Explain osmotic water uptake and loss in organisms
Apply water-potential comparisons to roots and a changed environment.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall water potential and root hair cell structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What process explains root-hair water entry down a water-potential gradient?
What you will learn
- Apply water-potential comparisons to roots and a changed environment.
- Apply the idea to a supplied biological example or investigation.
A root hair takes up water when the gradient favours it
Water enters a root hair cell by osmosis when soil water has higher water potential than the cell contents. The membrane provides the partially permeable boundary; the long extension increases soil contact but does not create a one-way pump.
A plant must replace water lost from its surfaces. The uptake comparison depends on current conditions. A root does not guarantee inward osmosis if the surrounding soil solution has lower water potential than the root cell.
Changed surroundings can change direction
If dissolved substances make the external water potential lower, net water entry can decrease or water may leave cells. This explains why a concentrated external solution can reduce plant-cell water content under the stated conditions.
Water moves across cell membranes according to the water-potential comparison, whereas mineral ions can be actively transported against their own concentration gradients. Osmosis of water and active ion uptake are different processes occurring at the same root surface.
Sources for this lesson
Pause and explain
Can a root hair lose water by osmosis?
Worked example
Soil water is −0.1 MPa and a root hair is −0.5 MPa. Then the soil changes to −0.8 MPa while the initial cell value is unchanged. Predict the initial directions.
Show the worked solution
- Initially soil is higher, so water enters.
- After the change, the cell is higher than the soil.
- Net water initially leaves; later cell conditions may change.
Answer Initially inward; after the change initially outward.
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.
Soil is −0.2 MPa and root contents −0.6 MPa. Predict water movement.
Give me a hint
Higher potential is outside.
Compare my reasoning
- −0.2 is higher.
- Net movement crosses inward.
Into the root hair by osmosis.
Look for these in your work
- Name process and direction.
Outside is −0.9 MPa and root contents −0.4 MPa. Predict the initial direction.
Give me a hint
Compare the values.
Compare my reasoning
- Inside is higher.
- Net water moves outside.
Out of the root hair.
Look for these in your work
- Avoid a universal inward claim.
Explain why inward mineral-ion uptake and outward water movement could occur simultaneously.
Give me a hint
Each substance follows a different mechanism/comparison.
Compare my reasoning
- Ions can be actively transported using respiratory energy.
- Water direction follows water potential.
- They need not share a direction.
Active ion uptake and osmosis of water have different driving conditions.
Look for these in your work
- Distinguish ion concentration from water potential.
Common mistakes
- A root hair is not an automatic one-way water pump.
- Predict an initial direction, not a fixed state forever.
What can you explain now?
Can a root hair lose water by osmosis?
Compare with the explanation
Yes, if outside water potential is lower.
Direction follows the current comparison.
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Make it stick
Explain why root structure aids contact but does not fix osmotic 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.