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
Correct — C, osmosis. Osmosis is the net movement of solvent (water) across a selectively (semi-) permeable membrane from a region of higher water concentration — the dilute solution — to a region of lower water concentration — the concentrated solution. The membrane lets water pass but holds back the dissolved solute, so water moves down its own concentration gradient until the two sides reach equilibrium.
- (a)Diffusion — Diffusion is the movement of any particles (gases or solutes) from a region of high concentration to one of low concentration and does not need a membrane; osmosis is the special case that involves water crossing a selectively permeable membrane.
- (b)Dispersion — Dispersion means the scattering or spreading of particles through a medium; it is not the membrane-controlled flow of water described here.
- (d)Absorption — Absorption is the general uptake of substances (such as water and minerals by roots); it is a broader term and not specifically the net flow of water across a selectively permeable membrane.
Osmosis is the net diffusion of water across a selectively permeable membrane. Water always moves from the side with more free water (the dilute solution, higher water potential) to the side with less free water (the concentrated solution, lower water potential). The membrane blocks the solute, so only water can equalise the two sides.
The trap is the direction. Since it is the water that moves, it flows toward the more concentrated (higher-solute) side, which is 'thirstier' for water. If there were no selectively permeable membrane, the same spreading of particles would simply be diffusion.
- Osmosis is the movement of solvent (water) across a selectively permeable membrane.
- Water moves from the dilute solution (higher water potential) to the concentrated solution (lower water potential).
- The membrane allows water through but blocks the dissolved solute.
- In reverse osmosis an external pressure is applied to the concentrated side to push water the opposite way, and it is used to purify water.
- Reversing the direction — water moves toward the concentrated (higher-solute) side, not away from it.
- Calling any movement of water osmosis; without a selectively permeable membrane it is simple diffusion.
Asked as a definition-match (name the process) or as statements on the direction of water flow and on 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
Same concept — in osmosis the solvent moves from the dilute to the concentrated solution (so statement 1 is reversed), and reverse osmosis applies pressure to the concentrated side. Reinforces the direction of water flow tested here.
- practice — not a real PYQ
In reverse osmosis, water is made to move
- (a)from the dilute to the concentrated solution on its own
- (b)from the concentrated solution to the dilute solution by applying external pressure
- (c)from a hot region to a cold region
- (d)out of the membrane into the air
Answer(b) from the concentrated solution to the dilute solution by applying external pressure — the reverse of natural osmosis.
- practice — not a real PYQ
Which one of the following does NOT require a selectively permeable membrane?
- (a)Osmosis
- (b)Reverse osmosis
- (c)Simple diffusion of a gas
- (d)Turgor in a plant cell
Answer(c) Simple diffusion of a gas — diffusion needs only a concentration gradient, not a membrane.