Edexcel IGCSE Biology · Spec 2.55B-2.58B
Transpiration
What transpiration is, the four factors that affect its rate, and how a potometer measures water uptake.
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Transpiration
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Explained
Water leaving the leaves, and measuring how fast
Transpiration is the loss of water vapour from a plant, mainly through the stomata on the underside of the leaves.
It happens as a consequence of gas exchange. The stomata have to open to let carbon dioxide in for photosynthesis, and water vapour escapes through the same openings. Water evaporates from the surfaces of the mesophyll cells inside the leaf and diffuses out.
As water leaves the top of the plant, more is pulled up the xylem to replace it. That continuous column is the transpiration stream, and it is what delivers water and mineral ions from the roots to the leaves.
The four factors
Light intensity. More light opens the stomata wider for photosynthesis, so more water escapes. In the dark most stomata close and transpiration nearly stops.
Temperature. Warmer air gives the water molecules more kinetic energy, so they evaporate and diffuse faster.
Wind speed. Moving air carries away the water vapour that has collected just outside the leaf, maintaining a steeper concentration gradient, so diffusion continues faster.
Humidity. Humid air already contains a lot of water vapour, so the gradient between the leaf and the air is shallower and transpiration slows.
Three of those four work through the concentration gradient, so if you understand the gradient you can reason your way to the answer rather than recalling it.
The potometer
A potometer measures the rate at which the shoot takes water up, by timing how far an air bubble moves along a capillary tube.
Strictly it measures uptake rather than transpiration. Most of the water taken up is transpired, but a small amount is used in photosynthesis and retained in the cells, so uptake is an estimate. Saying so is often a mark on its own.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for describing a potometer investigation credits six things: setting the apparatus up underwater or cutting the stem underwater and drying the leaves; using a fan at different distances or different speeds; leaving it for a stated time; measuring the distance the bubble moves; keeping the other factors constant; and repeating, resetting the bubble using the reservoir.
The first of those is the one students omit. Cutting the stem and assembling the apparatus underwater stops air entering the xylem, because an air bubble in the water column would break it and the shoot would stop taking water up. Drying the leaves matters because surface water would evaporate and exaggerate the reading.
On a data question about stomata, the same paper credits an inverse relationship or negative correlation between a hormone and water loss, that the hormone closes the stomata, and that closing the stomata reduces transpiration and prevents wilting. It also credits noting that light, humidity or wind may have changed, which is the standard way to question a conclusion drawn from field data.
Adaptations that reduce water loss
Most stomata are on the lower surface, away from direct sun, and a waxy cuticle on the upper surface is waterproof.
Plants in dry places go further: fewer stomata, stomata sunk into pits where humid air collects, hairs that trap moist air near the surface, rolled leaves, and in a cactus leaves reduced to spines with a thick stem storing water.
Each of those reduces the concentration gradient or the area available for evaporation, and naming the mechanism is what turns a list into an explanation.
Spec 2.55B-2.58B
What you need to know
- Explain what transpiration is
- Describe four factors that affect its rate
- Describe how a potometer measures it
Active recall
Quick check
Answer each question before opening the answer.
Define transpiration.
The loss of water vapour from the leaves by evaporation followed by diffusion through the stomata.
Name three factors that increase the rate of transpiration.
Higher temperature, higher light intensity, more air movement (wind), and lower humidity.
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