Potato Osmosis Practical
Potato cylinders are cut with a cork borer and trimmed to equal length. Four cylinders are blotted and weighed: each is 2.0 g. They are placed for 30 minutes in labelled beakers containing 0%, 5%, 10%, and 20% sucrose solution. After 30 minutes the cylinders are removed, blotted, and reweighed. The masses are 2.3 g, 2.1 g, 1.8 g, and 1.5 g respectively. The 20% cylinder was left on the bench for 10 minutes before weighing and feels limp; the 0% cylinder was weighed immediately. A classmate says, 'The 20% potato gained water because it was in the strongest solution, so osmosis moved water into the cells.' The teacher asks the group to calculate percentage changes, check the direction of water movement, and identify any result that should not be trusted without repeating the trial.
1. Calculate the percentage mass change for each cylinder and state the direction of water movement in the 0% and 20% solutions.
Read the worked answer
Percentage change = (final mass − initial mass) ÷ initial mass × 100.
- 0%: (2.3 − 2.0) ÷ 2.0 × 100 = +15%
- 5%: (2.1 − 2.0) ÷ 2.0 × 100 = +5%
- 10%: (1.8 − 2.0) ÷ 2.0 × 100 = −10%
- 20%: (1.5 − 2.0) ÷ 2.0 × 100 = −25%
In 0% sucrose (distilled water), the external solution has a higher water potential than the potato cells, so water enters the cells by osmosis and mass increases. In 20% sucrose, the external solution has a lower water potential, so water leaves the cells and mass decreases. The direction is outward in 20%, not inward.
2. Critique the classmate's conclusion about the 20% cylinder. Include the effect of leaving it on the bench.
Read the worked answer
The conclusion is wrong. A 0.2 g decrease, or −25%, shows that the 20% cylinder lost water; it did not gain water. The 20% sucrose solution is more concentrated than the potato cell sap, so water moved out of the cells by osmosis. That water loss makes the cells flaccid or plasmolysed, which explains why the cylinder feels limp.
Leaving it on the bench for 10 minutes makes the exact value less trustworthy. Water could evaporate from the surface, adding to the apparent mass loss, and the cells may continue to lose water. The −25% figure may therefore overstate the osmotic effect. The trial should be repeated with immediate blotting and weighing, the same timing for all cylinders, and several replicates.
3. Explain why the 0% cylinder gained mass and what would happen to a potato cylinder placed in distilled water for several hours.
Read the worked answer
The 0% solution is distilled water, which has a very high water potential compared with the potato cell sap. Water therefore enters the cells across the partially permeable membrane by osmosis, increasing the mass. The potato becomes firmer as the cells become turgid.
If left in distilled water for several hours, the cylinder would gain more water until the cells are fully turgid. It would not burst like an animal cell because the rigid cellulose cell wall resists expansion and exerts pressure back on the cell. The mass gain would level off rather than continue indefinitely. Very long soaking could also leach some solutes and soften the tissue, so 'firmer and slightly heavier' is a better prediction than unlimited swelling.