Describe osmosis through a partially permeable membrane
Identify water movement without confusing it with movement of a solute.
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall diffusion and the cell membrane.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What moves by osmosis?
What you will learn
- Identify water movement without confusing it with movement of a solute.
- Apply the idea to a supplied biological example or investigation.
Identify the moving substance and boundary
Osmosis is water diffusion through a partially permeable membrane. Such a membrane lets some particles cross more readily than others. Water moves into and out of cells by osmosis through the cell membrane; the supporting plant cell wall is not the selectively controlling boundary.
In a simple comparison at equal pressure with a solute that cannot pass, water has a net movement from the more dilute solution toward the more concentrated one. Water molecules move both ways, but the opposing transfers are unequal. The Extended lesson gives the formal water-potential explanation.
Use the assumptions
The membrane must permit water, and the supplied solute must have the stated permeability. Do not assume every dialysis membrane blocks every sugar or ion. Read the model conditions before predicting a result.
A cell in an external solution may gain or lose water depending on the comparison between inside and outside. Osmosis does not mean all water always moves inward, and water transfer does not require the cell to supply energy from respiration.
Pause and explain
Which boundary controls osmosis into a plant cell?
Worked example
A membrane permits water but not a stated solute. At equal pressure, one side is dilute and the other concentrated. Predict net water movement.
Show the worked solution
- Water is the moving substance.
- The membrane has the required permeability.
- In this simple model net water moves from dilute to concentrated.
Answer Toward the more concentrated solution through the membrane.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the moving substance and membrane condition required for osmosis.
Give me a hint
Give both defining features.
Compare my reasoning
- The substance is water.
- The boundary is partially permeable.
Water; a partially permeable membrane.
Look for these in your work
- Include both.
In the stated equal-pressure, solute-blocking model, predict net water movement between dilute and concentrated solutions.
Give me a hint
Keep the solute stationary in the model.
Compare my reasoning
- Water can cross.
- The dilute-to-concentrated direction is the net direction.
From dilute to concentrated solution.
Look for these in your work
- Use the model assumptions.
A student labels dissolved sugar movement osmosis. What must be corrected?
Give me a hint
Identify the moving substance.
Compare my reasoning
- Osmosis names water movement.
- A permeating sugar might diffuse, but that is not osmosis.
Call the water transfer osmosis; distinguish any sugar diffusion separately.
Look for these in your work
- Do not use osmosis for all solutes.
Common mistakes
- Osmosis refers to water, not sugar pumping.
- State membrane permeability and solution conditions.
What can you explain now?
Which boundary controls osmosis into a plant cell?
Compare with the explanation
Cell membrane.
It is partially permeable.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
State two features needed to identify osmosis.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.