NDA Biology · Cell Biology

Osmosis and Tonicity — Water Across the Membrane

Osmosis is the net movement of water across a selectively permeable membrane from a dilute (high-water) solution to a concentrated (low-water) one; tonicity decides whether a cell swells, shrinks or bursts.

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Why this matters

A 4-PYQ cluster — the only part of the chapter that asks you to REASON about a process, not just recall a name (including the chapter's one HARD question). The key skills: define osmosis precisely (dilute → concentrated, through a semi-permeable membrane), and predict the outcome — plasmolysis in a hypertonic plant cell, water loss / haemolysis in animal cells. EASY to HARD.

Concept 1 of 3: Osmosis — the direction water moves

Water always moves to even out concentration. Across a membrane that lets water through but not solute, water flows from the side with MORE water (dilute) to the side with LESS water (concentrated). Thinking in 'water concentration' avoids every tonicity trap.

Definition

Osmosis is the net movement of water across a selectively (semi-) permeable membrane, from a region of higher water concentration (dilute / lower solute) to lower water concentration (concentrated / higher solute).

It is a special case of diffusion — but of water, through a membrane.
'Lower concentration of water' outside = the surrounding solution is hypertonic (more solute) → water leaves the cell.
'Higher concentration of water' outside = the solution is hypotonic → water enters the cell.

Osmosis — direction of water flow

\text{water: high water conc.} \;\longrightarrow\; \text{low water conc. (across a semi-permeable membrane)}

Worked example

A cell's interior has a higher water concentration than the fluid surrounding it. Which way does water move, and what happens to the cell?

Practice this concept3 quick reps

Concept 2 of 3: Naming the water-movement processes

The bank often gives a one-line description and asks you to name the process. The trick is the membrane: water moving across a semi-permeable membrane to a concentrated solution is OSMOSIS specifically, not plain diffusion.

Definition

Process names you must distinguish:

Osmosis — net water movement across a semi-permeable membrane, dilute → concentrated.
Diffusion — movement of any particles from high to low concentration (no membrane required).
Absorption / dispersion — distractor terms, not the membrane-water process.
When a cell is in a medium with LOWER water concentration (hypertonic), water leaves and the cell loses water.

Description	Process
Water moves dilute → concentrated through a semi-permeable membrane	Osmosis Membrane + water = osmosis, not plain diffusion.
Particles spread from high to low concentration (no membrane)	Diffusion
Animal cell in lower-water (hypertonic) medium	Cell loses water

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Sep 2019 · Q77Easy

Example 2 · Cell Biology · Osmosis and Tonicity

Net movement of water from a dilute to a concentrated solution through a selectively permeable membrane is called

Osmosis vs diffusion — the membrane is the tell

Diffusion needs no membrane and can be any particle; osmosis is specifically WATER across a selectively permeable membrane. If a semi-permeable membrane is mentioned, the answer is osmosis.

Drill 1 more on naming the water-movement processes

Concept 3 of 3: Tonicity outcomes — plasmolysis and haemolysis

Put a cell in the wrong solution and it changes. In a hypertonic solution water leaves, so a plant cell's membrane pulls away from the wall (plasmolysis). In a hypotonic solution — or one that wrecks the membrane — water rushes in and an animal cell swells and bursts (haemolysis).

Definition

What happens to a cell by surrounding solution:

Hypertonic (more solute outside) — water leaves; an animal cell shrinks, a plant cell undergoes plasmolysis (membrane pulls away from the wall).
Hypotonic (more water outside) — water enters; the cell swells. An animal cell (no wall) can swell and burst; a plant cell becomes turgid (wall stops bursting).
A detergent solution dissolves the RBC's lipid membrane, so the cell takes in water and swells and bursts (haemolysis).

Water moves toward the more concentrated (lower-water) side.

Surrounding solution	Plant cell	Animal cell
Hypertonic (water leaves)	Plasmolysis (membrane shrinks from wall)	Shrinks / crenates Epidermal leaf peel in a hypertonic solution → plasmolysis.
Hypotonic (water enters)	Becomes turgid (wall holds)	Swells and may burst (haemolysis) RBC in 2% detergent → membrane disrupted → swells and bursts.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2023 · Q104Easy

Example 3 · Cell Biology · Osmosis and Tonicity

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 :

Plasmolysis is a PLANT-cell word

Plasmolysis = the membrane shrinking away from the cell WALL, so it applies to walled (plant) cells in a hypertonic solution. An animal cell, with no wall, simply shrinks (crenates) or in extreme cases the opposite — bursts in hypotonic solution (haemolysis).

Detergent bursts the RBC — it doesn't shrink it

A 2% detergent solution dissolves the RBC's lipid bilayer, so water floods in and the cell SWELLS and BURSTS. The trap answer 'the RBC will shrink' describes a hypertonic salt solution, not a detergent.

Drill 1 more on tonicity outcomes — plasmolysis and haemolysis

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Osmosis — the direction water moves
Osmosis — direction of water flow
$\text{water: high water conc.} \;\longrightarrow\; \text{low water conc. (across a semi-permeable membrane)}$

Reference tables (2)

Naming the water-movement processes3 rows

Description	Process
Water moves dilute → concentrated through a semi-permeable membrane	Osmosis Membrane + water = osmosis, not plain diffusion.
Particles spread from high to low concentration (no membrane)	Diffusion
Animal cell in lower-water (hypertonic) medium	Cell loses water

Tonicity outcomes — plasmolysis and haemolysis2 rows

Surrounding solution	Plant cell	Animal cell
Hypertonic (water leaves)	Plasmolysis (membrane shrinks from wall)	Shrinks / crenates Epidermal leaf peel in a hypertonic solution → plasmolysis.
Hypotonic (water enters)	Becomes turgid (wall holds)	Swells and may burst (haemolysis) RBC in 2% detergent → membrane disrupted → swells and bursts.

Watch out for (3)

Osmosis vs diffusion — the membrane is the tell
→ Naming the water-movement processes
Plasmolysis is a PLANT-cell word
→ Tonicity outcomes — plasmolysis and haemolysis
Detergent bursts the RBC — it doesn't shrink it
→ Tonicity outcomes — plasmolysis and haemolysis

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