Science
Carry out the following 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. Place each of the potato cups in a beaker containing water (Fig. 2.22). Now, set up the experiment as follows:
(a) Keep Cup A empty.
(b) Add one teaspoon sugar in Cup B.
(c) Add one teaspoon salt in Cup C.
(d) Add one teaspoon sugar in the boiled potato in Cup D.
Observe the four potato cups for at least two hours and answer the following questions:
(i) Explain why water gathers in the hollowed portion of Cup B and Cup C.
(ii) Why is Cup A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed portions of Cups A and D.
Cell
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Answer
(i) Cups B and C contain sugar and salt respectively, which form a concentrated (hypertonic) solution inside the potato cups. The water in the beaker is a dilute (hypotonic) solution. Water therefore moves through the living potato cells from the beaker (higher water concentration) into the cup (lower water concentration, higher solute concentration) by the process of osmosis. As a result, water gathers in the hollowed portions of Cups B and C.
(ii) Cup A serves as the control for the experiment. It has no sugar or salt, so it shows what happens when there is no concentration difference. This provides a baseline against which the changes in Cups B, C and D can be compared.
(iii) In Cup A, no water gathers because there is no solute (sugar or salt) inside the cup, so the water inside and outside the potato are nearly equal in concentration (isotonic) and there is no net movement of water by osmosis. In Cup D, the potato is boiled, which kills its cells. The cell membranes are destroyed and can no longer act as selectively permeable membranes, so osmosis cannot take place and water does not gather in the cup.
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