Carry out the following osmosis experiment: Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,

(a) Keep cup A empty

(b) Put one teaspoon sugar in cup B

(c) Put one teaspoon salt in cup C

(d) Put one teaspoon sugar in the boiled potato cup D. Keep these for two hours. Then observe the four potato cups and answer the following:

(i) Explain why water gathers in the hollowed portion of B and C.

(ii) Why is potato A necessary for this experiment?

(iii) Explain why water does not gather in the hollowed out portions of A and D.

(i) Potato cups B and C have sugar and salt respectively in them. The sugar and salt form a concentrated or hypertonic solution in the potato cups. This will cause osmosis to occur in which the water molecules will move from their higher concentration (outside) to their lower concentration (inside the cup) i.e., from hypotonic solution to hypertonic solution. Therefore, the level of water will increase in the potato cups B and C.

(ii) Potato A serves as a control for the experiment.

(iii) Water does not collect in cup A because cell sap and water become isotonic.

Water does not collect in cup D because the cells of potato died due to boiling. After boiling the cell membrane cannot function like a semi-permeable membrane and hence there will be no passage of water through it.