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Transpiration is the loss of water vapour from leaves through stomata, pulling water up from the roots through xylem. Free notes and 13 questions answered.
Transpiration and Water Movement is part of Plant tissues, organs and systems in GCSE AQA Higher Biology.
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Transpiration is the loss of water vapour from leaves through stomata, pulling water up from the roots through xylem. Free notes and 13 questions answered.
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Use this lesson to stay inside Plant tissues, organs and systems while connecting the detail back to the wider Organisation topic.
Read the core explanations from Transpiration and Water Movement before testing yourself from memory.
Picture a 30-metre oak tree. Water enters through tiny root hairs in the soil and somehow travels all the way to the topmost leaves. No pump, no heart - yet water defies gravity every second of every day. This is transpiration in action, and it's one of the most elegant transport systems in nature.
Understanding transpiration isn't just about plants - it connects to water cycles, climate, and even how we design greenhouses. Get this right, and you'll unlock how plants survive, thrive, and shape our world.
Here's how the process works step by step. Water evaporates from mesophyll cells inside the leaf, creating water vapour. This vapour diffuses out through stomata - tiny pores on the leaf surface. As water leaves, it creates a 'pull' that draws more water up through the xylem vessels from the roots.
Trap: Many students think plants actively pump water upwards. Actually, transpiration creates suction that pulls water up - like drinking through a very long straw. The plant doesn't push; evaporation pulls.
Guard cells are the gatekeepers. When they absorb water, they become turgid (swollen) and bend outwards, opening the stomata. When they lose water, they become flaccid and close the stomata. This gives plants precise control over when to lose water and when to conserve it.
Four key factors control how fast plants lose water. Understanding these helps explain why plants wilt on hot days and why greenhouses need careful climate control.
Temperature: Higher temperature gives water molecules more kinetic energy, so they evaporate faster from leaf cells. More evaporation means faster transpiration. Think of a kettle - hot water turns to steam much quicker than cold water.
Humidity: High humidity means lots of water vapour already in the air around the leaf. This reduces the concentration gradient, slowing diffusion of water vapour out of stomata. Low humidity creates a steep gradient and speeds up transpiration.
Air movement (wind): Moving air removes water vapour from around the leaf surface, maintaining a steep concentration gradient. Still air allows water vapour to build up, reducing the gradient and slowing transpiration.
Light intensity: Bright light causes stomata to open wider for photosynthesis (plants need CO₂). Open stomata allow more water vapour to escape. In darkness, stomata close to conserve water, reducing transpiration.
Transpiration rate is a compound measure - it combines the amount of water lost with the time taken. The basic formula helps us compare different conditions and understand how environmental factors affect water loss.
Worked example: Calculating Transpiration Rate
A plant loses 15 cm³ of water in 30 minutes. Calculate the transpiration rate. [2 marks]
Answer: 0.5 cm³/minute
While xylem carries water upwards, phloem carries dissolved sugars in both directions. Leaves make glucose during photosynthesis - these are 'sources'. Growing tips, roots, and storage organs need this glucose - these are 'sinks'. Phloem transports sugars from sources to sinks.
Unlike transpiration (which is passive), translocation requires energy. Companion cells actively load sugars into phloem sieve tubes, creating pressure that pushes the sugar solution along.
Key difference: Transpiration moves water up through xylem (one direction). Translocation moves sugars through phloem (both directions, depending on where they're needed).
The potometer is the classic method for measuring transpiration rate. It doesn't directly measure water loss - instead, it measures water uptake, which equals water loss under steady conditions.
Worked example: Using a Potometer
Describe how to use a potometer to investigate the effect of wind speed on transpiration rate.
Answer: Measure bubble movement over time at different wind speeds, keeping other factors constant
Transpiration questions often test your understanding of factors affecting rate and your ability to interpret experimental data. Here's how to tackle the common question types and avoid the traps.
Common 6-mark question structure: 'Explain how environmental factors affect the rate of transpiration.' Start with a clear definition of transpiration, then work through each factor (temperature, humidity, air movement, light) explaining the mechanism. Link each factor to evaporation rate or diffusion gradients.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Transpiration is the loss of water vapour from plant leaves through stomata, creating a transpiration stream that pulls water up from the roots through xylem vessels. Water evaporates from mesophyll cells, diffuses out through stomata, and this creates a pull that draws more water up the xylem.
Transpiration moves water up through xylem in one direction and is passive, driven by evaporation. Translocation moves dissolved sugars through phloem in both directions, from sources such as leaves to sinks such as roots, and requires energy because companion cells actively load sugars into phloem sieve tubes.
Four factors affect the rate of transpiration: temperature, humidity, air movement and light intensity. Higher temperature, low humidity, more wind and bright light all speed up transpiration, because they speed up evaporation, keep a steep concentration gradient or open the stomata wider.
Transpiration rate = volume of water lost ÷ time. For example, a plant that loses 15 cm³ of water in 30 minutes has a transpiration rate of 15 ÷ 30 = 0.5 cm³/minute. Always include units in the final answer.
A potometer measures water uptake, which equals water loss under steady conditions. The plant stem is cut underwater and inserted into a water-filled tube, a small air bubble is let into the graduated tube, and the distance the bubble moves in a set time gives the rate: rate = distance moved ÷ time.
Higher temperature speeds up transpiration because water molecules gain more kinetic energy and evaporate faster from leaf cells. More evaporation means faster transpiration, just as hot water turns to steam more quickly than cold water.
Bright light speeds up transpiration because it causes stomata to open wider so the plant can take in CO₂ for photosynthesis, letting more water vapour escape. In darkness, stomata close to conserve water, reducing transpiration.
Wind speeds up transpiration because moving air removes water vapour from around the leaf surface, maintaining a steep concentration gradient. Still air lets water vapour build up, reducing the gradient and slowing transpiration.
The rate of transpiration falls as humidity in the air rises. High humidity means lots of water vapour is already in the air around the leaf, which reduces the concentration gradient and slows diffusion of water vapour out of the stomata, while low humidity creates a steep gradient and speeds transpiration up.
Plants do not actively pump water upwards. Transpiration creates suction that pulls water up through the xylem: the plant does not push; evaporation from the leaves pulls.
Translocation requires energy, unlike transpiration, which is passive. Companion cells actively load sugars into phloem sieve tubes, creating pressure that pushes the sugar solution along from sources to sinks.
The plant stem is cut underwater when a potometer is set up to prevent air bubbles entering the xylem. The stem is then inserted into the potometer tube filled with water, and a small air bubble is allowed into the graduated tube to measure water uptake.
The effect of wind speed on transpiration is investigated with a potometer, changing only wind speed using a fan while keeping temperature, humidity and light constant. The distance the air bubble moves in a set time, such as 10 minutes, is measured at each wind speed, and rate = distance moved ÷ time.