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Osmosis & Tonicity

Osmosis & Tonicity

4 min read Updated Apr 18, 2026

Osmosis is the movement of water across a semipermeable membrane from low solute concentration to high solute concentration. It is passive - driven by the concentration gradient of water itself. Tonicity describes what happens to a cell placed in a solution relative to the cell’s own interior.

Osmosis: three solutions, three outcomes

Tonicity
Osmosis is diffusion of water across a membrane the solute cannot cross. Water always moves toward the solute. Hypotonic less solute outside
Water moves in. The cell swells and may burst (lyse). A red cell in pure water does exactly this.
 A plant cell instead
A plant cell becomes turgid instead, because its wall resists.
 Isotonic equal effective solute
Water moves both ways equally. No net change. This is what an intravenous saline drip is for.
 A plant cell instead
A plant cell goes flaccid: no inward pressure to hold it firm.
 Hypertonic more solute outside
Water moves out. The cell shrinks and crenates. This is how salt preserves food and dehydrates bacteria.
 A plant cell instead
A plant cell plasmolyses: the membrane pulls away from the wall.
 The distinction that trips people Osmolarity
Counts every dissolved particle, permeant or not. A physical measurement of the solution alone.
 Tonicity
Counts only the particles that cannot cross the membrane. It is a statement about a solution and a particular cell together.
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Water moving in Water moving out The cell The surrounding solution

Osmolarity is not tonicityOsmolarity counts every dissolved particle. Tonicity counts only the ones that cannot cross the membrane. A urea solution can have the same osmolarity as the cell and still be hypotonic, because urea crosses freely and water follows it in. Only impermeant solutes pull water.

Water follows soluteWater moves toward the side with more solute, which is the side with lower water concentration. Saying water moves down its own concentration gradient and saying it moves toward solute are the same sentence.

Why it matters clinicallyGive pure water intravenously and red cells lyse. Give concentrated saline and they shrivel. Isotonic saline at about 0.9% exists precisely so that neither happens, and it is why a drip bag says what it says.

Name the solution, then move the water toward the solute. Hypotonic outside means water goes in and the cell swells; hypertonic outside means water goes out and it shrinks. The prefix always describes the solution, never the cell.

Osmosis

A semipermeable membrane allows water through but blocks solute. Water moves from the side with less solute (high water concentration) to the side with more solute (low water concentration) until the solute concentrations equalize or until a physical pressure stops the flow.

Water can cross the bilayer directly at a slow rate, but most physiologic water movement goes through aquaporins - specialized channel proteins that let water pour through at much higher rates while excluding ions and other solutes. Kidney collecting-duct cells express aquaporin-2 under vasopressin (ADH) control, which is how urine is concentrated on demand.

Osmotic Pressure

The pressure needed to prevent osmosis is called osmotic pressure. For dilute solutions:

Π=iMRT\Pi = iMRT

  • Π = osmotic pressure.
  • i = van’t Hoff factor (number of particles a solute dissociates into; NaCl → Na+ and Cl-, i = 2).
  • M = molar concentration of solute.
  • R = gas constant.
  • T = absolute temperature.

A solution with more dissolved particles exerts more osmotic pressure. This is why IV solutions are designed to match blood osmolarity (~300 mOsm/L).

The Three Tonicities

Always stated from the perspective of the SOLUTION compared to the CELL’s cytoplasm.

| Tonicity | Solution vs. cell | Water flows | RBC outcome |
|----------|-------------------|-------------|-------------|
| Hypertonic | Higher solute | Water OUT of cell | Cell shrinks (crenation) |
| Hypotonic | Lower solute | Water INTO cell | Cell swells (lysis, hemolysis) |
| Isotonic | Equal solute | Balanced | No net change |

Plant Cells and Turgor Pressure

Plant cells have a rigid cell wall outside the membrane. When placed in a hypotonic solution, they absorb water and expand against the wall until the wall resists further expansion. That pressure inside the cell is turgor pressure - it is what keeps non-woody plants upright. In a hypertonic solution, plant cells lose water and the membrane pulls away from the wall (plasmolysis) - wilting.

What happens to a red blood cell placed in a hypotonic solution, and why?
Click to reveal answer
Water flows INTO the cell because the interior has a higher solute concentration than the hypotonic surrounding solution. The cell swells. RBCs lack a cell wall, so if the volume increases enough, the membrane ruptures - hemolysis. This is why pure water cannot be given intravenously; it would hemolyze RBCs.
What is the van’t Hoff factor for NaCl, and why does it matter for osmotic pressure?
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i = 2 because NaCl dissociates into two ions (Na+ and Cl-). Osmotic pressure depends on the total concentration of dissolved PARTICLES. A 0.5 M NaCl solution has the same osmotic pressure as a 1 M glucose solution (glucose does not dissociate, i = 1). Always multiply by i when comparing osmotic effects.
Why do plant cells placed in a hypotonic solution not burst?
Click to reveal answer
Plant cells have a rigid cell wall outside the plasma membrane. As water flows into the cell, the cell expands until the wall resists further expansion. The pressure inside the cell (turgor pressure) rises to balance the osmotic driving force. This is what keeps non-woody plants rigid and upright; wilted plants have lost turgor.
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