Osmosis and Cell Tonicity

What is osmosis?

Particles in a liquid are always moving. When a substance is more concentrated in one place than another, random motion spreads it out until it is evenly mixed. This is diffusion. Osmosis is a special case of diffusion: the diffusion of water across a selectively permeable membrane — a barrier that lets water through but blocks many dissolved particles (solutes) such as salt ions, sugars and proteins.

Solute particles attract water molecules and reduce how freely water can move. So a solution withmore solute effectively has less free water. When two solutions are separated by a membrane that solutes cannot cross, water moves toward the side with the higher solute concentration. It keeps moving until the concentrations are equal or until something, such as a cell wall, pushes back.

Water crosses cell membranes in two ways: slowly through the lipid bilayer itself and much faster through protein channels called aquaporins. Their discovery earned Peter Agre a share of the 2003 Nobel Prize in Chemistry. Cells that move a lot of water, such as red blood cells and kidney cells, have many aquaporins.

Hypotonic, isotonic and hypertonic solutions

Tonicity describes how a solution affects a cell’s volume. It is always a comparison between the outside solution and the cell’s contents.

Human blood plasma has an osmolality of roughly 275–295 mOsm per kilogram of water. That is why intravenous “normal saline” is 0.9% sodium chloride: it is close to isotonic with blood. In the simulator, about 300 mOsm/L is isotonic to the default cell.

Animal cells vs. plant cells

Animal cells

Animal cells have only a flexible membrane. In a hypotonic solution, water keeps entering and the cell swells. A red blood cell first loses its dimpled, biconcave shape and becomes a sphere; if it gains still more water, the membrane ruptures. The bursting of red blood cells is called hemolysis. In a hypertonic solution, red blood cells lose water and shrink into a spiky shape described as crenated.

Plant cells

Plant cells are surrounded by a rigid cell wall made mostly of cellulose. The wall lets water and solutes pass freely, but it resists stretching. In a hypotonic solution, the cell’s contents (theprotoplast, including the large central vacuole) swell and press against the wall. The wall pushes back with turgor pressure, which stops further net water entry and keeps the cell firm. This is what keeps leaves and young stems upright.

In an isotonic solution there is no turgor and the cell is flaccid; a plant made of flaccid cells wilts. In a hypertonic solution the protoplast loses water and shrinks away from the wall. This isplasmolysis, and it is usually damaging or fatal to the cell.

How the controls change the model

• Outside solution sets the solute concentration around the cell. Lowering it makes the solution hypotonic; raising it makes it hypertonic.
• Inside the cell (starting) sets how much solute the cell contains. Because solutes cannot leave, this amount stays fixed. As water enters or leaves, the same solute is diluted or concentrated — watch the “Inside now” readout change while the cell’s solute particles spread out or crowd together.
• Membrane water permeability changes how easily water crosses. It changes the speed of osmosis but not where it ends up.

The model calculates the rate of volume change from the difference between inside and outside concentrations (minus turgor pressure for plant cells). As water moves, that difference shrinks, so the cell changes size quickly at first and then levels off — the curved shape you see on the graph.

Key terms

Solute and solvent

The solute is the dissolved substance (such as salt); the solvent is the liquid it dissolves in (here, water).

Selectively permeable membrane

A membrane that allows some substances, such as water, to pass while blocking others.

Osmolarity

The total concentration of dissolved particles, in osmoles (or milliosmoles, mOsm) per litre of solution.

Tonicity

The effect a solution has on a cell’s volume: hypotonic, isotonic or hypertonic.

Aquaporin

A membrane channel protein that lets water molecules pass rapidly in single file.

Turgor pressure

The pressure of a plant cell’s contents against its cell wall.

Hemolysis, crenation and plasmolysis

Bursting of red blood cells; shrinking of red blood cells; and the protoplast pulling away from a plant cell wall.

Water potential (Ψ)

A measure, used mainly in plant biology, of water’s tendency to move. Water flows from higher (less negative) to lower (more negative) water potential.

Formulas

Osmotic pressure of a dilute solution (the van ’t Hoff equation):

where i is the number of particles each formula unit releases (about 2 for NaCl), M is molarity,R is the gas constant and T is absolute temperature. At 25 °C, each 1 mOsm/L of solute corresponds to about 2.48 kPa of osmotic pressure.

Water potential of a plant cell, the sum of its solute and pressure components:

Adding solute makes Ψs more negative; turgor pressure makes Ψp positive. Net water movement stops when Ψ inside equals Ψ outside — check the “Ψ inside / outside” readout for a plant cell in distilled water.

The simulator’s equilibrium volume follows the Boyle–van ’t Hoff relationship:

where b is the fraction of the cell’s volume that does not take part in osmosis (0.4 for the red blood cell in this model).

Real-world applications

• Intravenous fluids. Fluids given into veins are formulated with their tonicity in mind, because the wrong concentration could shift water into or out of blood cells. (This is an educational example only — see our medical disclaimer.)
• Food preservation. Salting meat and fish or packing fruit in sugar creates a hypertonic environment that draws water out of bacteria and fungi, preventing them from growing.
• Watering plants. Wilting is what happens when plant cells lose turgor. Too much fertilizer or salty soil can make soil water hypertonic to root cells, pulling water out of the roots (“fertilizer burn”).
• Fish and other aquatic animals. Freshwater fish live in a hypotonic environment and constantly gain water, so they produce lots of dilute urine. Marine fish lose water to salty seawater, so they drink water and excrete excess salt.
• Kidneys. The kidneys recover water from urine by osmosis, using aquaporins whose numbers are regulated by hormones.
• Desalination. Reverse osmosis plants apply pressure greater than seawater’s osmotic pressure to push water through a membrane, leaving the salt behind.

Common misconceptions

• “At equilibrium, water stops moving.” Water keeps crossing in both directions; it is thenet flow that becomes zero. The crossings readout shows this directly.
• “In osmosis, the salt moves.” In osmosis, only water moves across the membrane. The solute stays where it is.
• “Water moves toward where there is more water.” The opposite: net water movement is toward the side with more solute.
• “Hypertonic” describes the cell. Tonicity describes the solution relative to the cell. A solution is hypertonic to a cell.
• “Plant cells burst in pure water.” The cell wall prevents bursting. Turgid cells are healthy.

What the model simplifies

This is a teaching model, not a precise physiological one. The cell is drawn as a two-dimensional cross-section, and the number of particles is far smaller than in reality. Time is slowed down: a real red blood cell reaches osmotic equilibrium in a fraction of a second. The model assumes no solute can cross the membrane, whereas real cells actively pump ions and some solutes (such as urea) cross membranes easily, which changes a solution’s tonicity. The bursting threshold is approximate; in laboratory tests, red blood cells typically begin to lyse in salt solutions of roughly half normal strength.

Key takeaways

• Osmosis is the net movement of water across a selectively permeable membrane toward higher solute concentration.
• Tonicity compares the outside solution with the cell: hypotonic solutions cause swelling, hypertonic solutions cause shrinking.
• Animal cells can burst in hypotonic solutions; plant cells become turgid because their wall resists expansion.
• Water always crosses in both directions — equilibrium means equal rates, not no movement.
• Membrane permeability controls how fast osmosis happens, while concentrations control how far it goes.