Q.44 Given below are two statements, one is labelled as Assertion (A) and other one
labelled as Reason (R).
Assertion (A) : Osmosis is a special type of diffusion in which movement of
substances occurs from higher solute concentration to lower solute concentration.
Reason (R) : Diffusion is dominant mode of transport at the scale of a cell.
In light of the above statements, choose the correct answer from the options given
below.
1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
3. (A) is true but (R) is false.
4. (A) is false but (R) is true.
Osmosis involves water moving from low solute (high water) to high solute (low water) concentration, making Assertion (A) false due to reversed direction. Reason (R) correctly identifies diffusion as the primary passive transport at cellular scales. Correct answer: Option 4 – (A) is false but (R) is true.
Correct Answer
Option 4: (A) is false but (R) is true.
Assertion (A) incorrectly reverses osmosis direction—water moves from lower solute concentration (higher water potential) to higher solute concentration (lower water potential) across semipermeable membranes. Reason (R) accurately states diffusion dominates cellular transport (no energy required for small molecules, ions across membranes).
Core Concepts
Osmosis Direction
Water moves down its concentration gradient (high water → low water), which equals low solute → high solute. The assertion wrongly states “higher solute → lower solute” for substances (should specify water).
Diffusion at Cellular Scale
Diffusion suffices for:
-
O₂, CO₂ exchange (alveoli → blood)
-
Small ions (K⁺ across membranes)
-
Nutrients across 1-10 μm distances
Active transport needed only for larger gradients/concentrations.
| Process | Particle | Direction | Membrane | Energy |
|---|---|---|---|---|
| Diffusion | Solutes | High → Low conc. | Permeable | None |
| Osmosis | Water | Low solute → High solute | Semipermeable | None |
Options Analysis
| Option | A Status | R Status | Explains A? | Correct? | Reason |
|---|---|---|---|---|---|
| 1 | True | True | Yes | ❌ | A false (wrong direction) |
| 2 | True | True | No | ❌ | A false |
| 3 | True | False | N/A | ❌ | R true (diffusion IS dominant) |
| 4 | False | True | N/A | ✅ | Perfect match |
Common Exam Trap
Flipped gradients: Students confuse water movement (low solute → high solute) with solute diffusion (high → low). Assertion tests this distinction. R tests passive transport hierarchy.
Memory Aid
“Osmosis = Opposite of Diffusion”
-
Diffusion: Solutes high → low
-
Osmosis: Water low solute → high solute
Visual: Saltwater (high solute) pulls pure water across membrane.
Exam Strategy: Always specify which particle moves in transport questions. Practice AR by evaluating statements independently first.
