What is transpiration, and what makes a plant lose water faster or slower?
Transpiration as the loss of water vapour from a plant, the transpiration stream, the factors that affect the rate of transpiration, and the role of stomata and guard cells.
A focused answer to the WJEC GCSE Biology section 1.5 topic on transpiration, covering transpiration as the loss of water vapour from a plant, the transpiration stream, the factors affecting its rate, and the role of stomata and guard cells in controlling water loss.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC wants you to define transpiration, describe the transpiration stream, explain the factors that affect the rate of transpiration, and describe the role of stomata and guard cells.
What transpiration is
As water evaporates from the cells inside the leaf and escapes through the stomata, it pulls more water up behind it. This continuous flow of water from the roots, up the xylem and out of the leaves is called the transpiration stream. It helps the plant in several ways: it transports water for photosynthesis, carries dissolved mineral ions to the leaves, supports the plant by keeping cells turgid, and helps cool the leaves.
Factors affecting the rate of transpiration
Four conditions change how fast a plant transpires.
- Temperature. A higher temperature gives the water molecules more energy, so they evaporate faster, and warm air holds more water vapour. The rate increases.
- Light intensity. In brighter light the stomata open wider for photosynthesis, so more water vapour escapes. The rate increases.
- Humidity. In humid air there is already a lot of water vapour outside the leaf, so the concentration gradient is less steep and water diffuses out more slowly. The rate decreases.
- Air movement (wind). Moving air carries away the water vapour from around the stomata, keeping a steep concentration gradient, so water diffuses out faster. The rate increases.
Stomata and guard cells
The stomata are tiny pores, mostly on the lower surface of the leaf. Each is surrounded by two guard cells that can open or close it.
- Most stomata are on the lower surface because it is cooler and shadier, which reduces water loss.
- When the plant has plenty of water, the guard cells become turgid and the stoma opens, letting carbon dioxide in for photosynthesis.
- When water is short, the guard cells lose water and become flaccid, so the stoma closes, reducing further water loss.
This means the plant must balance two needs: open stomata let carbon dioxide in but lose water; closed stomata save water but stop photosynthesis.
Measuring transpiration with a potometer
A potometer measures the rate at which a cut shoot takes up water, which is a good estimate of the rate of transpiration. You record how far a bubble of air moves along a tube in a set time, under different conditions. To investigate a factor, you change one condition (such as wind from a fan, or a brighter lamp) while keeping the others the same, and compare the rates.
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 marksDescribe how an increase in temperature and an increase in wind speed each affect the rate of transpiration, and explain why.Show worked answer →
A 4-mark question: two factors, each with a reason.
An increase in temperature increases the rate of transpiration, because the water molecules have more energy and evaporate faster from the leaf cells, and warm air can hold more water vapour.
An increase in wind speed increases the rate of transpiration, because moving air carries away the water vapour from around the stomata, keeping a steep concentration gradient so water diffuses out faster.
Markers reward: temperature increases evaporation; wind removes water vapour and keeps the gradient steep. Just saying each factor "increases transpiration" without a reason loses the explanation marks.
WJEC style3 marksExplain why most stomata are found on the lower surface of a leaf and how guard cells reduce water loss.Show worked answer →
A 3-mark question on stomata and guard cells.
Most stomata are on the lower surface, which is cooler and shadier than the upper surface, so less water evaporates and is lost. The guard cells around each stoma can close it, for example when the plant is short of water, which reduces the water vapour escaping and so reduces water loss.
Markers reward: lower surface is cooler/shadier so less evaporation; guard cells close the stomata to reduce water loss. Saying guard cells "let gases in" is true but does not answer the question about reducing water loss.
Related dot points
- The structure of a leaf and how it is adapted for photosynthesis, and the structure and function of the xylem and phloem in transporting water, minerals and sugars.
A focused answer to the WJEC GCSE Biology section 1.5 topic on leaf structure and plant transport, covering the structure of a leaf and its adaptations for photosynthesis, the structure and function of xylem and phloem, and the uptake of water by root hair cells.
- Photosynthesis as the process that makes glucose using light energy, its word and symbol equations, the limiting factors of light, carbon dioxide and temperature, and the required practicals.
A focused answer to the WJEC GCSE Biology section 1.5 topic on photosynthesis, covering photosynthesis as the process that uses light energy to make glucose, its word and symbol equations, the limiting factors of light intensity, carbon dioxide and temperature, and the required practicals on starch production and rate.
- Feeding relationships in ecosystems, food chains and food webs, pyramids of number and biomass, the transfer of energy along a food chain and why energy is lost at each trophic level.
A focused answer to the WJEC GCSE Biology section 1.6 topic on feeding relationships, covering food chains and food webs, pyramids of number and biomass, the transfer of energy along a food chain, why energy is lost at each trophic level, and the implications for food production.
- The carbon cycle and the nitrogen cycle, the roles of photosynthesis, respiration, combustion and decomposition, the types of bacteria in the nitrogen cycle, and the conditions affecting decay.
A focused answer to the WJEC GCSE Biology section 1.6 topic on nutrient cycles, covering the carbon cycle and the roles of photosynthesis, respiration, combustion and decomposition, the nitrogen cycle and its bacteria, the role of decomposers, and the conditions that affect the rate of decay.
Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)