NEET Biology · Cell Biology
Osmosis
Simulator
Change solute concentration on each side of a membrane — watch water movement, cell volume change, and understand exactly why.
Section 1 of 5 · Foundations
What you'll understand after this lab
Osmosis is one of the most tested topics in NEET Biology. Four labs take you from the basic definition through to osmotic pressure calculations — with every cell animation driven by real concentration gradients.
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Water movementSee animated water molecules crossing the membrane — always from low solute (high water potential) to high solute (low water potential).
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Real cell responsesWatch animal cells lyse or crenate. Watch plant cells undergo plasmolysis or become turgid. Different structures, different limits.
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Osmotic pressure formulaInteractive van't Hoff equation: π = iCRT. Change concentration and temperature — see how osmotic pressure responds.
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6 NEET trapsDiffusion vs osmosis, water potential sign conventions, imbibition, plasmolysis — the concepts most often tested incorrectly.
Hypotonic
Outside solution has
less solute than cell
Water enters cell → cell swells
less solute than cell
Water enters cell → cell swells
Isotonic
Equal solute concentration
on both sides
No net water movement
on both sides
No net water movement
Hypertonic
Outside solution has
more solute than cell
Water exits cell → cell shrinks
more solute than cell
Water exits cell → cell shrinks
Definition
Osmosis
Movement of water (solvent) across a selectively permeable membrane from a region of lower solute concentration (higher water potential, less negative ψ) to higher solute concentration (lower water potential, more negative ψ).
Water Potential (ψ)
ψ = ψₛ + ψₚ
Water potential = solute potential (ψₛ, always negative) + pressure potential (ψₚ). Water moves from less negative to more negative ψ. Pure water has ψ = 0 (maximum).
Semipermeable Membrane
H₂O passes · solute stays
The cell membrane allows water to pass freely but restricts solute molecules. This selectivity is what drives osmosis — solute particles cannot equalise by moving through the membrane.
Key Distinction
Osmosis ≠ Diffusion
Diffusion: movement of any substance (solute or solvent) down its concentration gradient, no membrane required. Osmosis: specifically water, specifically across a semipermeable membrane.
Section 1 of 5
Section 2 of 5 · Main Simulation
Cell Osmosis Simulator
Set the solute concentration inside and outside the cell. The cell membrane is selectively permeable — watch water molecules cross in the direction of lower water potential (higher solute). The cell volume responds in real time.
1
Set concentrationsInside conc. = cytoplasm solute. Outside conc. = surrounding solution. Both in molar (M) units.
2
Watch water flowBlue dots = water molecules. They flow toward the more concentrated side (higher osmotic pressure). Arrow direction + speed changes live.
3
Read the cell stateThe tonicity badge tells you if the solution is hypo-, iso-, or hypertonic. The cell volume meter shows % change from baseline.
Live Cell Simulation
Cell Membrane — Cross Section
Water molecules animated in real time
Isotonic ⇌
Inside (cytoplasm)
0.3 M
Outside (solution)
0.3 M
Net Water Flow
—
direction
Cell Volume
—
% of normal
Conc. Gradient
—
Δ M
Ψ outside−in
—
rel. units
—
Understanding the simulation
Direction of flow: Water moves from low solute (high ψ, less negative) to high solute (low ψ, more negative). If outside > inside, water leaves the cell.
Concentration gradient: The larger the difference, the faster the net flow. At equal concentrations, individual water molecules still cross both ways — but at equal rates (dynamic equilibrium).
Animal cell response: No cell wall. Shrinks (crenation) in hypertonic solution, swells and may lyse in hypotonic. The membrane cannot resist turgor pressure.
Net flow = zero at isotonic. Both gross flows are equal. This is called dynamic equilibrium — not that osmosis has stopped, but that entry = exit rates.
NEET TIPWater moves toward lower water potential (more negative ψ), which is toward higher solute concentration. A common trap: students say "water moves toward high solute" which is correct, but the reason is that high solute = low ψ — always justify with water potential.
Section 2 of 5
Section 3 of 5 · Comparative Response
Plant Cell vs Animal Cell
Plant and animal cells respond very differently to osmotic stress — the cell wall changes everything. Drag the external concentration slider and watch both cells respond simultaneously. Same solution, very different outcomes.
1
Both cells, same solutionThe external concentration applies to both cells. Internal concentration is fixed at 0.3 M (normal cell osmolarity).
2
Watch the key differencesAnimal cell: unlimited swelling → lysis. Plant cell: cell wall resists swelling → turgor pressure builds. In hypertonic: animal crenates vs plant plasmolysis.
3
Spot the special casesFlaccid cell (ψₚ = 0), plasmolysis (protoplast pulls away from wall), incipient plasmolysis (just starting).
Comparative Response
External Solution Concentration
0.3 M
← Hypotonic (0 M)Isotonic (0.3 M)Hypertonic (0.9 M) →
Animal Cell
No cell wall · membrane only
Normal
Balanced: normal biconcave shape
Plant Cell
Has rigid cell wall · turgor pressure
Turgid
Cell wall prevents lysis; turgor pressure builds
Plant vs Animal — key differences
Plant cell — hypotonic: Water enters → turgor pressure (ψₚ) builds → cell wall resists → turgid state. The wall prevents lysis. Turgidity keeps plants upright (wilting = low turgor).
Animal cell — hypotonic: Water enters → cell swells with no wall resistance → may burst (haemolysis in red blood cells, cytolysis in others).
Plant cell — hypertonic: Water exits → protoplast (living contents) shrinks → plasmolysis: protoplast pulls away from cell wall. Cell wall remains but protoplast detaches.
Animal cell — hypertonic: Water exits → cell shrinks → crenation: spiky, shrunken appearance. Cell membrane wrinkles but does not detach from anything.
Isotonic: Both cells in equilibrium. Animal cell: normal shape. Plant cell: flaccid (no turgor pressure, ψₚ = 0, but no plasmolysis yet).
NEET TIPPlasmolysis is reversible (deplasmolysis) if cell is returned to a hypotonic solution early enough. Incipient plasmolysis = the point just when plasmolysis begins = ψₚ = 0 = flaccid state. At this point ψ_cell = ψₛ (no pressure potential). This exact condition appears repeatedly in NEET.
Section 3 of 5
Section 4 of 5 · Osmotic Pressure
Osmotic Pressure — van't Hoff Equation
Osmotic pressure (π) is the minimum pressure needed to prevent water entry across a semipermeable membrane. It depends on solute concentration, temperature, and the van't Hoff factor i (for electrolytes).
1
Adjust all variablesChange concentration C, temperature T, and van't Hoff factor i. See π update live with the full working shown.
2
Try electrolytesNaCl: i ≈ 2. CaCl₂: i ≈ 3. Glucose: i = 1. The i factor accounts for dissociation into ions — more particles = higher π.
3
Compare with simulationThe visualisation shows how osmotic pressure opposes water entry. Higher π on one side = stronger pull for water.
van't Hoff Equation Lab
π = iCRTvan't Hoff equation
π = osmotic pressure (atm)
i = van't Hoff factor (dissociation)
C = molar concentration (M)
R = 0.0821 L·atm mol⁻¹ K⁻¹
T = temperature (K)
—
atm
Osmotic Pressure Visualisation
π determines how strongly water is "pulled" in
Concentration C (M)
0.3 M
Temperature T (K)
298 K
van't Hoff factor i
1
1 = glucose2 = NaCl3 = CaCl₂4 = Al₂(SO₄)₃
π (atm)
—
osmotic pressure
π (kPa)
—
SI units
Effective conc.
—
i × C (M)
T (°C)
—
Celsius
—
Osmotic pressure — key concepts
Osmotic pressure definition: The minimum external pressure required to exactly oppose osmotic flow into a more concentrated solution. Higher solute concentration = higher π.
van't Hoff factor i: Electrolytes dissociate into multiple ions, creating more solute particles. NaCl → Na⁺ + Cl⁻, so i = 2. More particles = greater effective concentration = higher π.
Temperature effect: π ∝ T (Kelvin). Higher temperature → faster molecular motion → greater osmotic pressure. This is analogous to ideal gas pressure increasing with T.
Turgor pressure in plants: When water enters a plant cell, the cell wall exerts an inward pressure (ψₚ > 0). The cell reaches equilibrium when π_cell = ψₚ + π_outside.
NEET TIPFor NEET, remember π = CRT for non-electrolytes (i = 1). The van't Hoff factor i for ideal NaCl = 2, but in NEET problems the question will specify whether to account for dissociation. Also: osmotic pressure is a colligative property — it depends only on the number of solute particles, not their identity.
Section 4 of 5
Section 5 of 5 · Revision Sheet
Quick Reference — All Osmosis Terms & Formulae
Every definition, formula, and NEET trap for Osmosis and Water Relations in one place. Print or screenshot for last-minute revision.
Key Terms
| Term | Definition | NEET relevance |
|---|---|---|
| Osmosis | Movement of water across a semipermeable membrane from low to high solute concentration (high to low water potential) | Core definition — always tested |
| Water potential (ψ) | ψ = ψₛ + ψₚ. Tendency of water to move. Pure water ψ = 0 (maximum). Always ≤ 0 in solutions. | Sign convention trap: more negative = lower ψ |
| Solute potential (ψₛ) | ψₛ = −iCRT. Always negative. Reduces water potential. | Confused with osmotic pressure: π = −ψₛ |
| Pressure potential (ψₚ) | Usually positive (turgor). Can be negative (tension in xylem). Zero at flaccidity. | ψₚ = 0 at incipient plasmolysis |
| Turgor pressure | Outward pressure exerted by cell contents against cell wall. Keeps plants rigid. | Turgor = turgid; loss = wilting |
| Plasmolysis | Shrinkage of protoplast away from cell wall in hypertonic solution. Reversible if early. | Only in plant cells (wall present) |
| Crenation | Shrivelling of animal cell in hypertonic solution. Spiky/crenated RBC appearance. | Animal cells only (no wall) |
| Haemolysis | Bursting of RBC in hypotonic solution. No wall to resist swelling. | Why blood plasma must be isotonic |
| Flaccidity | Plant cell at ψₚ = 0. No turgor pressure. Isotonic with surroundings. Limp but not plasmolysed. | Intermediate state — often confused with plasmolysis |
| Imbibition | Absorption of water by colloidal substances (seeds, cell walls) without a membrane. NOT osmosis. | Distinct from osmosis — no membrane required |
| Diffusion | Movement of any substance down its concentration gradient. No membrane required. | Not the same as osmosis |
| Facilitated diffusion | Diffusion aided by membrane proteins (aquaporins for water). Faster than simple diffusion; still passive. | Aquaporins speed up osmosis |
Formulae
| Equation | Variables | Use when |
|---|---|---|
| ψ = ψₛ + ψₚ | ψ = water potential, ψₛ = solute potential, ψₚ = pressure potential | All water movement problems in plants |
| ψₛ = −iCRT | i = van't Hoff factor, C = molarity, R = 8.314 J/mol/K, T = temp in K | Calculating solute potential |
| π = iCRT | π = osmotic pressure (= −ψₛ) | Osmotic pressure of a solution |
| π = CRT | For non-electrolytes (i=1): glucose, sucrose, urea | Simpler NEET calculations |
| Water moves: high ψ → low ψ | Equivalently: low solute → high solute | Direction of osmosis |
| At equilibrium: ψ_inside = ψ_outside | No net water movement at equal water potentials | End-point of osmosis |
NEET Traps
TRAP 1Osmosis IS passive transport — no ATP required. Water moves spontaneously down its water potential gradient. Active transport moves solutes against gradient using ATP. Osmosis never requires energy.
TRAP 2Water moves toward MORE NEGATIVE ψ, not "toward higher concentration" alone. Always use water potential language in NEET answers. More solute = more negative ψₛ = lower ψ = water flows in.
TRAP 3Imbibition is NOT osmosis. Seeds absorbing water to germinate, wood swelling — these are imbibition. No semipermeable membrane is involved. NEET frequently asks you to distinguish these.
TRAP 4Plasmolysis only in plant cells; crenation only in animal cells. Plant cells have walls, so the membrane peels away from the wall. Animal cells have no wall, so the whole cell shrinks. Don't mix these terms.
TRAP 5Incipient plasmolysis = flaccid, ψₚ = 0. At this point, the cell's ψ = ψₛ only. The protoplast just starts to pull away but hasn't visibly separated yet. This specific state is heavily tested.
TRAP 6Osmotic pressure is a colligative property. It depends on the NUMBER of solute particles (moles), not the type. 1 M glucose = 1 M sucrose (same π). But 1 M NaCl ≠ 1 M glucose because NaCl gives 2 particles (i=2).
Section 5 of 5 · Complete!