How do plants move water up to their leaves and sugars to where they are needed?
Transport in plants by xylem (water and mineral ions) and phloem (dissolved sugars), the transpiration stream and translocation, the factors affecting the rate of transpiration, and the role of stomata and guard cells.
A focused answer to the OCR Gateway GCSE Biology A topic B2 on transport in plants, covering xylem and phloem, the transpiration stream and translocation, the factors affecting transpiration rate, and the role of stomata and guard cells.
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What this dot point is asking
OCR wants you to describe transport in plants by xylem and phloem, explain the transpiration stream and translocation, state the factors affecting transpiration rate, and describe the role of stomata and guard cells.
Xylem and phloem
Plants are large multicellular organisms, so (like animals) they need transport systems. They have two:
The transpiration stream
The transpiration stream is useful to the plant: it brings water (needed for photosynthesis and to keep cells turgid) and mineral ions up to the leaves, and helps cool the plant.
Factors affecting the rate of transpiration
The rate of transpiration increases when:
- Light intensity is higher: the stomata open in the light (for photosynthesis), so more water evaporates.
- Temperature is higher: water molecules have more energy and evaporate faster.
- Humidity is lower: drier air around the leaf keeps the concentration gradient steep, so water diffuses out faster.
- Wind speed is higher: moving air removes water vapour from around the leaf, keeping the gradient steep.
You can measure transpiration rate with a potometer, which measures how fast a plant takes up water (a close estimate of transpiration).
Stomata and guard cells
The stomata are pores, mostly on the underside of the leaf, that let carbon dioxide in for photosynthesis and let water vapour and oxygen out. Each stoma is surrounded by two guard cells that change shape to open or close it.
In the light the guard cells take in water, become turgid and bend apart, opening the stoma. In the dark, or when the plant is short of water, the guard cells lose water, become flaccid and close the stoma, reducing water loss. This is how the plant balances getting carbon dioxide for photosynthesis against losing too much water.
Translocation
Translocation is the movement of dissolved sugars (and other substances) in the phloem, from sources (where sugars are made or stored, such as the leaves) to sinks (where they are used or stored, such as growing roots, shoots and fruits). Because sources and sinks change with the seasons, phloem moves sugars in both directions, unlike the one-way xylem.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksCompare the structure and function of xylem and phloem in a plant.Show worked answer →
A 4-mark Compare question.
Xylem: carries water and dissolved mineral ions; movement is one way, from the roots up to the leaves; made of dead, hollow cells strengthened with lignin, forming continuous tubes.
Phloem: carries dissolved sugars (such as sucrose) made in photosynthesis; movement is in both directions (up and down the plant), called translocation; made of living cells.
Markers reward the paired points: what each carries, the direction of movement, and whether the cells are living or dead. A strong answer notes xylem moves substances from roots to leaves while phloem moves sugars from sources (leaves) to sinks (roots, fruits).
OCR 20214 marksDescribe what is meant by transpiration, and explain how increasing the temperature and the wind speed each affect the rate of transpiration.Show worked answer →
A 4-mark question on transpiration.
Transpiration: the evaporation of water from the leaves (mainly through the stomata), which pulls water up the xylem from the roots in the transpiration stream.
Temperature: a higher temperature gives the water molecules more energy, so water evaporates from the leaves faster and the rate of transpiration increases.
Wind speed: faster moving air removes the water vapour from around the leaf, keeping the concentration gradient steep, so water evaporates and diffuses out faster and the rate of transpiration increases.
Markers reward the definition plus a clear cause-and-effect explanation for each factor.
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