Biology 11–12 · Year 11

Osmosis in potato cylinders across a sucrose concentration series

Module 1: Cells as the Basis of Life (Cell Function)

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The idea

Water crosses a partially permeable membrane down its own concentration gradient, so tissue gains mass in dilute solutions and loses mass in concentrated ones.

What you need

Sucrose solutions 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/L, 50 mL of each per group (the source method uses diluted blackcurrant squash at 0 to 100 percent; sucrose gives a known molarity)
Large potatoes, 2; cork borer 8 to 10 mm; scalpel on a white tile; ruler
Six labelled test tubes or 100 mL beakers
Electronic balance reading to 0.01 g; paper towel; stop clock
Three potato cylinders per solution (18 in total), each trimmed to 40 mm

How to do it

Cut 18 cylinders from the same potato with the cork borer, trim each to 40 mm, blot dry and record the initial mass of each to 0.01 g.
Place three cylinders in each sucrose solution, note the time, and leave for at least 30 minutes (the source suggests a minimum of 15 minutes; longer gives a larger change).
Remove each cylinder, blot with the same number of wipes each time, and record the final mass.
Calculate percentage change in mass = (final minus initial) divided by initial, times 100, for each cylinder, then the mean for each concentration.
Plot mean percentage change against sucrose concentration and read the concentration at which the line crosses zero: this estimates the concentration of the cell sap (the isotonic point).
Repeat with a second potato to check whether the isotonic point is consistent.

What you should see

Cylinders in water and dilute sucrose gain mass and feel firm; cylinders in 0.8 to 1.0 mol/L lose mass and become floppy. The curve crosses zero between the two concentrations that bracket no change; for example a gain of 4.0 percent at 0.2 mol/L and a loss of 2.0 percent at 0.4 mol/L gives an isotonic estimate of 0.33 mol/L by linear interpolation (computed). The learner knows it worked when the sign of the mass change reverses across the series and replicate cylinders in the same solution agree to within about 1 percentage point.

What changes

What you change: sucrose concentration (mol/L)
What you measure: percentage change in mass of the potato cylinder
What you keep the same: potato variety and tuber
cylinder length and diameter
time in solution
temperature
blotting method
volume of solution

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Sucrose moves into the potato; sucrose cannot cross the membrane at any useful rate, water does.
Water stops moving at the isotonic point; water still crosses in both directions, with no net change.
Mass change is caused by the solution soaking into the tissue like a sponge rather than by water crossing cell membranes.

Safety card

Low riskLearners carry it out

Hazards

Cuts from the cork borer and scalpel
Slips from spilt solution

Controls

Bore into a tile or board, never towards the hand
Wipe spills at once
No food from the laboratory is eaten

Note

Sucrose solutions are low hazard: RiskAssess (https://www.riskassess.com.au/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-8 BIO11/12-3 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities