Trace evaporation and diffusion through a leaf
Follow liquid water at mesophyll surfaces to vapour in air spaces and out through stomata.
Section B8.3 · Transpiration
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 mesophyll, stomata and diffusion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is diffusion?
What you will learn
- Locate the evaporation step
- Explain the subsequent vapour-diffusion route
Evaporation creates the vapour
Mesophyll cells present moist surfaces to intercellular air spaces. Water evaporates from those surfaces into the spaces. Evaporation is a liquid-to-gas change; it is not the movement of mineral ions out of the leaf.
The spaces between cells connect with stomatal pores. They provide a route for water vapour to reach the leaf surface. An air space is a gap between cells, not an empty xylem vessel or a chloroplast.
Diffusion carries vapour out
When the water-vapour concentration is higher inside than outside, vapour diffuses through stomata to the surrounding air. Diffusion is net movement down a concentration gradient; it is distinct from the evaporation that supplied the vapour.
If the external air becomes very humid, the vapour gradient can decrease. Stomatal aperture and other conditions also affect loss. “Water evaporates at the stomata” alone omits the required internal mesophyll evaporation and air-space route.
Sources for this lesson
Pause and explain
Where does the required evaporation occur?
Worked example
Order: diffusion through stomata; evaporation at mesophyll; vapour in air spaces.
Show the worked solution
- Evaporation at moist mesophyll creates vapour.
- Vapour occupies air spaces and diffuses out through stomata when a gradient exists.
Answer Moist mesophyll → evaporation → vapour in air spaces → diffusion through stomata.
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.
Give the evaporation site and the exit route.
Give me a hint
Separate creation of vapour from its movement.
Compare my reasoning
- Water evaporates from mesophyll surfaces into air spaces.
- Vapour diffuses from air spaces through stomata.
Evaporation: moist mesophyll surfaces. Exit: air spaces → stomata → outside air by diffusion.
Look for these in your work
- Locate evaporation
- Trace the vapour exit
Correct “diffusion turns liquid water into gas in xylem”.
Give me a hint
Both the process and site need correction.
Compare my reasoning
- Evaporation changes liquid water into vapour at mesophyll surfaces.
- Diffusion then moves the vapour down its concentration gradient.
Evaporation at mesophyll creates vapour; diffusion carries it through air spaces and stomata.
Look for these in your work
- Correct the state-change process
- Correct the site and diffusion role
External water-vapour concentration rises while leaf conditions stay comparable. Predict the effect on the diffusion gradient.
Give me a hint
Compare internal and external concentrations.
Compare my reasoning
- A higher external concentration narrows the inside-outside difference.
- A smaller gradient can reduce net outward diffusion, with other factors matched.
The outward vapour gradient decreases, so net vapour loss can decrease under matched conditions.
Look for these in your work
- Describe a smaller gradient
- Qualify the prediction with matched conditions
Common mistakes
- Using evaporation and diffusion as synonyms.
- Drawing liquid water directly flowing out through a stomatal pore as the main transpiration account.
What can you explain now?
Where does the required evaporation occur?
Compare with the explanation
At moist mesophyll surfaces into air spaces
The resulting vapour can then diffuse through stomata.
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Make it stick
Draw three arrows and label which step is evaporation and which is diffusion.
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.