Drop a red blood cell into pure water and it bursts. Drop a plant cell in and it simply firms up. Water potential explains both, and a bag of potato cylinders lets you measure it.
Water potential (ψ) is the tendency of water to move out of a solution, measured in kPa. Pure water has the highest water potential, 0 kPa; dissolving solutes lowers it, so all solutions have negative values. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. An animal cell in a solution of higher water potential gains water and may burst; in a lower one it loses water and shrinks. A plant cell's wall stops it bursting, so it becomes turgid; in a solution of lower water potential the protoplast pulls away from the wall: plasmolysis. Soaking plant tissue in a range of solutions and finding where there is no change in mass gives the water potential of the tissue.
Fig. 1: percentage change in mass of potato cylinders after soaking in sucrose solutions; the line crosses zero at 0.30 mol dm−3
Section 01
Water potential.
Set pure water on the left and 0.50 mol dm−3 sucrose on the right, press Start and read both water potentials. Then make the two sides equal and see what happens to the net movement.
Pure water: ψ = 0 kPa, the highest value possible.
Solutions: solute molecules attract water molecules and leave fewer free to move, so ψ falls below zero. The more concentrated the solution, the more negative ψ.
Direction: water always moves from higher (less negative) to lower (more negative) ψ. From −400 kPa to −900 kPa, for example, never the other way.
Equilibrium: when the water potentials are equal, water still crosses both ways at the same rate, so there is no net movement.
Non-living model: a Visking tubing bag of sucrose solution in water gains mass, or pushes liquid up a capillary tube, because the tubing lets water through but not sucrose.
Examiner trap
Say "from a higher water potential to a lower water potential", not "from a high concentration to a low concentration". "Concentration" is ambiguous (of water or of solute?), and AS mark schemes expect osmosis to be explained in terms of water potential.
Section 02
Animal and plant cells.
Use the Pure water, Isotonic for blood and 0.8 mol dm−3 buttons, then drag the Sucrose slider slowly. Watch the red blood cell and the plant cell side by side and read the water potential of the solution.
Solution compared with the cell
Red blood cell (no wall)
Plant cell (cell wall)
higher ψ (for example pure water)
water enters, the cell swells and bursts: haemolysis
water enters, the protoplast pushes against the wall: turgid, but it does not burst
equal ψ
no net movement, normal shape
no net movement
lower ψ (concentrated)
water leaves, the cell shrinks and becomes crenated
water leaves the vacuole, the protoplast shrinks away from the wall: plasmolysis
In the sim: the red blood cell contents are about −750 kPa, matched by about 0.30 mol dm−3 sucrose; the plant cell sap is about −1000 kPa.
Incipient plasmolysis: the point where the protoplast just begins to pull away from the wall. The cell is then flaccid.
The cell wall is fully permeable, so in a plasmolysed cell the external solution fills the space between the wall and the shrunken protoplast.
Why it matters: turgid cells support non-woody plants, and animal cells rely on the blood and tissue fluid being kept close to their own water potential.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the osmosis sim, pure water is on the left and 0.50 mol dm−3 sucrose on the right. Which way is the net movement of water?
Quick check+10 XP
What is the water potential of pure water?
Quick check+10 XP
In the cells sim, a red blood cell is put into pure water. What happens?
Quick check+10 XP
In the cells sim, the plant cell in 0.8 mol dm−3 sucrose shows its protoplast pulled away from the cell wall. What is this called?
Section 03
Measuring tissue water potential.
Press Soak for 24 hours, then Line of best fit. Read the concentration where the line crosses zero and the water potential it gives. Press New potato to see a different result.
Method: cut cylinders of the same diameter and length from one potato with a cork borer, blot and weigh each one, and place one in each of a range of sucrose solutions (for example 0 to 0.6 mol dm−3). Leave for a fixed time at constant temperature, blot dry the same way and reweigh.
Percentage change: (final mass − initial mass) ÷ initial mass × 100. Using percentages allows for cylinders with slightly different starting masses.
Graph: plot percentage change in mass against concentration and draw a line of best fit. Where it crosses zero, the solution has the same water potential as the tissue (Fig. 1).
Convert: read the water potential of that concentration from a table or graph. In the lab model 0.30 mol dm−3 is about −740 kPa; real sucrose tables give slightly more negative values.
Reliability: repeat each concentration, use cylinders from the same potato, and blot consistently, because surface liquid adds mass.
Examiner trap
A cylinder that gains mass was in a solution of higher water potential than its cells. Do not say the potato "absorbed sucrose": the membrane is partially permeable, and the mass change is due to water moving by osmosis.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Cell A has a water potential of −450 kPa and the neighbouring cell B has a water potential of −800 kPa. Which statement is correct?
Exam style+20 XP
Why does a plant cell placed in pure water not burst, while a red blood cell does?
Exam style+20 XP
Potato cylinders in 0.20 mol dm−3 sucrose gained 4% in mass and those in 0.40 mol dm−3 lost 6%. Assuming a straight line, at what concentration would there be no change in mass?
Exam style+20 XP
Why are results in the potato investigation recorded as percentage change in mass rather than change in mass?
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Osmosis and water potential, mastered.
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