Potato cylinders in sucrose solutions are the osmosis practical that IGCSE Biology Paper 5 and Paper 6 return to again and again. By hand, push the cork borer into the potato, cut each core to 3.0 cm against the ruler, blot it, weigh it and drop it into its labelled beaker. Run the stopwatch (the speed lever saves you the real half hour), lift each cylinder out with forceps, blot it and reweigh it. Then work out the percentage changes, plot your own graph and read off the concentration that matches the cells. Every attempt uses a different potato. Allow about 35 minutes.
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Headings with quantity / unit, every initial and final mass exactly as the balance showed it (2 decimal places), percentage changes calculated correctly to a consistent number of decimal places, a minus sign for every loss, cylinders cut to 3.0 cm, and at least 30 minutes in solution.
A straight line of best fit through your own points, the concentration read where your line crosses 0 %, and an accuracy mark for a value close to what careful technique gives.
No change means the solution and the cells have the same water potential (the same concentration); water moves in by osmosis from a dilute solution, and out of the cells in a concentrated one, leaving them flaccid.
Why percentage change is used, two control variables, a real source of error such as uneven blotting, and an improvement such as repeats at each concentration.
The marking follows the conventions in published Cambridge mark schemes. The experiments, readings and wording on this page are our own.