To find a sucrose solution with about the same water potential as potato tissue, look for the concentration where the percentage change in mass crosses zero. If the two nearest readings are on opposite sides of zero, use them to estimate where the crossing lies.
The 2027 SEC G3 Biology K325 syllabus includes explaining osmosis in terms of water potential. This guide interprets a practical result. It uses concentration to locate the crossing and does not calculate water potential numerically.
- Record each sample’s initial and final mass.
- Divide the mass change by its initial mass to find the percentage change.
- Find the two concentrations that bracket zero.
- Explain zero change in terms of net water movement.
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Compare percentage changes
Potato pieces can start at slightly different masses. A piece that rises from 2.50 g to 2.55 g gains 0.05 g, or 2% of its starting mass. Blot surface solution consistently before weighing so droplets on the outside do not dominate the measurement.
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Worked example | The crossing sits between two readings
Suppose the mean mass change is +2% at 0.20 mol dm⁻³ sucrose and −3% at 0.30 mol dm⁻³. On a graph of percentage mass change against concentration, zero lies between these points. If you approximate that part of the graph by a straight line, moving from +2% to zero covers two fifths of the vertical change. The estimated concentration is 0.20 + (2/5 × 0.10) = 0.24 mol dm⁻³.
At this concentration the tissue shows approximately zero net change in mass in the test. This is consistent with little or no net movement of water by osmosis and similar water potentials under the stated conditions.
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Check the estimate with another run
Repeat with concentrations closer to the crossing, such as 0.23, 0.24 and 0.25 mol dm⁻³. Use comparable pieces, the same immersion time and temperature, and several pieces at each concentration. The original 0.24 mol dm⁻³ is an estimate based on the points you have.
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Frequently asked questions
Can water still move at the crossing?
Water can move in both directions. Zero net movement means that those movements balance overall.
Is the crossing the sucrose concentration inside the potato?
It estimates an external concentration that gives a similar water potential. Potato cells contain other dissolved substances, so the experiment does not identify their internal sucrose concentration.