Explain osmosis using water potential
Osmosis is the net movement of water through a partially permeable membrane from higher to lower water potential.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In which direction does net osmosis occur?
What you will learn
- Define osmosis in terms of water potential and a partially permeable membrane
- Predict the direction of net water movement between solutions
Water potential predicts net water movement
Water potential, ψ, describes the tendency of water to move. For a membrane-permeable water molecule, net movement is from a region of higher water potential to lower water potential when a partially permeable membrane separates the regions.
Pure water has a water potential of zero under the stated reference conditions. Dissolved solutes make water potential more negative. Compare the water potentials on both sides rather than saying water simply moves toward “more concentrated” in every context.
Osmosis is net, not one-way molecular motion
A partially permeable membrane allows water molecules through but restricts at least some solute particles. Water molecules move in both directions; osmosis describes the net movement down the water-potential gradient.
At equal water potentials, water continues crossing in both directions but there is no net osmosis. Do not confuse water potential with solute concentration alone when pressure or other conditions differ; this course uses the given water-potential values.
Pause and explain
Which side has the higher water potential: −20 kPa or −90 kPa?
Worked example
Side A has ψ = −40 kPa and side B has ψ = −160 kPa, separated by a partially permeable membrane. State the direction of net osmosis.
Show the worked solution
- The higher water potential is the less negative value, −40 kPa.
- Water moves toward the lower water potential, −160 kPa.
Answer Net water movement is from side A (−40 kPa) to side B (−160 kPa), because water moves from higher to lower water potential.
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.
Define osmosis.
Give me a hint
Include net movement, water, water potential and the membrane.
Compare my reasoning
- Water moves across a partially permeable membrane.
- Net movement is from higher to lower water potential.
Osmosis is the net movement of water molecules across a partially permeable membrane from higher water potential to lower water potential.
Look for these in your work
- Net movement stated
- Membrane and water-potential direction included
A cell has ψ = −120 kPa and the surrounding solution has ψ = −70 kPa. State the direction of net water movement.
Give me a hint
The less negative value is the higher water potential.
Compare my reasoning
- The solution at −70 kPa has higher water potential.
- Water moves toward the cell at −120 kPa.
Net water movement is from the surrounding solution (−70 kPa) into the cell (−120 kPa).
Look for these in your work
- Values ordered correctly
- Direction named
At equilibrium across a partially permeable membrane, explain why there is no net osmosis even though water molecules still move.
Give me a hint
Compare opposing molecular movements.
Compare my reasoning
- Water molecules cross in both directions.
- Equal opposing movements give zero net movement.
Water molecules continue to cross in both directions, but the opposing movements balance on average because water potential is equal on each side. Therefore there is no net osmosis.
Look for these in your work
- Molecular movement retained
- No-net-flow explained
Common mistakes
- Comparing negative values by magnitude instead of numerical order
- Saying water moves from lower to higher water potential
- Treating osmosis as movement of solute through the membrane
What can you explain now?
In which direction does net osmosis occur?
Compare with the explanation
From higher to lower water potential across a partially permeable membrane
This states the water-potential direction and membrane condition.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
State the definition of osmosis and compare negative water potentials to predict direction.
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.