Drop one chip in pure water and it swells. Drop an identical chip in strong syrup and it goes limp. Somewhere between the two is a concentration where nothing changes, find it, and you have measured what is inside the cells.
Every cell sits behind a partially permeable membrane: it lets water through but holds back larger solutes like sucrose.
Water potential measures how free the water is to move. Pure water is the maximum, set at zero; any solute drops it below zero.
Osmosis is the consequence: water crosses from higher potential to lower. No pump, no decision, down the gradient, every time.
So a chip in water gains; a chip in strong sucrose loses. The practical hunts for the solution where the gradient disappears.
A potato cube goes into concentrated sucrose. Within minutes it shrinks and softens.
You can see what happens. The marks are for why.
The potato shrinks. Which statement best explains what is happening?
Option B. Nothing pulls the water. Sucrose lowers the solution's water potential below that inside the cells, so water moves down the gradient, outward. The sucrose stays put; the membrane keeps it out.
Drop the "sugar pulls the water" picture. No attraction, no pump, just a difference in water potential, and water drifting toward the lower side.
IB exam tip: A question that asks you to explain this needs the mechanism and the consequence, solute lowers water potential, so water moves down the gradient out of the cell. A question that asks you to state the direction only needs "water leaves the cell."
Small water molecules cross both ways; large sucrose molecules bounce off. The net drift is your mass change.
Sucrose concentration is your independent variable. It sets the bath's water potential, and that decides which way water flows.
Drag from water to strong sucrose and watch the % mass change respond.
Near the middle the chip stops changing. That concentration is what the graph hunts for.
At the crossover, mass holds steady, no net water moves. So the water potential inside equals that of the bath: the two are isotonic.
Read the concentration off the x-axis where the line crosses zero. That is the potato's osmolarity.
But osmolarity is a concentration (mol per litre); water potential is an energy (MPa). Not the same number.
Take your crossover concentration to the worksheet table to read the matching MPa. Concentration in, potential out.
The shape your graph will take: % mass change falls as concentration rises. Drag the marker to where the best-fit line crosses zero.
≈ 0.30 mol dm⁻³. Where the line crosses zero is the potato's osmolarity. On the worksheet table, that maps to about −0.83 MPa.
The whole experiment in one number, off a line through your own points.
A worked question on your data shape. Try each part before revealing the mark scheme.
A student bathes identical potato cylinders in sucrose solutions from 0 to 1.0 mol dm⁻³ for 60 minutes, then calculates the percentage change in mass for each. Their line of best fit crosses 0% mass change at 0.30 mol dm⁻³. The worksheet table gives the water potential of a 0.30 mol dm⁻³ sucrose solution as −0.83 MPa.
Outline = a brief account of the main points. Award 1 mark per point, max 2.
Accept: "controls for differences in initial mass". Do not accept: "it is more accurate" with no reason.
Explain = give reasons / mechanism, both what and why. Award 1 mark per point, max 3.
Accept: "water potential" or "Ψ". Do not accept: "the sugar attracts / pulls the water out".
Deduce = reach a conclusion from the data; State = give a specific answer. Award 1 mark per point, max 4.
Accept: −0.8 to −0.9 MPa. Do not accept: a positive value, or quoting the concentration (0.30 mol dm⁻³) as the water potential.
Fill in the blanks. Stuck? Tap Reveal answers.
Osmosis is the net movement of down a gradient of water , across a partially permeable . The concentration at which a chip shows no net change is the point, which gives the potato's .
No microscope, just mass, a range of solutions, and the one concentration where the gradient vanishes.
Water moves down a water-potential gradient. Solute lowers the potential; nothing is pulled.
IB: State = direction of water. Explain = solute lowers Ψ, so water moves down the gradient.
Where % mass change hits zero, tissue and solution are isotonic. That concentration is the osmolarity.
IB: Deduce needs "no net movement → equal water potential".
Osmolarity is a concentration; water potential is an energy in MPa. The table converts between them.
IB: don't quote the molarity as the water potential, convert it.