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 $D$.

(N/A) $(i)$ Osmosis is the process responsible for the gathering of water in the hollowed portion of $B$ and $C$. Since the concentration of solute (sugar in cup $B$ and salt in cup $C$) is higher inside the cup as compared to the water outside,water moves from its higher concentration (outside the cup) towards the lower concentration (inside the cup) through the semipermeable membrane of the potato cells. This process of inward movement of solvent is called endosmosis.
$(ii)$ Potato $A$ acts as a control for the experiment. It is necessary to provide a baseline for comparing the results and to demonstrate that the movement of water is due to the concentration gradient of the solute.
$(iii)$ Water does not gather in the hollowed portions of $A$ and $D$ because:
In potato $A$,the hollowed portion is empty. Since there is no concentration difference (no solute),no osmosis occurs.
In potato $D$,although it contains sugar,it is boiled. Boiling kills the potato cells and destroys the semipermeable membrane,which is essential for osmosis to occur. Therefore,no water movement takes place.