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) Water gathers in B and C because in both the situations there is difference in the concentration of water in the trough and water in the potato cup. Hence endosmosis (i.e. water enters into the cell) takes place as the potato cells act as a semi-permeable membrane.
(ii) Potato cup A is necessary in the experiment as a ‘control’ for providing comparison with situations created in potato cups B, C and D. It indicates that the potato cavity alone does not induce any movement of water.
(iii) Water does not gather in hollowed out portions of A. This can be explained as follows. For osmosis to take place, a concentration gradient is necessary to develop between the two solutions on either side of a semi- permeable membrane (in this case potato strip). Here water is present only on one side i.e., convex side of potato cup whereas concave side of the cup is empty. Water will fill in the concavity only on account of endosmosis that can take place when some solution having concentration higher than that of water is present it. Hence, the hollowed out portion of A remained empty. In cup D cells become dead due to boiling, hence semi permeability of membrane is lost so no osmosis will take place in it.