Explain evaporation and diffusion in transpiration
Transpiration begins when water evaporates from internal leaf surfaces; water vapour then diffuses to the atmosphere.
Section 7.2 · Transport mechanisms
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the first event in the leaf during transpiration?
What you will learn
- Explain transpiration as evaporation from internal leaf surfaces followed by diffusion of water vapour to the atmosphere
- Distinguish transpiration from water uptake and liquid guttation
Liquid becomes vapour inside the leaf
Water delivered to leaf tissues evaporates from moist internal surfaces, especially mesophyll cell walls, into the air spaces. Evaporation is a change from liquid to gas at a surface; it can occur below the boiling point.
The vapour then diffuses through connected leaf air spaces and usually out through open stomata to the atmosphere when the water-vapour concentration is higher inside the leaf than outside.
Keep the process boundaries clear
Transpiration is the loss of water vapour from a plant, mainly from leaves. Root uptake supplies replacement water but is not itself transpiration. Guttation is liquid water exuded from specialised structures and is not water-vapour loss.
Opening or closing stomata affects the diffusion route, but leaf water loss also depends on the internal-to-external vapour gradient and environmental conditions. Avoid claiming that any one factor determines the rate in every situation.
Pause and explain
Which process follows evaporation during transpiration?
Worked example
Put the main leaf events of transpiration in order.
Show the worked solution
- Liquid water evaporates from moist internal surfaces.
- Water vapour diffuses through air spaces and out to the atmosphere.
Answer Water evaporates from internal leaf surfaces into the air spaces, then water vapour diffuses through the leaf and usually out through stomata to the atmosphere.
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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.
Describe the two linked steps that make up transpiration from a leaf.
Give me a hint
Start at an internal moist surface.
Compare my reasoning
- Water evaporates from internal leaf surfaces into air spaces.
- Water vapour diffuses through the leaf and out to the atmosphere.
Water evaporates from moist internal leaf surfaces into the air spaces. Water vapour then diffuses through the leaf, usually out through open stomata, to the atmosphere.
Look for these in your work
- Evaporation stated
- Diffusion stated
- Direction correct
A student observes liquid droplets at a leaf edge and calls them transpiration. Correct the description.
Give me a hint
Distinguish vapour loss from liquid exudation.
Compare my reasoning
- Transpiration is water-vapour loss after evaporation and diffusion.
- Liquid droplets are not transpiration; the structure and conditions should be identified before naming guttation.
Transpiration is water-vapour loss: water evaporates from internal surfaces and vapour diffuses to the atmosphere. Liquid droplets at a leaf edge are not transpiration; they may be guttation, depending on their route and conditions.
Look for these in your work
- Transpiration sequence correct
- Liquid distinction accurate
- Guttation appropriately qualified
Internal water-vapour concentration is high, external humidity rises, and stomatal aperture is unchanged. Predict how the outward diffusion gradient changes.
Give me a hint
Compare inside with the more humid outside air.
Compare my reasoning
- Higher external humidity increases outside water-vapour concentration.
- The inside-to-outside difference becomes smaller, so the outward gradient is reduced.
The outward water-vapour concentration gradient becomes smaller because external water-vapour concentration rises while stomatal aperture is unchanged. This predicts a reduced diffusion drive under the stated conditions; it does not alone determine every whole-plant rate.
Look for these in your work
- Gradient direction compared
- Effect predicted
- Condition limit retained
Common mistakes
- Calling root water uptake transpiration
- Reversing evaporation and diffusion
- Describing transpiration as droplets of liquid water leaving the leaf
What can you explain now?
What is the first event in the leaf during transpiration?
Compare with the explanation
Water evaporates from internal surfaces into air spaces
The liquid-to-vapour change occurs before outward diffusion.
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Make it stick
What evaporates during transpiration, where does it evaporate, and how does the vapour reach the atmosphere?
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