Use water potential to predict osmosis
Apply the Extended definition from higher to lower water potential.
Section B3.2 · Osmosis
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall osmosis, signed numbers and plant-cell pressure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which water potential is higher?
What you will learn
- Apply the Extended definition from higher to lower water potential.
- Apply the idea to a supplied biological example or investigation.
Use the formal comparison
Osmosis is the net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane. Water potential expresses the tendency for water to move; compare the supplied values rather than assuming water always moves into a cell.
In simple solutions at equal pressure, a more dilute solution has higher water potential than a more concentrated solution. Dissolved solutes lower water potential. The pressure condition matters: a pressurised plant cell cannot always be compared using solute concentration alone.
Read negative values correctly
A value of −0.2 MPa is higher than −0.7 MPa because it is less negative. Water therefore has net movement from the −0.2 side toward the −0.7 side if the membrane permits water. Numerical water-potential examples are supporting reasoning practice, not a separate required calculation formula.
At equal water potentials there is no sustained net osmosis in the model, although water still moves in both directions. Do not confuse the sign of water potential with the direction of a diagram arrow.
Sources for this lesson
Pause and explain
Water potential is equal on both sides. What happens in the ideal model?
Worked example
A cell has water potential −0.6 MPa; its surroundings are −0.2 MPa. Predict initial net water movement.
Show the worked solution
- −0.2 is higher than −0.6.
- The membrane is partially permeable.
- Net water moves from surroundings into the cell.
Answer Into the cell, 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.
Water potentials are −0.3 MPa outside and −0.9 MPa inside. Predict initial net osmosis.
Give me a hint
Compare numerical values.
Compare my reasoning
- −0.3 is higher.
- Water moves toward −0.9.
Into the cell.
Look for these in your work
- State higher-to-lower direction.
Water potentials are −1.0 MPa outside and −0.4 MPa inside. Predict initial net osmosis.
Give me a hint
The less negative value is higher.
Compare my reasoning
- Inside is higher.
- Water moves toward the lower outside value.
Out of the cell.
Look for these in your work
- Compare correctly.
Why can solute concentrations alone be insufficient to predict water movement in a pressurised plant cell?
Give me a hint
Recall the equal-pressure condition of the simple rule.
Compare my reasoning
- Water potential also reflects pressure.
- A resisting wall can create internal pressure.
- Use the overall water-potential comparison.
Pressure can alter the water-potential comparison; use supplied water potentials.
Look for these in your work
- Include pressure and water potential.
Common mistakes
- Higher means less negative for these values.
- Do not ignore pressure in a real plant-cell comparison.
What can you explain now?
Water potential is equal on both sides. What happens in the ideal model?
Compare with the explanation
No sustained net osmosis, but water still moves.
Opposing transfers balance.
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Make it stick
State the Extended definition without replacing water potential with particle count.
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.