UPPSC 2021 Prelims GS Paper-I · Question 77 of 150

Correct — C, Osmosis. When salt is sprinkled on cut cucumber, it dissolves in the surface film of water and creates a region of high solute concentration (low water potential) OUTSIDE the cucumber cells. The water inside the cells has a higher water potential, so it moves out across the selectively (semi-)permeable cell membranes towards the salty, more concentrated exterior. This net movement of the solvent (water) across a semipermeable membrane from higher to lower water potential is osmosis — which is why water oozes out of the salted cucumber (the cells lose turgor and plasmolyse).

• (a)Active transport — Wrong — active transport moves solute particles AGAINST their concentration gradient and needs energy (ATP) and carrier proteins. Here it is water, not a solute, that moves, and it moves passively down its water-potential gradient with no energy spent.
• (b)Passive transport — Not the specific term. Passive transport is a broad umbrella (diffusion, facilitated diffusion and osmosis) for movement down a gradient without energy. Osmosis IS a kind of passive transport, but the question asks for the precise process by which water crosses the membrane — that exact term is osmosis, so (c) is the expected answer.
• (d)Diffusion — Wrong — diffusion is the general net movement of any particles from higher to lower concentration. When the moving substance is specifically water crossing a semipermeable membrane, the process is given the special name osmosis; the outflow of water here is osmosis, not ordinary diffusion.

Osmosis is the net movement of solvent (water) molecules across a selectively permeable membrane from a solution of higher water potential (more dilute) to one of lower water potential (more concentrated), until equilibrium. Cell membranes are semipermeable, so cells lose water by osmosis in a hypertonic (saltier) surrounding and gain water in a hypotonic one. Sprinkling salt makes the cucumber's surroundings hypertonic, so its cells lose water and plasmolyse.

The trap lies in three overlapping terms. Diffusion is the general case; osmosis is the special case for water across a semipermeable membrane; passive transport is the umbrella that contains both. Since the substance moving here is water and it crosses cell membranes, the exact and expected answer is osmosis. Active transport is easily eliminated because there is no energy-driven uphill movement.

• Osmosis = movement of water (solvent) across a semipermeable membrane from higher to lower water potential.
• Salt makes the outside hypertonic, so water leaves the cucumber cells (they lose turgor / plasmolyse).
• Osmosis is a type of passive transport — it requires no metabolic energy.
• Diffusion is the general term; osmosis is specifically the diffusion of water across a semipermeable membrane.

• Choosing 'diffusion' when the moving substance is water crossing a membrane (that is osmosis).
• Choosing 'passive transport', the umbrella term, instead of the specific process osmosis.

UPPSC and UPSC frame osmosis through everyday examples (salted vegetables, raisins swelling in water, plasmolysis) or via statement-pairs on osmosis vs reverse osmosis — identify the direction of water movement and the semipermeable membrane.

• practice — not a real PYQ

  A dried raisin placed in pure water swells up. This is best explained by:

  • (a)Osmosis (endosmosis)
  • (b)Active transport
  • (c)Plasmolysis
  • (d)Evaporation

  Answer(a) Osmosis (endosmosis) — water enters the raisin's cells from the higher-water-potential surroundings across the semipermeable membrane.

• practice — not a real PYQ

  Which process is used in desalination plants to obtain fresh water from sea water?

  • (a)Simple diffusion
  • (b)Reverse osmosis
  • (c)Active transport
  • (d)Osmosis

  Answer(b) Reverse osmosis — external pressure greater than the osmotic pressure forces water from the concentrated (salty) side through a membrane to the dilute side.