OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(c) (i) the process of transpiration and the environmental factors that…
Full specification wording
Transport in plants: (c) (i) the process of transpiration and the environmental factors that affect transpiration rate
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 4 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. Water evaporates from the moist surfaces of mesophyll cells into air spaces, then diffuses out through stomata down a water-potential gradient into the surrounding air. This creates tension that pulls water up the xylem in the transpiration stream. The rate depends on environmental factors: higher temperature increases evaporation and diffusion; increased light intensity opens stomata; greater air movement (wind) removes humid air near the leaf, steepening the gradient; and higher humidity reduces the gradient, lowering the rate. Lower atmospheric pressure can also increase the rate. These factors act by changing the water-potential gradient between leaf air spaces and the external air, or by changing stomatal aperture.
What earns the marks
- Defines transpiration as loss of water vapour from aerial parts, chiefly via stomata.
- Describes evaporation from mesophyll surfaces into air spaces followed by diffusion out through stomata down a water-potential gradient.
- Explains that rising temperature increases the rate by increasing evaporation and diffusion.
- Explains that increased light intensity opens stomata, raising the rate.
- Explains that greater air movement removes humid air, steepening the gradient and increasing the rate.
- Explains that higher humidity reduces the water-potential gradient and lowers the rate.
Common mistakes and how to put them right
Describing transpiration as a passive 'pumping' of liquid water.
Put it right: It is loss of water vapour, with the transpiration stream driven by tension; correct the wording.
Saying wind 'dries' the leaf without mechanism.
Put it right: Wind removes humid air, increasing the diffusion gradient; state the gradient.
Claiming humidity increases transpiration.
Put it right: High humidity reduces the gradient and lowers the rate; correct the direction.
Exam tips
- Always link each environmental factor to the mechanism — evaporation, diffusion gradient or stomatal aperture.
- Use the term water-potential gradient rather than vague 'concentration' when explaining diffusion of water vapour.
- For data questions, read the axis and units carefully before comparing rates.
Try the lesson
The start of the Transport in plants: (c) (i) the process of transpiration and the environmental factors that affect transpiration rate lesson, made from checked teaching notes for this specification point.
Have a go first
Through which leaf structures does most water vapour leave a plant during transpiration?
Listen
Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves. Water evaporates from the moist surfaces of mesophyll cells into air spaces, then diffuses out through stomata down a water-potential gradient into the surrounding air. This creates tension that pulls water up the xylem in the transpiration stream. The rate depends on environmental factors: higher temperature increases evaporation and diffusion; increased light intensity opens stomata; greater air movement (wind) removes humid air near the leaf, steepening the gradient; and higher humidity reduces the gradient, lowering the rate. Lower atmospheric pressure can also increase the rate. These factors act by changing the water-potential gradient between leaf air spaces and the external air, or by changing stomatal aperture. A strong answer earns marks for these points. Defines transpiration as loss of water vapour from aerial parts, chiefly via stomata. Describes evaporation from mesophyll surfaces into air spaces followed by diffusion out through stomata down a water-potential gradient. Explains that rising temperature increases the rate by increasing evaporation and diffusion. Explains that increased light intensity opens stomata, raising the rate. Exam tip: Always link each environmental factor to the mechanism — evaporation, diffusion gradient or stomatal aperture.
Key words
Say what each word means before you turn the card.
Transpiration
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Water-potential gradient
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Transpiration stream
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Stomatal aperture
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See every definition
- Transpiration
- The loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves.
- Water-potential gradient
- The gradient down which water vapour diffuses out through stomata into the surrounding air, and which drives the transpiration stream.
- Transpiration stream
- The movement of water up the xylem, pulled by tension created by transpiration.
- Stomatal aperture
- The opening of stomata, which environmental factors change to affect the transpiration rate.
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