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Q9(ii):
Carry out the following osmosis experiment: Take four peeled potato halves and scoos 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: (ii) Why is potato A necessary for this experiment?

Solution :

Theoretical Foundation of the Osmosis Control Experiment

In experimental biology, specifically when investigating the movement of solvent molecules across a semi-permeable membrane (osmosis), it is imperative to establish a control group. Potato cup A serves as the negative control in this investigative setup.

Step 1: Establishing the Experimental Variable

The core objective of this experiment is to observe the movement of water (the solvent) into the potato cups based on the concentration gradient established by solutes (sugar and salt). The independent variables are the presence of solute ($S$) and the integrity of the potato cell membrane ($M$).

  • Experimental Cups (B, C, D): These cups contain solutes that create a hypertonic environment relative to the water in the trough, intended to drive endosmosis.
  • Control Cup (A): This cup contains no solute, meaning the water potential ($\psi_w$) inside the cup is equal to the water potential of the surrounding trough water.

Step 2: Analysis of the Control Function

Potato cup A is necessary to provide a baseline for comparison. By keeping it empty, we isolate the effects of the solutes in cups B and C. If water levels were to rise in all cups, we would be unable to distinguish between genuine osmosis and external environmental factors like evaporation, atmospheric pressure, or capillary action.

Trough (Water) Cup A (Control) No Solute $\Delta \text{Level} \approx 0$

Step 3: Logical Justification [Per the Scientific Method]

Cup Conditions Expected Outcome Role
A Empty No net movement Control Group
B/C Solute present Endosmosis (Level rises) Experimental Group

The necessity of Cup A is justified by the requirement for comparative validation. Without Cup A, the researcher cannot confirm that the rise in water level in Cups B and C is specifically due to the osmotic gradient established by the sugar or salt particles. It proves that the potato tissue itself does not automatically absorb water at an accelerated rate when placed in a liquid medium, thus validating that the results of Cups B and C are caused by the solute-induced change in osmotic potential ($\Psi_s$).

Final Solution: Potato cup A acts as the control in this experiment. It provides a baseline for comparison, proving that the rise in water level observed in the other cups is due to the presence of solutes (sugar/salt) and the resulting osmotic gradient, rather than any intrinsic property of the potato or experimental artifacts.


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