During a laboratory experiment, a student immerses epidermal leaf peel in a hypertonic solution. After some time, the student examined the cells under a microscope and observed that :
- (a)the cells swelled.
- (b)the cells were plasmolysed.
- (c)the cells built up turgor pressure.
- (d)the cells size was unaffected.
Correct — B, the cells were plasmolysed. A hypertonic solution has a higher solute concentration than the cell sap, so water leaves the cell by osmosis. The protoplast shrinks and pulls away from the cell wall — this is plasmolysis — so under the microscope the epidermal cells appear plasmolysed.
- (a)the cells swelled. — Cells swell only in a hypotonic solution, where water enters; a hypertonic solution drives water out, so the cells shrink rather than swell.
- (c)the cells built up turgor pressure. — Turgor builds up when water enters in a hypotonic or pure-water medium; in a hypertonic solution the cell loses water and turns flaccid, so turgor falls.
- (d)the cells size was unaffected. — Size stays unchanged only in an isotonic solution; a hypertonic solution causes a net outflow of water, so the cells visibly shrink.
Plant cells respond to their surroundings by osmosis, the movement of water across a semipermeable membrane from a region of higher water potential to lower. In a hypertonic solution water leaves the cell, the protoplast contracts, and it detaches from the rigid cell wall — the shrinkage called plasmolysis.
The single word 'hypertonic' decides the answer: the outside solution is more concentrated, so water exits and the cell shrinks. Options describing swelling or turgor describe the opposite (hypotonic) case, and 'unaffected' describes an isotonic medium.
- A hypertonic solution has a higher external solute concentration, so water leaves the cell.
- Plasmolysis is the shrinking of the protoplast away from the cell wall due to water loss.
- In a hypotonic solution water enters and the cell becomes turgid.
- In an isotonic solution there is no net movement of water.
- Swapping the effects of hypertonic and hypotonic solutions.
- Confusing plasmolysis (reversible water loss) with cell death.
NDA/UPSC give a cell placed in a solution and ask what happens — trace the water movement from higher to lower water potential to decide swelling, plasmolysis or no change.
Consider the following statements: 1. During the process of osmosis, the solvent travels from the concentrated solution to the dilute solution. 2. In the reverse osmosis, external pressure is applied to the dilute solution. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer(d) Neither 1 nor 2 — in osmosis the solvent moves from the dilute (higher water potential) to the concentrated solution, and reverse osmosis applies pressure to the concentrated side, so both statements are wrong.
UPSC prelims 2005 tested the direction of water flow in osmosis — the very principle behind plasmolysis. Water moves toward the more concentrated (hypertonic) solution, which is why a cell in a hypertonic medium loses water and plasmolyses.
- practice — not a real PYQ
A plant cell placed in pure water (a hypotonic medium) will
- (a)become plasmolysed
- (b)become turgid
- (c)shrink and die
- (d)stay exactly the same size
Answer(b) become turgid — water enters the cell by osmosis, building up turgor pressure.
- practice — not a real PYQ
The shrinkage of the protoplast away from the cell wall when a cell loses water is called
- (a)turgidity
- (b)plasmolysis
- (c)imbibition
- (d)diffusion
Answer(b) plasmolysis — loss of water in a hypertonic solution makes the protoplast pull away from the wall.