Study guides & original practice
Osmosis and tonicity practice
Tonicity predicts how a surrounding solution affects cell volume. Focus on solutes that do not freely cross the membrane and state whether pressure differences matter.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
In a simple equal-pressure model, water moves net toward the side with more nonpenetrating solute. Hypertonic surroundings tend to draw water out; hypotonic surroundings tend to drive water in. Individual water molecules move both ways even when there is no net flow.
Worked example, step by step
An animal cell contains 0.30 M nonpenetrating solute. It is placed in a large 0.10 M solution of the same solute, with equal initial pressure.
- Compare concentrations: the surrounding solution has less nonpenetrating solute.
- Classify the surroundings as hypotonic relative to the cell.
- Predict initial net water movement into the cell through its water-permeable membrane.
- Predict swelling; do not assume unlimited safe expansion, because an animal cell lacks a rigid cell wall.
A mistake to watch for
A solute that can freely cross changes the problem. Total dissolved-particle concentration and tonicity are related but not interchangeable when permeability differs.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. What happens initially if the same cell enters 0.50 M nonpenetrating solute?
Show answer and explanation
Water moves net out and the cell tends to shrink under the stated equal-pressure conditions.
2. Does isotonic mean water molecules stop crossing?
Show answer and explanation
No. Opposing movement continues; there is no sustained net volume change from the concentration comparison.
3. Why might a plant cell respond differently from an animal cell?
Show answer and explanation
Its cell wall can generate opposing pressure as water enters. Pressure can affect the eventual balance, so concentration alone is not the full endpoint explanation.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Biology 2e provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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