one with more. Wall-less cells swell or shrink; walled cells turn turgid or plasmolyse; HL adds the numbers.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
· How these pages are made
Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D2.3.1 How water dissolves ions and polar molecules
In solvation, solute particles are separated and surrounded by water molecules.
Water is polar: partial negative oxygen, partial positive hydrogens.
Na⁺ ions attract the oxygen end of water; Cl⁻ ions attract the hydrogen ends.
Polar molecules such as glucose dissolve by hydrogen bonding at their hydroxyl groups.
Non-polar molecules such as triglycerides have no such groups, so water cannot bond to them.
Students often think ions dissolve by forming hydrogen bonds with water. In fact ions are held by attraction between their charge and water's partial charges.
Students often think dissolving breaks a solute into atoms. In fact the ions or molecules stay intact; they are only separated and surrounded.
D2.3.2 Osmosis: water moves towards the higher solute concentration
Osmosis is net water movement across a partially permeable membrane.
Water moves from lower solute concentration to higher solute concentration.
The lower-solute solution is hypotonic; the higher-solute one is hypertonic.
Two solutions with equal solute concentration are isotonic: no net movement.
Students often say water moves from concentrated to dilute. In fact state it in solute terms: from hypotonic to hypertonic.
Students often think the solute crosses to even things out. In fact the solute usually cannot cross; water moves instead.
D2.3.3 Predicting the direction of water movement for a cell
In a hypotonic environment, water enters the cell.
In a hypertonic environment, water leaves the cell.
In an isotonic environment there is dynamic equilibrium: equal flow both ways, no net change.
Osmosis is passive; a cell cannot pump water to choose the direction.
Students often think water stops moving in an isotonic solution. In fact molecules cross both ways at equal rates.
Students often think cells pump water in or out as needed. In fact water follows the gradient on its own.
D2.3.4 Finding a tissue's isotonic concentration, and judging the data
Bathe tissue samples in a range of concentrations; measure change in mass or length.
Plot percentage change against concentration; the isotonic concentration is where the line crosses zero.
It usually lies between two tested concentrations, not at one of them.
Standard deviation shows spread of repeats; standard error shows how reliable the mean is.
Standard error estimates how close the sample mean is to the true mean; it shrinks with more repeats and is drawn as error bars.
Students often think the isotonic value must be one of the concentrations tested. In fact it is read from where the plotted line crosses zero.
Students often think standard error measures mistakes. In fact it estimates how close the sample mean is to the true mean.
D2.3.5 Cells without walls swell, burst, shrink or crenate
In a hypotonic medium a wall-less cell swells and can burst.
In a hypertonic medium it shrinks and its membrane crinkles: crenation.
Freshwater unicells such as Paramecium use a contractile vacuole to expel incoming water.
Multicellular organisms keep tissue fluid isotonic so their cells neither swell nor shrink.
Students often think an animal cell swells until tight, then stops, like a plant cell. In fact with no wall it keeps taking in water and can burst.
Students often think the contractile vacuole stores water. In fact it expels water that keeps entering by osmosis.
D2.3.6 Cells with walls become turgid or plasmolyse
In a hypotonic medium water enters until the wall resists: turgor pressure builds.
The cell is then turgid; turgor makes soft plant tissue firm.
In a hypertonic medium the protoplast shrinks and the membrane pulls from the wall: plasmolysis.
The wall keeps its shape; external solution fills the gap.
Students often think a plasmolysed cell crumples wall and all. In fact the wall holds; only the protoplast shrinks away from it.
Students often think a plant cell in pure water bursts. In fact the wall resists, and turgor pressure stops net uptake.
D2.3.7 Isotonic solutions in medicine
Intravenous fluid such as 0.9% sodium chloride is isotonic with blood plasma.
So blood cells neither burst nor crenate; pure water would make them burst.
Organs for transplant are bathed in isotonic solution to prevent swelling or shrinkage.
Students often think pure water is the safest drip. In fact it is hypotonic to plasma and would burst blood cells.
Students often think the salt is there to replace lost salt. In fact it makes the fluid isotonic with plasma.
D2.3.8 Water potential: energy per unit volume, measured against pure water HL
Water potential (ψw) is the potential energy of water per unit volume, in kPa.
Absolute values cannot be measured; pure water at atmospheric pressure and 20 °C is zero.
Dissolving solutes lowers water potential, so solutions have negative values.
Students often think adding solute makes water potential positive. In fact it lowers it below zero.
Students often think zero means no energy at all. In fact zero is a chosen reference, not an absolute.
D2.3.9 Water moves from higher to lower water potential HL
Water moves down a water potential gradient, losing potential energy as it goes.
No energy input from the cell is needed.
Net movement stops when both regions have the same water potential.
−300 kPa is higher than −900 kPa, so water moves from −300 towards −900.
Students often think −900 kPa is higher because 900 is bigger. In fact −300 is closer to zero and so higher.
Solute potential (ψs) is zero for pure water and more negative with more solute.
Pressure potential (ψp) is usually positive inside cells, as the wall presses back.
In xylem carrying sap under tension, pressure potential is negative.
Add them: ψs of −700 kPa and ψp of +200 kPa give ψw of −500 kPa.
Students often add the magnitudes and get −900 kPa. In fact ψp is positive: (−700) + (+200) = −500 kPa.
Students often think pressure potential is always positive. In fact it is negative in xylem under tension.
D2.3.11 Explaining turgor and plasmolysis with potentials HL
In pure water, ψp rises until ψs + ψp reaches zero: full turgor.
The cell's water potential now equals the water outside, so net uptake stops.
In a hypertonic solution water leaves; at incipient plasmolysis ψp has fallen to zero.
Then ψw equals ψs; a lower outside potential pulls the membrane off the wall.
Students often think the wall physically blocks more water at full turgor. In fact the gradient is gone: ψw inside equals zero outside.
Students often think a plasmolysed cell has negative pressure potential. In fact the wall exerts no pressure, so ψp is zero.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Sodium chloride dissolves when it is placed in water. Which statement describes the attractions that hold the ions in solution?
Answer and reasoning
Na⁺ ions are attracted to the partially negative oxygen atoms of water molecules, and Cl⁻ ions to the partially positive hydrogen atoms. — Water is polar. Each positively charged Na⁺ ion is attracted to the partially negative oxygen atoms of surrounding water molecules, and each negatively charged Cl⁻ ion to the partially positive hydrogen atoms, so the ions are separated and held in solution.
Hydrogen bonds form between each ion and the water molecules that surround it, just as they form between water molecules. — A student who uses hydrogen bonding to explain every property of water picks this. Hydrogen bonds form between water and polar molecules such as glucose; an ion is held in solution by the attraction between its charge and the partial charges on water.
Both kinds of ion are attracted to the oxygen end of the water molecules, because that is the end that carries a charge. — A student who has only seen a hydrated sodium ion drawn picks this. The hydrogen atoms of water carry partial positive charges, so it is the hydrogen end that is attracted to a negatively charged chloride ion.
Water molecules react chemically with the sodium chloride, breaking it down into separate sodium and chlorine atoms. — A student who thinks of dissolving as a chemical change picks this. No reaction occurs: the ions stay as Na⁺ and Cl⁻ and are simply separated and surrounded by water molecules.
2 Two sucrose solutions, 0.2 mol dm⁻³ and 0.6 mol dm⁻³, are separated by a membrane that is permeable to water but not to sucrose. Which statement is correct?
Answer and reasoning
Water moves by osmosis from the 0.6 mol dm⁻³ solution into the 0.2 mol dm⁻³ solution, down the concentration gradient. — A student who applies the diffusion rule 'high to low' to the solution rather than the solute picks this. Water moves from the solution with the lower solute concentration to the solution with the higher solute concentration.
Water moves by osmosis from the 0.2 mol dm⁻³ solution, which is hypertonic, into the 0.6 mol dm⁻³ solution, which is hypotonic. — A student who has swapped the prefixes picks this. The direction is right, but the 0.2 mol dm⁻³ solution has the lower solute concentration and is hypotonic; the 0.6 mol dm⁻³ solution is hypertonic.
Water moves by osmosis from the hypotonic 0.2 mol dm⁻³ solution into the hypertonic 0.6 mol dm⁻³ sucrose solution. — Water moves from the less concentrated to the more concentrated solution. The 0.2 mol dm⁻³ solution has the lower solute concentration, so it is hypotonic; the 0.6 mol dm⁻³ solution is hypertonic and gains water.
Sucrose moves from the 0.6 mol dm⁻³ solution into the 0.2 mol dm⁻³ solution until the two solutions are isotonic. — A student who expects the solute to equalise the concentrations picks this. The membrane is not permeable to sucrose, which is exactly why it is water that moves.
3 A cell is placed in a solution that is isotonic with its cytoplasm. Which statement describes the movement of water across its membrane?
Answer and reasoning
Water molecules cross the membrane in both directions at equal rates, so there is a dynamic equilibrium with no net movement. — In an isotonic environment water molecules continue to enter and leave the cell, but at the same rate. This is a dynamic equilibrium: there is no net movement of water, not an absence of movement.
Water molecules stop crossing the membrane in either direction, because there is no concentration gradient to drive them. — A student who drops the word 'net' picks this. Individual water molecules move randomly through the membrane whatever the concentrations; the gradient only decides whether there is a net direction.
Solute molecules cross the membrane in both directions, keeping the concentrations on the two sides of the membrane equal. — A student who thinks of concentration differences as being managed by the solute picks this. The membrane is partially permeable, and the question concerns water, which crosses in both directions at equal rates.
The cell stops pumping water across the membrane, because it no longer needs to use energy to adjust its water content. — A student who believes the cell actively controls osmosis picks this. Osmosis is passive and uses no energy; water crosses the membrane continuously in both directions in an isotonic solution.
4 Five potato cylinders were bathed in 0.4 mol dm⁻³ sucrose. Their percentage changes in mass had a mean of −3.0%, a standard deviation of 1.0% and a standard error of 0.45%. What does the standard error indicate?
Answer and reasoning
The average size of the mistakes made in weighing the cylinders, which would be zero if the balance had been used correctly. — A student who reads 'error' as 'mistake' picks this. Standard error reflects natural variation between samples, not faults in technique; perfectly weighed cylinders that vary in their response still give a standard error above zero.
The spread of the five individual percentage changes around their mean, which is the same thing that the standard deviation shows. — A student who treats the two statistics as one picks this. The spread of the individual values is described by the standard deviation of 1.0%; the standard error is a property of the mean, which is why it is smaller.
How close the sample mean of −3.0% is likely to be to the true mean; it would decrease if more cylinders were measured. — Standard error indicates the precision of the sample mean as an estimate of the true mean for this concentration. It is smaller than the standard deviation and falls as the number of repeats rises, and it can be shown graphically as error bars.
The maximum and minimum percentage changes recorded, so every cylinder lost between 2.55% and 3.45% of its mass. — A student who reads error bars as a range picks this. Standard error is not the limits of the data: with a standard deviation of 1.0%, individual cylinders lie well outside 0.45% of the mean.
5 Which statement about the tissue fluid that bathes the cells of a mammal is correct?
Answer and reasoning
It is kept slightly hypotonic to the cytoplasm of the cells, so that they are kept fully hydrated by a steady inflow of water. — A student who thinks more water is always better for cells picks this. A steady inflow of water into cells that lack walls would make them swell and eventually burst.
It is kept isotonic with the cytoplasm of the cells, so that they neither swell and burst nor shrink through net osmosis. — Cells without walls are damaged by net water movement in either direction. Multicellular organisms maintain isotonic tissue fluid so that there is a dynamic equilibrium and no harmful change in cell volume.
It is kept isotonic with the cytoplasm of the cells, so that no water molecules cross their membranes at all. — A student who thinks isotonic means no movement picks this. Water molecules cross the membranes continuously; in an isotonic fluid they do so at equal rates in each direction, giving no net movement.
Its concentration does not need regulating, because cells that take in excess water become turgid and then stop. — A student who expects animal cells to become turgid picks this. Turgidity depends on a cell wall; animal cells have none, so excess water makes them swell and burst.
6 A strip of onion epidermis is mounted in a concentrated sucrose solution and viewed under a microscope. Which statement describes what happens to its cells?
Answer and reasoning
Water leaves each cell by osmosis and the whole cell, wall included, shrinks and crumples inward, just as an animal cell crenates. — A student who transfers crenation to plant cells picks this. The rigid cellulose wall keeps its shape; only the protoplast inside it shrinks.
Water leaves each cell by osmosis and an empty space forms between the wall and the membrane, because the wall lets nothing through. — A student who treats the wall as a barrier picks this. The cell wall is freely permeable, so the space that opens up is filled by the sucrose solution.
Water leaves each cell by osmosis, the protoplast shrinks and its membrane pulls away from the wall: the cells are plasmolysed. — The concentrated sucrose solution is hypertonic to the cell sap, so water leaves by osmosis. The protoplast shrinks and the membrane pulls away from the wall, which keeps its shape: this is plasmolysis.
Water enters each cell by osmosis, because the concentrated solution is hypotonic to them, and the cells become turgid. — A student who has swapped hypo- and hyper- picks this. A concentrated solution has the higher solute concentration, so it is hypertonic and water leaves the cells.
7 A kidney removed for transplantation is bathed in a solution while it is stored. Which type of solution is used, and why?
Answer and reasoning
Sterile distilled water, so that the cells stay well hydrated and no solutes can enter the organ before it is transplanted. — A student who regards pure water as the safest fluid picks this. Distilled water is hypotonic to the cells, which would take in water by osmosis, swell and burst.
An isotonic solution, so that the kidney's cells neither gain nor lose water by net osmosis while the organ is stored. — A solution with the same solute concentration as the cytoplasm of the kidney's cells prevents net osmotic movement of water in either direction, so the cells are not damaged by swelling or shrinkage before transplantation.
A slightly hypotonic solution, so that the cells are kept fully hydrated by a steady inward flow of water. — A student who thinks extra water is good for cells picks this. A hypotonic solution would cause a net inflow of water into cells that lack walls, and they would swell and burst.
A hypotonic solution, so that the cells take in water and become turgid, keeping the organ firm during storage. — A student who expects animal cells to become turgid picks this. Turgidity needs a cell wall; the cells of a kidney have none, so water entering them causes swelling and bursting, not firmness.
8 Which statement correctly describes water potential and the scale on which it is measured? HL
Answer and reasoning
It is the absolute potential energy of water per unit volume, so pure water at 20°C has a value of zero because its molecules have no potential energy. — A student who reads zero as 'none' picks this. The absolute potential energy of water cannot be measured; zero is a chosen reference point, and values are given relative to it.
It is the potential energy of water per unit volume, which is zero for pure water and rises to positive values as more and more solute is dissolved in it. — A student who expects 'more solute' to give a bigger number picks this. Dissolving solute lowers water potential, so solutions at atmospheric pressure have negative values.
It is the potential energy of water per unit volume, measured in kPa relative to pure water at atmospheric pressure and 20°C, which is set at zero. — Water potential is the potential energy of water per unit volume. Because its absolute quantity cannot be measured, values are expressed relative to pure water at atmospheric pressure and 20°C, defined as zero, and the units are usually kilopascals.
It is the concentration of water in a solution, measured in mol dm⁻³, which is highest in pure water and is lower in every other solution. — A student who carries the 'water concentration' model into Higher Level picks this. Water potential is an energy per unit volume, measured in kPa; it also takes account of pressure, which a concentration cannot.
9 Two adjacent plant cells have water potentials of −300 kPa (cell A) and −900 kPa (cell B). Which statement describes the net movement of water between them? HL
Answer and reasoning
From B to A, because −900 kPa is the larger number, so the water in B has the greater potential energy. — A student who compares magnitudes and ignores the sign picks this. −900 kPa is further below zero than −300 kPa, so B has the lower water potential and its water has less potential energy.
There is no net movement, because both values are negative, so neither cell's water has any potential energy. — A student who treats the scale as absolute picks this. Negative values are relative to pure water; the water in both cells has potential energy, and there is more of it in A than in B.
From A to B only if the cells use energy to pump it, because water cannot move down a gradient by itself. — A student who thinks water movement is active picks this. Water moves down a water potential gradient passively, without any input of energy from the cells.
From A to B, because water moves from higher to lower water potential and loses potential energy. — −300 kPa is the higher water potential, so cell A's water has more potential energy per unit volume than cell B's. Water moves from higher to lower water potential, losing potential energy as it does so, until the two are equal.
10 Which statement about the solute potential and pressure potential of water in plant structures is correct? HL
Answer and reasoning
Pressure potential is positive throughout the plant, including in the xylem vessels, because a pressure lower than zero cannot exist in a liquid. — A student who has only met pushing pressures picks this. A column of xylem sap being pulled upward is under tension, and tension is a negative pressure potential.
Solute potential is zero for pure water and negative for any solution; pressure potential is usually positive in cells but negative in xylem sap under tension. — Solute potential ranges from zero downwards. Pressure potential is generally positive inside cells, where the wall presses on the contents, but in xylem vessels the sap is pulled up under tension, which is a negative pressure.
Solute potential is positive in cell sap, because dissolving solutes adds potential energy to the water, and it is negative only in xylem sap. — A student who expects solute to raise the value picks this. Solutes lower the free energy of water, so solute potential is negative in cell sap and in every other solution.
Pressure potential makes no contribution to the water potential of a cell; only the solute potential of the cell sap determines its value. — A student who keeps the Standard Level solute-only model picks this. For a cell with a wall, ψw = ψs + ψp, and the pressure potential can be several hundred kilopascals.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
11 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A student writes: "Glucose and sodium chloride both dissolve in water because water molecules form hydrogen bonds with them." How should this claim be evaluated?
Answer and reasoning
Correct for both: hydrogen bonds are the attraction that holds every kind of dissolved particle in solution, whether it is a molecule or an ion. — A student who treats hydrogen bonding as the explanation for all solvation picks this. Ions do not form hydrogen bonds; they are held by the attraction between their full charge and the partial charges of polar water molecules.
Correct for glucose, whose hydroxyl groups form hydrogen bonds with water, but not for sodium chloride, whose ions are attracted to water's partial charges. — Glucose is a polar molecule with –OH groups that hydrogen-bond to water, so the claim is right for it. Sodium chloride dissolves because Na⁺ and Cl⁻ ions are attracted to the oppositely charged ends of polar water molecules, which is a different kind of attraction.
Incorrect for both: glucose dissociates into ions when it dissolves, and ions are held in solution by attraction to the partial charges on water, not by hydrogen bonds. — A student who believes only charged particles can dissolve picks this. Glucose molecules stay intact in solution; their polar hydroxyl groups form hydrogen bonds with water, so the claim is right for glucose.
Incorrect for both: water dissolves each of these solutes by reacting with it and breaking its molecules or ions apart into smaller particles. — A student who regards dissolving as a chemical reaction picks this. Solvation is physical: glucose molecules and sodium and chloride ions are surrounded by water molecules but are not broken down or changed.
2 The cytoplasm of a red blood cell has a solute concentration equivalent to 0.9% sodium chloride. A red blood cell is placed in a 3% sodium chloride solution. What is the net movement across its membrane, and what happens to the cell?
Answer and reasoning
Water moves into the cell and it swells, because water moves from the more concentrated solution to the less concentrated cytoplasm. — A student who moves water down the solution's concentration gradient picks this. Water moves from the lower solute concentration (the cytoplasm) to the higher solute concentration (the 3% solution), so it leaves the cell and the cell shrinks.
Water moves into the cell and it swells, because the 3% solution is hypotonic to the cytoplasm of the cell. — A student who has swapped hypo- and hyper- picks this. The 3% solution has the higher solute concentration, so it is hypertonic to the cytoplasm; water moves out of the cell and it shrinks.
Sodium chloride moves into the cell until the solute concentrations on the two sides are equal, and the cell's volume is unchanged. — A student who expects the solute to equalise the concentrations picks this. The cell membrane is partially permeable: the net movement that reduces the difference is water leaving the cell, which therefore shrinks.
Water moves out of the cell, because the solution is hypertonic to the cytoplasm, so the cell shrinks and becomes crenated. — The 3% solution has a higher solute concentration than the 0.9%-equivalent cytoplasm, so it is hypertonic to the cell. Net movement of water by osmosis is out of the cell into the hypertonic solution; the cell, which has no wall, shrinks and its membrane develops the crinkled outline called crenation.
3 Potato cylinders were bathed for 24 hours in sucrose solutions and their mean percentage change in mass recorded: 0.0 mol dm⁻³, +8.0%; 0.2 mol dm⁻³, +2.5%; 0.4 mol dm⁻³, −3.0%; 0.6 mol dm⁻³, −8.5%; 0.8 mol dm⁻³, −12.0%. Which conclusion is supported by these data?
Answer and reasoning
The isotonic concentration is exactly 0.2 mol dm⁻³, because that is the tested concentration with the change in mass closest to zero. — A student who thinks the answer must be one of the tested values picks this. Mass still increased by 2.5% at 0.2 mol dm⁻³, so that solution was hypotonic to the tissue; the isotonic concentration lies between 0.2 and 0.4 mol dm⁻³.
The potato cells are isotonic with sucrose of about 0.3 mol dm⁻³, the concentration at which the change in mass would be zero. — Mass rose in 0.2 mol dm⁻³ and fell in 0.4 mol dm⁻³, so the line joining the points crosses zero between them. Interpolating between +2.5% and −3.0% gives about 0.29 mol dm⁻³: the concentration isotonic with the cell contents.
The 0.6 mol dm⁻³ solution was hypotonic to the potato cells, which is why the cylinders bathed in it lost mass. — A student who has swapped the prefixes picks this. The cylinders lost mass because water left their cells, which means the solution had the higher solute concentration: it was hypertonic to the cells.
The cylinders changed in mass because sucrose diffused across their membranes until its concentration was equal on each side. — A student who expects the solute to equalise the concentrations picks this. The cell membranes are partially permeable and sucrose cannot cross them; the changes in mass are water entering the cells by osmosis in the hypotonic solutions and leaving them in the hypertonic ones.
4 Red blood cells are placed in distilled water. What happens to them, and why?
Answer and reasoning
Water enters by osmosis and the cells swell until their membranes are stretched taut, and then they remain turgid. — A student who expects animal cells to behave like plant cells picks this. A cell membrane cannot resist the pressure that a cell wall resists, so the swelling continues until the cell bursts.
Water leaves by osmosis, because the cytoplasm is more concentrated than the distilled water, and the cells shrink. — A student who moves water from the more concentrated side to the less concentrated side picks this. Water moves towards the higher solute concentration, so it enters the cells.
Water leaves by osmosis, because distilled water is hypertonic to the cytoplasm, and the cells crenate. — A student who has swapped the prefixes picks this. Distilled water contains no solute, so it is hypotonic to the cytoplasm; crenation happens in a hypertonic medium such as concentrated salt solution.
Water enters by osmosis and the cells swell and burst, because no wall resists their expansion. — Distilled water is hypotonic to the cytoplasm, so water enters the cells by osmosis. Red blood cells lack a cell wall, so nothing resists the increase in volume and the cells swell and burst.
5 Paramecium is a unicellular organism that lives in fresh water. Its contractile vacuole repeatedly fills with water and then expels it from the cell. Why is this necessary?
Answer and reasoning
The cytoplasm is hypertonic to fresh water, so water continually enters by osmosis and must be expelled to prevent swelling and bursting. — Fresh water has a much lower solute concentration than the cytoplasm, so water enters the cell by osmosis all the time. Paramecium has no cell wall, and the contractile vacuole removes the water so that the cell does not swell and burst.
The vacuole stores the water the cell absorbs, keeping a reserve that the cell can draw on when the pond water becomes scarce. — A student who associates vacuoles with storage picks this. The contractile vacuole expels water from the cell; its job is removal of excess water, not storage.
The vacuole takes in water to replace the water the cell loses by osmosis to the fresh water, which is more dilute than the cytoplasm. — A student who moves water from the more concentrated side to the less concentrated side picks this. Water moves into the more concentrated cytoplasm, so the cell gains water and the vacuole has to remove it.
The vacuole is the pump by which the cell moves water in by osmosis, using energy whenever the cytoplasm needs more water. — A student who thinks the cell controls osmosis actively picks this. Osmosis is passive and brings water in unasked; the energy used by the contractile vacuole goes into pushing that water out.
6 A plant cell is placed in pure water. What happens to it?
Answer and reasoning
Water enters by osmosis and the cell swells until it bursts, like an animal cell in pure water. — A student who transfers the fate of a red blood cell to a plant cell picks this. The cellulose wall resists expansion, so pressure builds up inside and net entry of water stops before the cell can burst.
Solutes leave the cell into the pure water until the cell sap is as dilute as the water outside. — A student who expects the solute to equalise the concentrations picks this. The membrane is partially permeable; it is water that moves, entering the cell.
The cell pumps water in, using energy, until it is full and then it switches the pumping off. — A student who believes the cell drives osmosis actively picks this. Osmosis is passive: water enters down the gradient, and it is the pressure from the wall, not a decision by the cell, that stops net uptake.
Water enters by osmosis and the cell becomes turgid, its wall resisting further expansion. — Pure water is hypotonic to the cell sap, so water enters by osmosis. The cell wall resists expansion, turgor pressure develops, and net uptake of water stops when the cell is turgid.
7 A patient who has lost a large volume of blood is given fluid intravenously. Why is 0.9% sodium chloride solution chosen rather than pure water?
Answer and reasoning
It is isotonic with blood plasma, so the patient's blood cells neither take in water and burst nor lose water and crenate. — 0.9% sodium chloride has the same solute concentration as blood plasma. Adding it to the blood does not change the plasma's solute concentration, so there is no net osmotic movement of water into or out of the blood cells.
Pure water would be absorbed too slowly by the blood cells, whereas the sodium chloride solution enters them quickly. — A student who sees pure water as harmless picks this. The problem with pure water is the opposite: it is hypotonic to the cytoplasm, so water would enter the blood cells rapidly by osmosis and burst them.
The patient has lost salt along with the blood, and the purpose of the solution is to replace that salt in the plasma. — A student who looks for a nutritional reason picks this. The concentration is chosen for an osmotic reason: it matches the plasma, so fluid can be added without changing its solute concentration.
Pure water is hypertonic to blood plasma, so it would draw water out of the patient's blood cells, shrinking them. — A student who has swapped the prefixes picks this. Pure water contains no solute, so it is hypotonic to plasma; the danger is that water enters the cells and they burst.
8 A plant cell has a solute potential of −700 kPa and a pressure potential of +200 kPa. What is its water potential? HL
Answer and reasoning
-500 kPa — ψw = ψs + ψp = (−700 kPa) + (+200 kPa) = −500 kPa. The positive pressure potential raises the water potential of the cell above its solute potential.
-900 kPa — A student who adds the two values as magnitudes picks this. The pressure potential is positive and the solute potential negative, so they partly cancel: −700 + 200 = −500 kPa.
-700 kPa — A student who ignores pressure and equates water potential with solute potential picks this. The wall's pressure on the contents contributes +200 kPa, giving −500 kPa.
+500 kPa — A student who expects the water potential of cell contents to be positive picks this. Solute potential is negative, and here it outweighs the pressure potential, so the water potential is −500 kPa.
9 Plant tissue is bathed in pure water. Which statement explains, in terms of solute and pressure potentials, why net uptake of water by its cells eventually stops? HL
Answer and reasoning
Water enters until the cell wall has been stretched to its limit, and the fully stretched wall then physically prevents any more water from passing into the cell. — A student who pictures the cell as a full container picks this. Water crosses the membrane according to water potential, not available space; it is the rising pressure potential that removes the gradient.
Uptake does not stop: water keeps entering until the pressure potential exceeds what the wall can withstand and the cell bursts. — A student who transfers bursting from animal cells picks this. The wall withstands the pressure; net uptake stops when the cell's water potential has risen to zero, well before any bursting.
Water entering raises the pressure potential as the wall resists expansion, until it cancels the solute potential and the cell's water potential is zero. — As water enters, the wall presses back with increasing force, so ψp rises. When ψp equals the magnitude of ψs, ψw = ψs + ψp = 0, equal to the pure water outside; there is no longer a gradient and the cell is at full turgor.
Water enters until the cell sap has become so dilute that its solute potential has risen to zero, matching the pure water outside. — A student who thinks only solutes matter picks this. The sap is diluted only slightly; its solute potential stays negative, and equilibrium is reached because the pressure potential rises to cancel it.
10 Plant tissue is bathed in a sucrose solution with a water potential of −2500 kPa. The cells initially have a solute potential of −900 kPa and a pressure potential of +600 kPa. Which statement explains what happens? HL
Answer and reasoning
Water leaves the cells; pressure potential becomes negative as the shrinking protoplast pulls inward on the cell wall that surrounds it. — A student who applies xylem tension to any cell under strain picks this. Once the protoplast shrinks away from the wall there is no wall pressure on it, so the pressure potential is zero, not negative.
Water leaves the cells; the wall contracts with the protoplast, so the pressure potential stays positive as the cell shrinks. — A student who thinks the whole cell shrinks like a crenating animal cell picks this. The rigid wall keeps its shape; the protoplast pulls away from it and the pressure potential falls to zero.
Water leaves the cells; the sap becomes more concentrated, so ψs rises above zero and the water potential of every cell becomes positive. — A student who expects concentrated solutions to have positive values picks this. Concentrating the sap makes its solute potential more negative, and the cell's water potential falls towards −2500 kPa.
Water leaves the cells; pressure potential falls to zero as the protoplast pulls away from the wall, so ψw equals ψs for each cell. — The cells start at ψw = −900 + 600 = −300 kPa, far above the solution's −2500 kPa, so water leaves. As the protoplast shrinks the wall stops pressing on it, ψp falls to zero and the cells plasmolyse; from incipient plasmolysis onward ψw = ψs, which becomes more negative as the sap concentrates.
11 Five potato cylinders were bathed in 0.4 mol dm⁻³ sucrose for 24 hours. The mean percentage change in mass was −3.0% with a standard deviation of 1.0%; the mean percentage change in length was −2.0% with a standard deviation of 2.5%. What do these data indicate?
Answer and reasoning
The mass measurements were more reliable than the length measurements, because the mass repeats were less widely spread around their mean. — Standard deviation describes the spread of the repeats around their mean. The smaller standard deviation for mass shows that those five repeats agreed more closely, so mass is the more reliable measure of the effect of the solution on the tissue; the length repeats were widely scattered.
The length measurements contained more mistakes, because a larger standard deviation means that more errors were made when the ruler was used. — A student who reads statistical 'error' as 'mistake' picks this. Standard deviation describes the natural spread of the repeats, not faults in technique; a small change in length is hard to resolve with a ruler, so the repeats scatter even when it is used correctly.
Every mass repeat lay within 1.0% of the mean and every length repeat within 2.5% of it, because the standard deviations give the range of each set. — A student who reads any measure of spread as a range picks this. Standard deviation is not the limits of the data: it describes the typical spread of the repeats, and some repeats usually lie further from the mean than one standard deviation, so a mass repeat may well have changed by more than 1.0% away from −3.0%.
The standard deviations show how close each sample mean is to the true mean, so the length mean is less accurate but the spread of the repeats cannot be compared. — A student who treats standard deviation and standard error as one statistic picks this. It is standard error that indicates how close a sample mean is to the true mean; standard deviation describes the spread of the repeats, which is exactly what allows the reliability of the two measurements to be compared.
That was your twenty minutes. Real practice on D2.3 is past-paper questions marked against the mark scheme.
What the exam asks of D2.3
Paper 1A asks you to predict the direction of water movement for a named cell and medium, or to pick the correct term. Paper 1B gives mass-change data with error bars and asks for the isotonic concentration, or asks what standard error tells you. Paper 2 uses *explain*: state the solute concentrations, give the direction of osmosis, then the effect on the cell. HL questions give ψs and ψp values and use *calculate* and *explain*; show the addition and state which way water moves.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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