Osmosis/Tonicity Predictor
Compare a cell's internal solute concentration to its surrounding solution to predict which way water will move by osmosis, and what happens to the cell as a result — for both animal cells (crenation/lysis) and plant cells (plasmolysis/turgor). Two 3D diagrams compare a shrinking cell in a hypertonic solution to a swelling cell in a hypotonic solution, and charts show water movement direction across a range of concentration differences.
Reviewed by the ToolNestr Editorial Team — July 2026
Shrinking vs. swelling cell
1. In a hypertonic solution
Water leaves the cell, causing it to shrink and shrivel (crenation).
2. In a hypotonic solution
Water enters the cell, causing it to swell (and potentially burst without a wall).
Osmosis charts
How it works
The core idea in one line: water always moves across a cell membrane toward whichever side has the higher solute concentration, and whether that movement helps or harms the cell depends entirely on which direction it's headed and whether the cell has a rigid wall to resist it.
Compare C_outside to C_inside
higher outside concentration → hypertonic (water leaves); lower → hypotonic (water enters); equal → isotonic
Water moves toward the higher solute concentration
osmosis moves water down its own concentration gradient, i.e., toward the more concentrated solute side
A cell membrane is selectively permeable to water, which means water can cross it freely even when the larger solute particles dissolved on either side cannot. Osmosis moves that water toward the side with a higher solute concentration, diluting the more concentrated side until equilibrium is reached (or, in a living cell, until the membrane's limits are tested). Comparing a cell's internal solute concentration to its surroundings — hypertonic (more concentrated outside), hypotonic (less concentrated outside), or isotonic (equal) — directly predicts which way water will move and by how much. Whether that movement helps the cell (turgor in a walled plant cell) or seriously harms it (lysis in an unwalled animal cell) depends entirely on whether a rigid cell wall is present to resist the resulting pressure.
Worked example 1 — a red blood cell in a hypertonic solution
Given: A red blood cell has an internal concentration of 300 mOsm/L, placed in a solution of 500 mOsm/L.
This is exactly why saltwater is dangerous to drink in excess — it draws water out of your cells rather than hydrating them.
Worked example 2 — a plant cell in a hypotonic solution
Given: A plant cell has an internal concentration of 300 mOsm/L, placed in pure water (0 mOsm/L, hypotonic).
This turgor pressure is exactly what keeps a healthy, well-watered plant's leaves and stems firm and upright — wilting happens when cells lose this turgor pressure instead.
Tonicity outcomes by cell type
The cell wall in plant cells fundamentally changes the outcome compared to animal cells.
| Tonicity | Animal cell result | Plant cell result |
|---|---|---|
| Hypertonic solution ★ | Crenation (shrinks, shrivels) | Plasmolysis (membrane pulls from wall) |
| Isotonic solution | No change | No change (wall not under tension) |
| Hypotonic solution | Swells, may lyse (burst) | Turgid (firm, does not burst) |
★ Reference row (worked example 1). The presence or absence of a rigid cell wall is the single biggest factor determining whether osmotic water entry is dangerous or beneficial to the cell.
Where osmosis and tonicity actually matter
💉 Medical IV fluid formulation
IV fluids given to patients must be carefully matched to blood tonicity (typically isotonic, like 0.9% saline) — an incorrectly hypotonic or hypertonic IV fluid can damage red blood cells throughout the body.
🥗 Food preservation (salting and pickling)
Salting or sugaring food creates a strongly hypertonic environment that draws water out of any bacterial or fungal cells present, inhibiting their growth — the same osmosis principle used for centuries in food preservation.
🌱 Plant irrigation and fertilizer management
Over-fertilizing soil can create a hypertonic environment around plant roots, drawing water out of root cells instead of into them — a phenomenon known as fertilizer burn.
🧫 Laboratory cell culture technique
Cell culture media must be carefully formulated to be isotonic to the cells being grown, since even small osmotic imbalances can stress or kill cultured cells.
Common misconceptions
"Hypertonic and hypotonic describe a fixed property of a solution, independent of what it's compared to."
Tonicity is always a relative, two-way comparison — the same solution can be hypertonic relative to one cell type and hypotonic relative to another, depending on each cell's own internal concentration.
"Water moves from an area of low water concentration to high water concentration."
It's more precise (and less confusing) to describe osmosis in terms of solute concentration: water moves toward the side with the higher solute concentration, which is mathematically the same as saying it moves toward lower water concentration, but avoids ambiguity.
"Plant cells can never be harmed by a hypotonic environment."
While the cell wall prevents plant cells from bursting in a hypotonic solution, they can still be damaged by a strongly hypertonic environment (plasmolysis), and severe or prolonged plasmolysis can be lethal to the cell.
"Osmosis and diffusion are the same process."
Osmosis specifically refers to the movement of water across a selectively permeable membrane; diffusion is the more general movement of any particle from high to low concentration. Osmosis is technically a special case of diffusion, applied specifically to water.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 5, "Structure and Function of Plasma Membranes" (free, peer-reviewed). openstax.org
- • Alberts et al., Molecular Biology of the Cell — Membrane transport and osmosis chapter.
- • Campbell & Reece, Biology — Cell membrane transport chapter.
Comparison-based prediction: water moves toward the higher solute concentration side. Results assume a selectively permeable membrane freely permeable to water.
How to use this calculator
Enter both concentrations
Intracellular and extracellular solute concentration, in mOsm/L.
Select cell type
Animal cell or plant cell — the cell wall changes the predicted outcome.
Read the prediction
Tonicity classification, water movement direction, and resulting cell behavior display instantly.
Related tools
Frequently asked questions
What is tonicity?
Tonicity describes how a surrounding solution's solute concentration compares to a cell's internal concentration, and therefore which way water will move across the cell membrane by osmosis.
What do hypertonic, hypotonic, and isotonic mean?
A hypertonic solution has a higher solute concentration than the cell (water leaves the cell). A hypotonic solution has a lower solute concentration (water enters the cell). An isotonic solution has equal concentration (no net water movement).
Why do plant cells not burst in a hypotonic solution the way animal cells can?
Plant cells have a rigid cell wall outside the cell membrane that resists expansion — as water enters, internal pressure (turgor pressure) builds up against the wall, making the cell turgid and firm rather than causing it to burst.
What happens to an animal cell in a hypertonic solution?
Water leaves the cell, causing it to shrink and shrivel — a process called crenation. In a plant cell, the same water loss causes the cell membrane to pull away from the rigid cell wall, a distinct process called plasmolysis.
Why does osmosis matter for medical IV fluids?
IV fluids must be isotonic to blood cells (or carefully managed if not) — an accidentally hypotonic IV fluid could cause red blood cells to swell and burst (hemolysis), while a hypertonic one could cause them to shrink and crenate, both of which are medically dangerous.