What is Osmosis? — Definition, Types & NEET Biology

Osmosis — Explained for NEET

Osmosis is a fundamental process in biology that governs water movement in and out of cells. Unlike simple diffusion, osmosis specifically involves the movement of solvent (water) molecules across a selectively permeable membrane.

The direction of osmosis depends on the water potential (ψ) of the solutions on either side of the membrane. Water always moves from a region of higher water potential to a region of lower water potential. Water potential is determined by two components: solute potential (ψs, always negative) and pressure potential (ψp, usually positive in plant cells).

There are three types of solutions relative to a cell: isotonic (same solute concentration as the cell), hypotonic (lower solute concentration — water enters the cell), and hypertonic (higher solute concentration — water leaves the cell). When a plant cell is placed in a hypotonic solution, it becomes turgid due to endosmosis. In a hypertonic solution, the cell undergoes plasmolysis as water leaves by exosmosis.

Plasmolysis is the shrinkage of the protoplasm away from the cell wall when a plant cell loses water in a hypertonic solution. The point at which plasmolysis is just about to begin is called incipient plasmolysis. This process is reversible — placing the plasmolyzed cell in a hypotonic solution causes deplasmolysis.

In animal cells (which lack a cell wall), osmosis can cause the cell to swell and burst (lysis) in a hypotonic solution or shrink (crenation) in a hypertonic solution. This is why maintaining osmotic balance is critical in the body — the kidneys play a major role in regulating this.

Osmosis is essential for water absorption by roots, opening and closing of stomata, and maintaining cell turgidity in plants.

Key Points from NCERT

• Osmosis is the movement of water from higher water potential to lower water potential through a semi-permeable membrane
• Water potential (ψ) = solute potential (ψs) + pressure potential (ψp); pure water has ψ = 0
• Plasmolysis occurs when a plant cell is placed in a hypertonic solution — protoplasm shrinks away from the cell wall
• Endosmosis (water enters cell) occurs in hypotonic solutions; exosmosis (water leaves cell) occurs in hypertonic solutions
• Root hair cells absorb water from soil by osmosis due to their higher solute concentration compared to soil water