Question
Explain osmosis using the potato experiment. What happens when potato strips are placed in (a) distilled water, (b) concentrated salt solution, and (c) isotonic solution? Explain the underlying mechanism.
Solution — Step by Step
Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).
In living cells, the cell membrane is the selectively permeable membrane. It allows water to pass freely but restricts large molecules like sucrose and NaCl.
Water potential () = Solute potential () + Pressure potential (). Pure water has the highest water potential (). Adding solutes lowers water potential (makes it more negative).
Cut potato strips of equal length and weight. Weigh and measure them. Place strips in:
- Beaker A: Distilled water (pure water, no solutes)
- Beaker B: Concentrated salt (NaCl) solution
- Beaker C: Potato juice or a solution isotonic to potato cells
Leave for 30-60 minutes. Remove, blot dry, and re-measure length/weight.
What happens: The potato strips become turgid — they gain water, become longer, heavier, and firm. They feel hard and resist bending.
Why: The potato cell cytoplasm contains dissolved sugars, salts, and other solutes — it has a lower water potential than distilled water. So water moves by osmosis from the distilled water (higher ) into the potato cells (lower ) through the cell membrane.
As water enters, the vacuoles expand and press against the cell wall → cells become turgid. The entire strip expands.
This is endosmosis — movement of water into the cell.
Key term: The distilled water is a hypotonic solution relative to the potato cell contents (lower solute concentration than cells).
What happens: The potato strips become plasmolyzed — they lose water, become shorter, lighter, and soft (flaccid). They bend easily and feel limp. If concentration is very high, the cell membrane visibly pulls away from the cell wall (plasmolysis).
Why: The concentrated salt solution has much lower water potential than the potato cell contents. Water moves by osmosis from the potato cells (higher relative to the salt solution) out into the surrounding solution.
The vacuoles shrink, cells lose turgor, and the strip shrinks and softens. At extreme concentrations, the cell membrane detaches from the cell wall — this is plasmolysis.
This is exosmosis — movement of water out of the cell.
Key term: The salt solution is a hypertonic solution relative to the potato cells (higher solute concentration than cells).
What happens: The potato strip shows no change in length, weight, or firmness.
Why: The isotonic solution has the same water potential as the potato cell contents. There is no net driving force for water to move in either direction. Water molecules still cross the membrane in both directions, but the rates are equal — no net movement.
Key term: Isotonic solution — same solute concentration as the cell’s cytoplasm (approximately 0.9% NaCl or appropriate sucrose concentration for potato).
Why This Works
Osmosis is driven by the difference in water potential. Water potential is essentially a measure of how much energy water molecules have to “want to move.” Pure water has maximum energy; adding solutes reduces this energy (lowers water potential) because solute-water interactions “tie up” water molecules.
The cell membrane’s selective permeability is the critical feature — if the membrane were freely permeable to all solutes, concentrations would equilibrate by diffusion of both solutes and water, and osmosis wouldn’t occur. The membrane acts as a filter that allows only water (and small gases) to pass readily.
This is why proper salinity matters for cell health. Too little salt (hypotonic environment) → cells swell (and can burst in animal cells — lysis). Too much salt (hypertonic) → cells shrink (and can die from dehydration — crenation in animal cells, plasmolysis in plant cells). The body maintains blood plasma at precise osmolarity for this reason.
Alternative Method
Simple osmoscope demonstration: Tie a semipermeable membrane (e.g., dialysis tubing, pig bladder) across the opening of a tube. Fill the tube with sugar solution, seal it, and immerse in water. Water enters by osmosis and the sugar solution rises up the tube (you can see the water level rising). The height of liquid rise is proportional to the osmotic pressure.
Reverse: fill with water, immerse in sugar solution → liquid level drops as water leaves.
For CBSE board exams (Class 11 Biology), this potato experiment is frequently asked as a 5-mark “design and explain” question. Structure your answer as: Aim → Materials → Procedure (with three beakers) → Observations (all three cases) → Inference/Conclusion. Specifically name the phenomenon in each case: turgidity/endosmosis (distilled water), plasmolysis/exosmosis (salt solution), no change (isotonic). Including a labeled results table earns full marks.
Common Mistake
Many students write that “water moves from low concentration to high concentration” in osmosis — this is imprecise and misleading. Water moves from higher water potential to lower water potential. Adding solutes lowers water potential, so water moves toward higher solute concentration (lower water potential). But the driving force is the water potential gradient, not the solute concentration gradient per se. Pure water = highest water potential; concentrated solution = lowest water potential; water always moves from pure/dilute to concentrated. This distinction matters for questions involving turgor pressure, where pressure potential can raise the water potential of a cell even if its solute concentration is high.