NCERT Question 9 - Chapter 5 Class 9 - The Fundamental Unit of Life

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 .