Which one of the following will be resulted when an animal cell is surrounded by a medium with lower concentration of water?
- (a)Cell will lose water
- (b)No change in movement of water
- (c)Cell will gain water
- (d)Cell will swell up
Correct — A, Cell will lose water. A medium with a lower concentration of water is a medium with a higher concentration of dissolved solute — a hypertonic solution. In osmosis, water crosses the selectively permeable plasma membrane from wherever water is more concentrated to wherever it is less concentrated, so here the net flow is outward, from the cell into the medium. The animal cell therefore loses water and shrinks; in a red blood cell this shrivelling is visible under a microscope and is called crenation.
- (b)No change in movement of water — Water traffic stops being one-sided only when the medium is isotonic, that is when the water concentration inside and outside is equal. The stem states the outside water concentration is lower, so a concentration difference exists and net outward movement must follow.
- (c)Cell will gain water — This is the hypotonic case, and it is the exact reverse of the stem. A cell gains water when the surrounding medium has a higher concentration of water — more dilute than the cell contents — not a lower one.
- (d)Cell will swell up — Swelling is the visible consequence of gaining water, so it belongs with the hypotonic case as well. An animal cell put in a very dilute medium can swell so far that it bursts, because it has no cell wall to resist the pressure — but that is not what a low-water medium does.
Osmosis is the net movement of a solvent, in living systems water, across a selectively permeable membrane from a region where the solvent is more concentrated to one where it is less concentrated. Describing the outside medium by its water concentration rather than its solute concentration is simply the same statement turned around: more solute means less water. Three labels cover every case — hypertonic (less water outside, cell loses water), hypotonic (more water outside, cell gains water) and isotonic (equal, no net movement).
The stem is deliberately phrased in terms of water rather than salt, and that inversion is the whole trap. A candidate who reads 'lower concentration' and reflexively thinks 'dilute outside' will choose gaining water or swelling. Translate the phrase first — lower water outside means more solute outside, which means hypertonic — and the answer falls out. It is also worth holding the animal-cell and plant-cell versions apart. An animal cell in a hypertonic medium simply shrinks, while a plant cell in the same medium shows plasmolysis, in which the protoplast pulls away from the rigid cell wall.
- A hypertonic medium has more solute and therefore less water than the cell; the cell loses water by osmosis.
- A hypotonic medium has more water than the cell; the cell gains water and swells, and an animal cell may burst because it has no cell wall.
- In an isotonic medium the water concentrations are equal and there is no net movement of water.
- The shrinking of a red blood cell in a hypertonic solution is called crenation; the equivalent shrinkage of a plant protoplast away from its wall is plasmolysis.
- In reverse osmosis an external pressure greater than the osmotic pressure is applied on the concentrated side to drive water the other way.
Read 'lower concentration of water' as 'more solute outside' and the direction of flow is settled — water leaves the cell.
- Reading 'lower concentration of water' as 'dilute solution' — it means the opposite, a concentrated one.
- Carrying the plant-cell answer of plasmolysis into an animal-cell question, where the right word is shrinking or crenation.
- Assuming an animal cell in a very dilute medium merely swells — without a cell wall it can rupture.
As this direction-of-flow one-liner, as a laboratory scenario with a leaf peel or blood cells in a named solution, or as a statement pair on osmosis and reverse osmosis.
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
Both statements are reversed, which is exactly the inversion this NDA item plays on. The solvent moves from the dilute to the concentrated side in osmosis, and in reverse osmosis the pressure is applied on the concentrated side — get the direction right and both papers are answered by one idea.
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.
Answer(b) the cells were plasmolysed.
The same hypertonic medium, but acting on a plant cell instead of an animal one. Water leaves in both cases; the plant cell shows plasmolysis because of its rigid wall, the animal cell simply shrinks. Reading the two together fixes the difference the examiners keep testing.
Net movement of water from a dilute to a concentrated solution through a selectively permeable membrane is called
- (a) Diffusion
- (b) Dispersion
- (c) Osmosis
- (d) Absorption
Answer(c) Osmosis
The bare definition that this 2022 item asks you to apply. Once the direction — dilute towards concentrated — is memorised as part of the definition, the low-water medium in this question is immediately identified as the concentrated side.
- practice — not a real PYQ
Human red blood cells placed in distilled water will
- (a)shrink and become crenated
- (b)swell and may burst
- (c)remain completely unchanged
- (d)lose water and then regain it
Answer(b) swell and may burst — distilled water is strongly hypotonic to the cell, so water rushes in and the cell, having no wall, ruptures.
- practice — not a real PYQ
In reverse osmosis, the external pressure is applied on
- (a)the dilute solution
- (b)the concentrated solution
- (c)both sides equally
- (d)the semipermeable membrane alone
Answer(b) the concentrated solution — a pressure greater than the osmotic pressure is applied on the saline side to push water through the membrane the wrong way.