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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,

  1. Keep cup A empty
  2. Put one teaspoon sugar solution in cup B
  3. Put one teaspoon salt solution in cup C
  4. Put one teaspoon sugar solution in the boiled potato cup D. Keep these for two hours. 

Then observe the four potato cups and answer the following:

  1. Explain why water gathers in the hollowed portion of B and C.
  2. Why is potato A necessary for this experiment?
  3. Explain why water does not gather in the hollowed out portions of A and D.

 

Answer:

Substances move from areas of high concentration to areas of low concentration through a semipermeable membrane . This process is called osmosis .

Water also follows this principle.

i. In the first cup ,

  • There is no movement of water through the potato alone .
  • Potato cup A is necessary in the experiment as a ‘ control experiment ’ for providing comparisons with situations created in potato cups B, C and D. 

Moreover, 

For osmosis, there should be a gradient in concentration

In this case there is water outside the cup but none inside the cup and hence no concentration gradient . Thus, water will not flow through the membrane.

ii. Cups B and C contain sugar and salt solutions

  • This means that the concentration of water inside the cups is lesser than that outside
  • Hence, water gathers in cup B and C due to Osmosis

iii. In the case of cup D ,

  • The potato is boiled . Heating the cell membrane (which is made of proteins) leads to the destruction of the cell membrane due to the proteins being denatured
  • The destroyed cell membrane cannot act as a semi permeable membrane
  • Without a semi permeable membrane osmosis will not take place .

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Economics, Accounting and English at Teachoo