Osmosis practice questions become easier when you separate three decisions: what can cross the membrane, which direction water moves overall, and how that movement changes the cell. Don’t label a solution hypertonic or hypotonic until you’ve identified what you are comparing it with.
This practice set is for introductory college biology. It uses fictional cells and simplified solutions, with assumptions stated in each exercise. You’ll predict water movement, explain cell changes, and work through questions where the available information is not enough. These are classroom models, not instructions for selecting medical fluids.
Read these assumptions before you start
For questions 1 to 6, assume water crosses the membrane freely, the listed solute cannot cross, and no active transport changes the concentrations during the observation. Compare the outside solution with the cell interior. Unless a question introduces a cell wall or pressure, consider an animal cell at the start of the experiment.
With these assumptions, an outside solution containing more nonpenetrating solute particles per unit volume is hypertonic relative to the cell. Water moves out overall. With fewer such particles, it is hypotonic and water moves in overall. An isotonic solution produces no sustained change in cell volume from net osmosis.
OpenStax’s passive transport chapter explains osmosis, selective permeability, and the basic cell responses. The word net matters: molecules can move in both directions while the total transfer favors one side.
In the numerical questions, mOsm/L describes the concentration of dissolved particles. You are given the particle concentrations directly, so you do not need to calculate dissociation. Do not compare percentages of different chemicals as though equal percentages necessarily contain equal numbers of particles.
Use a three-part answer
Write “outside solution relative to cell,” then give the tonicity, a net water arrow, and the expected volume change. For example: outside hypertonic; net water out; cell shrinks. Add one sentence explaining the concentration comparison.
Draw a circle for the membrane and label the inside and outside. Put the concentrations beside their labels before drawing an arrow. This small step catches a common reading error: using the larger number correctly but placing it on the wrong side of the membrane.
If your lecture uses a different diagram convention, note it explicitly. The biology subject page can help you find related material, but compare any shared explanation against your instructor’s assumptions about membrane permeability.
Questions 1 to 3: concentration and direction
1. A more concentrated outside solution
An animal cell contains 300 mOsm/L of nonpenetrating solute. The large surrounding solution contains 450 mOsm/L of nonpenetrating solute. Predict the initial net movement of water and the cell’s response.
Answer: The outside solution is hypertonic relative to the cell. Water moves out overall, and the cell shrinks. Compare 450 outside with 300 inside. The external solution has the greater concentration of particles that cannot cross this membrane.
Repair this wrong answer: “Solute moves into the cell until both sides match.” The prompt says the solute cannot cross. You must use the membrane rule supplied in the question rather than assume every concentration difference is corrected by solute movement.
2. A less concentrated outside solution
A second animal cell starts at 300 mOsm/L inside. Its large surrounding solution contains 150 mOsm/L of nonpenetrating solute. Give the same three-part answer.
Answer: The outside is hypotonic; net water movement is inward; the cell swells. Don’t write that it must burst immediately. Swelling and eventual membrane rupture are different outcomes, and the prompt gives no observation time or limit on expansion.
For your diagram, draw a larger cell outline with a dashed pencil line and label it “predicted swelling.” Keep that prediction separate from a claim that an experiment actually measured a particular final size.
3. Equal particle concentrations
A cell and its surrounding solution each contain 300 mOsm/L of the same nonpenetrating solute. There is no pressure difference. A student says, “All water molecules stop crossing.” Is that accurate?
Answer: No. The solution is isotonic relative to the cell under these assumptions, and there is no net water transfer that changes its volume. Individual water molecules still cross in both directions.
Correct the sentence to: “There is no net water movement, so the cell’s volume remains stable.” This answer preserves the difference between molecular motion and the overall result you can observe.
Questions 4 to 6: check the reference point
4. The label changes when the reference changes
Solution A contains 200 mOsm/L and solution B contains 400 mOsm/L of a solute that cannot cross the membrane between them. Ignore pressure differences initially. Describe A relative to B, then B relative to A.
Answer: A is hypotonic relative to B. B is hypertonic relative to A. Initial net water movement is from A toward B.
The physical arrangement has not changed when you reverse the sentence. Only the reference point changes. If your two sentences assign the same label to both sides, reread the comparison before looking at the answer choices.
5. Infer the outside condition from a shrinking cell
An animal cell loses water and shrinks after being placed in a solution. The experiment involves nonpenetrating solutes, and other processes are excluded. What can you infer about the outside solution at the start?
Answer: It was hypertonic relative to the cell. The observation tells you the direction of the effective osmotic difference. It does not tell you the exact starting concentration, the identity of the solute, or the percentage of volume lost.
A complete answer can be precise without being numerical. Write the relationship the observation supports, and leave unsupported values blank. Inventing a concentration would make the answer less accurate.
6. Same solute, different percentages
A model membrane separates two dilute sucrose solutions, one 2% and one 5%, prepared using the same concentration convention. Water crosses; sucrose does not. Ignore initial pressure differences. Which way does water move overall at first?
Answer: From the 2% side toward the 5% side. You can compare these percentages because they describe the same substance using the same convention. The 5% side contains more sucrose particles per comparable amount of solution.
If one side contained a different substance, the percentages alone would not necessarily support the same comparison. Molecular mass and particle formation could matter. State what makes your shortcut valid before using it.
Question 7: a plant cell has a wall
A healthy plant cell is placed in a hypotonic environment. A student predicts the same unrestricted expansion as an animal cell. What important structure is missing from that explanation?
Answer: The cell wall. Water entry increases turgor pressure as the cell contents press against the wall. The wall resists expansion, so a simple prediction of unrestricted swelling is inappropriate.
Now change the prompt to a strongly hypertonic environment. Water loss can reduce the volume of the cell contents and pull the plasma membrane away from the wall, a process called plasmolysis. Do not draw the rigid wall shrinking in exactly the same way as the membrane-bound contents. The plant-cell discussion in OpenStax Biology provides the structural context.
On your answer sheet, draw the wall as an outer box and the membrane as an inner boundary. Label them separately. The purpose is to make the two structures impossible to confuse when reviewing your work later.
Question 8: when total concentration is not enough
A problem gives a cell interior of 300 mOsm/L and an outside solution of 400 mOsm/L. It does not identify the solutes or say whether they cross the membrane. Can you confidently predict the cell’s final volume from those two numbers alone?
Answer: No. You need information about membrane permeability and the solutes involved. The outside solution has higher total osmolarity, but that alone does not establish its tonicity relative to this cell.
Osmolarity counts dissolved particles. Tonicity concerns the effect on cell volume and depends on solutes that do not freely penetrate the relevant membrane. Keep the two terms in separate entries on your correction sheet. A useful answer asks which solutes cross and whether the question concerns an initial response or an equilibrium outcome.
In your notes, place a question mark beside “final volume,” not a guessed arrow. Recognizing missing information is part of solving the problem. It prevents a rule from an impermeable-solute worksheet from being applied to a different membrane model.
Build a correction sheet you can reuse
For each missed question, keep four short entries: the given conditions, your first prediction, the corrected prediction, and the mistaken assumption. Examples of useful error labels are “reversed inside and outside,” “forgot the cell wall,” and “treated total osmolarity as tonicity.”
Then change one detail and try again. Swap the two concentrations in question 1. Replace the animal cell in question 2 with a plant cell. Remove the permeability information from question 6. Explain which part of the answer changes and which stays the same.
Use the guide to finding biology notes to check whether a document covers your level and assumptions. You can browse study documents for additional membrane diagrams. Students should upload their own notes to help other students, including original labeled diagrams and explanations they are allowed to share.
Frequently asked questions
Does osmosis require ATP?
Osmosis itself is passive transport. Other cellular processes can use energy to establish concentration differences, but that does not make water’s osmotic movement an ATP-driven pump. See OpenStax’s cell membrane overview for the transport distinction.
Does isotonic mean the solutions contain identical chemicals?
No. The label concerns the effect on cell volume relative to a particular cell and membrane. Matching chemical identities is not the definition. The simple equal-concentration exercise above deliberately supplied the same nonpenetrating solute to remove complications.
Should I always write that water moves toward more solute?
Only with the relevant assumptions. Check permeability, pressure, and whether the question concerns initial movement or a later state. For the simplified exercises, the stated nonpenetrating-solute comparison works; missing assumptions require a more careful answer.