Where the numbers come from
Each tissue gets a hidden isotonic concentration between 0.20 and 0.45 mol/dm³. The masses follow a curve through that point, flattening at the ends because cores can only gain or lose so much, with a little noise in every core. A set is only used if the line of best fit through the points either side of 0% lands within 0.015 mol/dm³ of the hidden value, and other sensible ways of reading the graph agree with it. The key comes from those same numbers, so any careful graph lands inside the stated range.
Example: from masses to water potential
In set SPUD42 the potato cores’ mean change falls from +5.2% at 0.2 M to −6.0% at 0.4 M, and the line crosses 0% at about 0.29 M. The room was 24 °C, so ψs = −1 × 0.29 × 0.0831 × 297 = −7.16 bars. The beaker is open, so the solution’s ψ equals its ψs, and at the crossing the potato cells have the same water potential.
Levels, and AP or AQA working
GCSE (AQA required practical 3) stops at percentage change, the mean and the graph against concentration.
A-level and AP add cells in beakers, red blood cells and neighbouring cells. Pick how they are worked:
- AQA graph, kPa: water potential only. The table gives each sucrose solution’s water potential, and you read the tissue’s from your graph.
- AP equation, bars: solute and pressure potential, ψs = −iCRT, a solute potential drill and a dialysis bag.
The printed sheet uses the same method.
Questions
How do you find the isotonic point from a potato osmosis graph?
Plot the mean percentage change in mass against the sucrose concentration and draw a line of best fit. Where it crosses 0%, the cores neither gained nor lost water, so the cells' sap has about the same water potential as that solution.
How do you calculate percentage change in mass?
Take the final mass minus the initial mass, divide by the initial mass and multiply by 100. A gain is positive and a loss is negative. Use percentage change because the cores start at different masses.
How do you use ψs = −iCRT?
Multiply the ionization constant i (1 for sucrose, 2 for NaCl), the molar concentration C, R = 0.0831 L·bar/(mol·K) and the temperature in kelvin (°C + 273), then make it negative. For 0.3 M sucrose at 22 °C: −1 × 0.3 × 0.0831 × 295 = −7.35 bars.
Which way does water move by osmosis?
From higher water potential to lower water potential. Pure water is 0 and solutions are negative, so water moves toward the more negative side. In a plant cell, add the pressure potential to the solute potential before comparing.
What is the difference between turgid, flaccid and plasmolysed?
A turgid cell is full, with the wall pushing back (positive pressure potential). A flaccid cell has lost enough water that the pressure potential is 0, but the membrane still touches the wall. A plasmolysed cell has lost more, and the membrane has pulled away from the wall.
Why do AP and AQA give different units?
The AP Biology equation sheet works in bars with ψs = −iCRT. AQA A-level works in kPa and reads the tissue's water potential off a graph of mass change against the solutions' water potential. 1 bar = 100 kPa = 0.1 MPa. Pick AP or AQA above the questions.
Sources, checked October 25, 2026: College Board, AP Biology Course and Exam Description, effective fall 2025, Topic 2.7 and the equation sheet (PDF); AQA AS and A-level Biology 7401/7402 specification, version 1.5 (2021), 3.2.3 and required practical 3; AQA GCSE Biology 8461 specification, version 1.0 (2016), 4.1.3.2 and required practical activity 3; B. E. Michel, Solute potentials of sucrose solutions, Plant Physiology 50:196 (1972), for the AQA water potentials, converted to mol/dm³ with the sucrose solution density in NIST SP 260-217 (2022, PDF).