Osmosis Simulator — Visualize Water Movement Across Membranes

Three Solution Types and Their Effects on Cells

How to Use the Osmosis Simulator

1. Open the Osmosis Simulator
2. Set the inside concentration (solute inside the cell) using the slider
3. Set the outside concentration (solute in the surrounding solution)
4. Click Simulate to watch water molecules move across the membrane
5. Observe the cell volume change and read the net water flux direction
6. Toggle between Animal cell and Plant cell to see different responses

Water Potential Explained

Water potential (Ψ) measures the tendency of water to move. Pure water has a water potential of 0. Adding solutes lowers water potential (makes it more negative). Water always moves from higher (less negative) to lower (more negative) water potential.

For plant cells: Ψ = Ψs + Ψp, where Ψs is the solute potential (always negative, lowers water potential) and Ψp is the pressure potential (positive when cell wall pushes back). A fully turgid plant cell has high Ψp that exactly counteracts Ψs — no net water movement.

Osmosis vs Diffusion vs Active Transport

Real-World Applications

IV fluids in medicine

Intravenous saline (0.9% NaCl) is isotonic to human blood — it does not cause net water movement into or out of red blood cells. Using pure water as IV fluid would cause red blood cells to swell and burst (cytolysis). Using overly concentrated saline causes cells to shrink (crenation) and damages blood vessel walls.

Food preservation

Salt and sugar preserve food by creating hypertonic environments that draw water out of microbial cells via osmosis, dehydrating and killing them. Salted fish, dried fruits, and jams rely on this principle. The same effect is why slugs die when salt is poured on them.

Kidney and urine concentration

The kidney's loop of Henle creates an osmotic gradient in the renal medulla. As filtrate descends into the hypertonic medulla, water leaves by osmosis (the descending limb is water-permeable). Antidiuretic hormone (ADH) controls water reabsorption in the collecting duct by regulating aquaporin water channels — more ADH means more water retained and more concentrated urine.

Common Questions

Why don't plant cells burst in hypotonic solutions?

Plant cells have a rigid cell wall that limits how much the cell can expand. As water enters by osmosis, the cell wall exerts an inward pressure potential (turgor pressure) that eventually prevents further water entry. Animal cells lack a cell wall, so they keep swelling until they lyse.

What is plasmolysis?

Plasmolysis occurs when a plant cell is placed in a hypertonic solution. Water leaves by osmosis, and the cell membrane pulls away from the cell wall. The cell becomes flaccid and the visible gap between membrane and wall is the hallmark sign. Plasmolysis is usually reversible if the cell is returned to an isotonic or hypotonic solution quickly enough.