What is transpiration, and which factors change the rate at which a plant loses water?
Transpiration as the loss of water vapour from a plant, the transpiration stream, and the factors affecting the rate of transpiration.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on transpiration, covering transpiration as water loss from the leaves, the transpiration stream up the xylem, and how light, temperature, humidity and wind affect the rate.
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What this dot point is asking
WJEC Double Award Unit 1 wants you to explain transpiration, describe the transpiration stream, and explain how environmental factors affect the rate of transpiration.
What transpiration is
Transpiration is largely an unavoidable consequence of the leaf needing open stomata for gas exchange: while carbon dioxide diffuses in, water vapour diffuses out. Some water loss is useful because it pulls water (and dissolved minerals) up to the leaves and helps to cool the plant.
The transpiration stream
The transpiration stream also carries dissolved mineral ions absorbed by the roots up to the rest of the plant, so transpiration is linked to mineral transport as well as cooling.
Factors affecting the rate of transpiration
The rate of transpiration changes with the conditions:
- Temperature: higher temperature gives water molecules more energy, so they evaporate and diffuse out faster.
- Humidity: in dry (low humidity) air there is a steep concentration gradient, so water diffuses out faster; in humid air the rate is slower.
- Wind (air movement): wind carries away the water vapour from around the stomata, keeping the gradient steep, so the rate increases.
- Light intensity: in bright light the stomata open for photosynthesis, so more water is lost.
Measuring water uptake
A potometer measures the rate at which a leafy shoot takes up water, which is used as a measure of transpiration. As the shoot transpires, it pulls water from the apparatus, and an air bubble moves along a capillary tube; the distance it moves in a set time gives the rate. Changing the conditions (using a fan, a lamp, or a bag over the leaves) shows how each factor affects the rate.
Wilting and water balance
A plant must balance the water it takes up against the water it loses by transpiration. On a hot, dry, windy day, transpiration can lose water faster than the roots can replace it. The cells then lose water, become flaccid, and the plant wilts - the leaves and stem droop because the cells are no longer turgid. Wilting actually reduces further water loss, because the drooping leaves expose less surface to the sun and the guard cells close the stomata. This links transpiration back to osmosis and turgor in the cells.
Why most stomata are on the underside
Counting stomata under a microscope shows that most are on the lower surface of the leaf. This is an adaptation to reduce water loss: the underside is shaded and cooler, so less water evaporates there than from the hot upper surface that faces the sun. Having the stomata mostly underneath lets the leaf exchange gases while keeping transpiration as low as possible, which is a neat example of structure suiting function.
Try this
Q1. Name the structures through which most water vapour leaves a leaf. [1 mark]
- Cue. The stomata.
Q2. State one condition that would decrease the rate of transpiration. [1 mark]
- Cue. Any one of: lower temperature, higher humidity, less wind, or darkness.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain why the rate of transpiration increases on a hot, windy day.Show worked answer →
A Unit 1 explain question worth 4 marks. Reward: a higher temperature gives the water molecules more energy so they evaporate and diffuse out faster (1), and increases the rate of evaporation inside the leaf (1); wind carries away the water vapour from around the stomata, keeping a steep concentration gradient so water diffuses out faster (1); so both factors together increase the rate of transpiration (1). Markers credit temperature increasing evaporation and wind maintaining the gradient. A common error is to say wind "blows water out" rather than maintaining the gradient.
WJEC style3 marksDescribe how a plant could be investigated to measure the rate of water uptake, and name a piece of apparatus used.Show worked answer →
A Unit 1 practical question. Reward: a potometer is used to measure the rate of water uptake (1); a leafy shoot is sealed into the apparatus and the movement of an air bubble along a capillary tube is timed and measured (1); the further or faster the bubble moves, the faster the water uptake, which is used as a measure of transpiration (1). Markers credit the potometer, the bubble movement and using it as a measure of rate. A common error is to confuse uptake with the actual water lost.
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