Transpiration — Cambridge IGCSE Biology
Water loss from leaves, the factors that affect its rate, and how transpiration pull draws water up the xylem.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Transpiration is the loss of water vapour from leaves. Water moves from the xylem into the mesophyll cells and evaporates from their surfaces into the air spaces. The water vapour then diffuses out through the stomata. The large internal surface area of the air spaces and the size and number of stomata affect how much water is lost.
The rate of transpiration increases with higher temperature and higher wind speed, and decreases in humid air. Higher temperature makes water evaporate and diffuse faster, and wind keeps the concentration gradient steep by blowing water vapour away. A potometer estimates the rate by measuring how far an air bubble moves as a cut shoot takes up water.
Transpiration pull and wilting
SupplementTranspiration pull is the pull created when water evaporates from mesophyll cells and is replaced by water drawn out of the xylem. Water molecules are attracted to each other, so they form a continuous column that is pulled up from the roots. Wilting happens when a plant loses water faster than it takes it up, so cells become flaccid.
Exam questions
7 exam-style questions (25 marks), each with its mark scheme.
Answer the questionsFlashcards
10 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of transport in plants on one page, so you can see where this subtopic fits.
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Method, variables, risks and a results table, with questions to answer.
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Quick check questions
Define transpiration.
Show answer
The loss of water vapour from leaves.
Name two factors that increase the rate of transpiration when they increase.
Show answer
Temperature and wind speed.
Supplement How does higher humidity affect the rate?
Show answer
It slows transpiration, because the concentration gradient is less steep.
Supplement What holds the water column together in the xylem?
Show answer
Forces of attraction between water molecules.
Frequently asked questions
How does temperature affect the rate of transpiration?
A higher temperature increases the rate of transpiration, so a leafy shoot loses water vapour from its leaves faster. You can show this with a potometer, using rooms or water baths at different temperatures and measuring how far the air bubble moves in a set time. Keep the shoot, light and wind speed the same.
Why does wind increase the rate of transpiration?
Supplement Wind increases the rate of transpiration because it blows water vapour away from around the stomata. This keeps the concentration gradient between the air spaces inside the leaf and the outside air steep, so water vapour diffuses out faster. Higher humidity has the opposite effect, because humid air makes the gradient less steep.
How does a potometer measure transpiration?
A potometer measures how much water a cut leafy shoot takes up. As the shoot takes up water, an air bubble moves along a capillary tube, and the distance moved in a set time gives the rate of water uptake. Most of the water taken up is lost by transpiration, so this estimates the rate of transpiration.
How does water move up the xylem?
Supplement Water moves up the xylem by transpiration pull. Water evaporates from the mesophyll cells and diffuses out through the stomata, and more water is drawn out of the xylem to replace it. Water molecules are attracted to each other, so they form a continuous column that is pulled up from the roots to the leaves.
Why do plants wilt?
Supplement Plants wilt when they lose water by transpiration faster than they can take it up from the soil. Water leaves the cells by osmosis, so the cells lose turgor and become flaccid. The cells no longer press against each other, so the leaves and stems are not supported and droop. Hot, dry or windy conditions make wilting more likely.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026