How does water move through a plant, and what affects the rate?
Explain how water and mineral ions are transported by transpiration including the role of stomata, how sucrose is moved by translocation, and the effect of environmental factors on the rate of water uptake.
A focused answer to Edexcel GCSE Biology 6.9, 6.10, 6.12 and 6.13, covering transpiration and the stomata, translocation of sucrose, the effect of light, temperature and air movement on water uptake, and rate calculations for transpiration.
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What this dot point is asking
Edexcel statements 6.9, 6.10, 6.12 and 6.13 want you to explain how water and mineral ions move up the plant by transpiration (including the role of the stomata), how sucrose moves by translocation, how environmental factors affect the rate of water uptake, and how to do rate calculations for transpiration.
Transpiration
Water enters the roots by osmosis, travels up the xylem, and evaporates from the leaves. This stream also carries the mineral ions the plant needs. So transpiration both supplies the leaves with water for photosynthesis and delivers minerals around the plant, while cooling the leaf as water evaporates.
The stomata
Translocation
Translocation is the transport of dissolved sugars, mainly sucrose, made in the leaves by photosynthesis, to all other parts of the plant in the phloem. The sugars go to growing regions (to be used for respiration and growth) and to storage organs (to be stored as starch). Unlike the xylem's one-way flow, translocation can move sugars in either direction, wherever they are needed.
Factors affecting the rate of water uptake
The rate of transpiration, and so of water uptake, increases when conditions speed up evaporation and diffusion from the leaf:
- Higher temperature: water evaporates and diffuses faster.
- More light: stomata open wider for photosynthesis, so more water escapes.
- More air movement (wind): moving air carries away water vapour, keeping a steep concentration gradient.
- Lower humidity: drier air gives a steeper gradient between the leaf and the air, so water diffuses out faster.
You can measure water uptake with a potometer, timing how far an air bubble moves along a tube as the plant draws up water.
Try this
Q1. Name the cells that open and close the stomata. [1 mark]
- Cue. Guard cells.
Q2. State two environmental factors that increase the rate of transpiration. [2 marks]
- Cue. Any two of: higher temperature, more light, more air movement (wind), lower humidity.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksExplain how an increase in temperature and air movement increases the rate of transpiration from a leaf.Show worked answer →
A 4-mark explain question rewards a reason for each factor.
Higher temperature: the water molecules in the leaf gain more energy, so they evaporate faster from the surfaces inside the leaf and diffuse out of the stomata more quickly, increasing the rate of transpiration.
More air movement (wind): moving air carries away the water vapour from around the stomata, keeping a steep concentration gradient between the inside of the leaf and the air, so water diffuses out faster.
Markers reward faster evaporation and diffusion for temperature, and the maintenance of a steep concentration gradient for air movement. Saying the wind blows the water off the leaf is too vague to score.
Edexcel 20213 marksIn a potometer experiment, an air bubble moved 60 mm in 5 minutes. Calculate the rate of water uptake in mm per minute and state one factor that would increase it.Show worked answer →
A 3-mark question rewards the calculation and a valid factor.
Rate distance time mm per minute.
A factor that would increase it: higher temperature, more air movement (wind), brighter light (more stomata open), or lower humidity.
Markers reward the correct value with its unit ( mm/min) and one factor that genuinely increases water uptake. Quoting a factor that would slow uptake (such as high humidity), or omitting the unit, loses marks.
Related dot points
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)