Define transpiration and distinguish vapour from liquid
Describe water-vapour loss from leaves without confusing it with water uptake or liquid droplets.
Section B8.3 · Transpiration
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 delivery to mesophyll.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the gaseous state of water?
What you will learn
- Define transpiration precisely
- Distinguish transpiration from uptake and liquid loss
Water leaves as vapour
Transpiration is the loss of water vapour from plant leaves. Water first reaches leaf tissues as liquid; evaporation creates vapour at moist internal surfaces, and vapour can leave the leaf through stomata. A complete definition includes the substance, its state and the leaf origin.
Root uptake supplies replacement water but is a different process. A potometer follows water entering a cut shoot, so it provides an uptake measure used as a proxy rather than directly collecting all vapour lost.
Do not rely on visible droplets
Water vapour is invisible. Droplets seen on a bag enclosing leaves may be condensed vapour, but the droplets themselves are liquid. A matched empty bag and appropriate handling help distinguish the plant contribution from other water sources.
Liquid droplets at a leaf edge can arise through other processes, such as guttation. Calling every form of water movement “transpiration” loses the required distinction. Use the observed state and the stated method before naming the process.
Sources for this lesson
Pause and explain
Which definition is precise?
Worked example
Improve “transpiration is when roots drink water”.
Show the worked solution
- Roots absorb water; transpiration concerns loss from leaves.
- Specify that the lost water is vapour.
Answer Transpiration is the loss of water vapour from leaves; roots absorb replacement water.
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.
Write a definition including material, state and origin.
Give me a hint
Use “water”, “vapour” and “leaves”.
Compare my reasoning
- Identify water in its vapour state.
- State loss from leaves.
Transpiration is the loss of water vapour from leaves.
Look for these in your work
- State water vapour
- State leaf loss
A bag around leaves contains liquid drops. Explain how transpiration can contribute without calling the drops vapour.
Give me a hint
Separate leaf loss from later condensation.
Compare my reasoning
- Leaves can release water vapour.
- The vapour can condense on the bag into liquid.
Leaf vapour loss can contribute; the visible drops are condensed liquid, not vapour itself.
Look for these in your work
- Separate the two states
- Describe condensation after loss
Why does a root gaining 1 mL water not by itself prove exactly 1 mL transpiration during the same interval?
Give me a hint
Consider retention and other uses of water.
Compare my reasoning
- Uptake and leaf vapour loss are different measured quantities.
- Some water can be stored or used while conditions change.
Uptake is a proxy, not automatic equality with contemporaneous vapour loss; storage/use and changing conditions can create differences.
Look for these in your work
- Distinguish uptake from loss
- Give a reason equality is not guaranteed
Common mistakes
- Defining transpiration as root water absorption.
- Treating all visible liquid leaf droplets as transpiration itself.
What can you explain now?
Which definition is precise?
Compare with the explanation
Loss of water vapour from leaves
It includes the origin and state.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Define transpiration in one sentence, then name a different process involving plant water.
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.