How to use the simulator
The picture shows one animal cell sitting in a surrounding solution. Coral dots outside the cell stand for solute in the solution, and navy dots inside stand for the cell's own solute. There are three buttons, and the simulator opens on Isotonic solution:
- Hypotonic solution (less solute outside → water enters). The solution shows 9 solute dots against 20 inside the cell.
- Isotonic solution (equal solute → no net movement). 20 dots outside, 20 inside.
- Hypertonic solution (more solute outside → water leaves). 34 dots outside, 20 inside.
After you click a button, the cell grows or shrinks smoothly to its new size, and teal arrows around it point in the direction of net water movement: inward for hypotonic, outward for hypertonic. Isotonic has no arrows. When the cell is fully swollen in hypotonic solution, its outline turns dashed to show that it is close to bursting.
The vol bar on the right shows cell volume, with a dashed line marking the normal volume. The readout box gives three results:
- net water flow: into cell, out of cell or balanced (no net).
- cell volume: increases, decreases or stable.
- animal-cell outcome: swells / may lyse, shrinks (crenation) or normal.
The dot counts are only a picture. What matters is how the solute concentration outside compares with the inside. The number of solute dots inside the cell stays at 20 in every case; it is water that moves, not solute.
The key ideas
Osmosis is the diffusion of water across a selectively permeable membrane. Water crosses the lipid bilayer slowly by itself and much faster through aquaporin channels. The solute particles in this model cannot cross the membrane, which is why only water moves.
Tonicity words always compare two solutions, and they describe the solution, not the water:
- Hypotonic: lower solute concentration than the other side. Net water moves out of a hypotonic solution.
- Hypertonic: higher solute concentration than the other side. Net water moves into a hypertonic solution.
- Isotonic: equal solute concentrations. Water still crosses the membrane, but at equal rates in both directions, so there is no net movement.
In short, water moves toward the side with more solute, because adding solute lowers water potential.
What happens next depends on the cell. An animal cell has no wall, so in hypotonic solution it keeps taking in water until it may burst (lysis), and in hypertonic solution it shrivels (crenation). A plant cell's wall pushes back, so in hypotonic solution it becomes firm (turgid) rather than bursting, and in hypertonic solution the membrane pulls away from the wall (plasmolysis). This simulator shows the animal-cell case only.
Osmoregulation is how organisms control their water and solute balance. A freshwater protist such as Paramecium lives in hypotonic water and uses a contractile vacuole to pump the extra water out.
Worked example
Scenario: Red blood cells contain about 0.3 M of solutes that cannot cross the membrane. Samples go into three beakers of a solution whose solute also cannot cross: A holds 0.1 M solution, B holds 0.3 M and C holds 0.5 M. Predict what happens in each beaker.
Beaker A (0.1 M). The solution has less solute than the cell, so it is hypotonic to the cell. Net water moves into the cells. They swell and may lyse. In the simulator, choose Hypotonic solution: the arrows point inward, cell volume increases and the outcome reads swells / may lyse.
Beaker B (0.3 M). The concentrations are equal, so the solution is isotonic. Water moves in and out at the same rate, and the cells keep their normal shape. Choose Isotonic solution: no arrows, volume stable, outcome normal.
Beaker C (0.5 M). The solution has more solute than the cell, so it is hypertonic. Net water moves out of the cells, and they shrink and crenate. Choose Hypertonic solution: the arrows point outward, volume decreases and the outcome reads shrinks (crenation).
Common mistakes on the AP exam
- Saying solute moves to balance the concentrations. In osmosis it is water that moves. The solute stays put because it cannot cross the membrane.
- Using "hypertonic" without saying what it is compared with. Tonicity is relative. Write "the solution is hypertonic to the cell," not just "the cell is hypertonic."
- Getting the direction backwards. Water moves toward the side with more solute (lower water potential), not toward the side with more water.
- Saying water stops moving at equilibrium. Water keeps moving both ways; only the net movement stops.
- Saying a plant cell bursts in pure water. The cell wall prevents lysis; the cell becomes turgid. Lysis is the animal-cell outcome.
When the AP exam uses this
Tonicity and osmoregulation are part of Unit 2 (Cell Structure and Function), alongside membrane transport and water potential. Expect questions that show a cell or dialysis bag in a solution and ask which way water moves, how mass or volume changes, and why. Lab-based questions, such as potato cores or dialysis tubing soaked in sucrose solutions, use the same reasoning and may add a water potential calculation.