How do plants move water and food between their roots, stems and leaves?
Describe transport in plants by xylem and phloem and explain transpiration
A focused answer to the O-Level Biology outcome on plant transport. The roles of xylem and phloem, the transpiration stream that pulls water up, and the factors that affect the rate of transpiration.
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What this dot point is asking
SEAB wants you to describe how plants transport substances: water and minerals up the xylem, and food (sugars) through the phloem. You should be able to define transpiration, describe the transpiration stream that pulls water up the plant, and explain how temperature, humidity, wind and light affect the rate of transpiration.
The answer
Two transport tissues
Plants have two transport tissues running through the roots, stem and leaves:
- Xylem transports water and dissolved mineral ions from the roots upward to the stem and leaves. Xylem vessels are made of dead, hollow cells joined end to end, strengthened to withstand the pull of water.
- Phloem transports food, mainly dissolved sugar (sucrose) made by photosynthesis, from the leaves to other parts of the plant such as growing tips, roots and storage organs. This movement can go in either direction.
Water uptake and the transpiration stream
Water enters the root hair cells from the soil by osmosis (the soil water has a higher water concentration than the cell sap). It then travels across the root to the xylem and up to the leaves. The continuous flow of water up the plant is the transpiration stream.
Transpiration
Transpiration is the loss of water vapour from the leaves (and other surfaces) of a plant, mainly through the stomata, by evaporation and diffusion. As water evaporates from the leaf cells and diffuses out through the stomata, more water is pulled up the xylem to replace it. So transpiration drives the upward transport of water.
Factors affecting the rate of transpiration
Transpiration is faster when:
- Temperature is higher: water evaporates faster and warm air holds more vapour.
- Humidity is lower: drier air outside the leaf steepens the water-vapour gradient, so more diffuses out.
- Wind speed is higher: moving air carries water vapour away, keeping the gradient steep.
- Light is brighter: stomata open in the light for photosynthesis, allowing more water vapour to escape.
Why transpiration matters
Transpiration pulls water up to the leaves for photosynthesis, carries mineral ions up the plant, and cools the leaves as water evaporates. But too much water loss can cause wilting, so plants close their stomata when water is short.
Examples in context
Example 1. A cut flower in dyed water. When a white flower is placed in coloured water, the dye is drawn up the xylem and colours the petals, showing the upward transport of water through the xylem and the transpiration pull.
Example 2. Wilting on a hot day. On a hot, dry, windy day, transpiration is fast and the plant may lose water faster than the roots can supply it. The cells become flaccid and the plant wilts, which is why plants close their stomata to conserve water.
Try this
Q1. State what the xylem transports and in which direction. [2 marks]
- Cue. Water and dissolved mineral ions, upward from the roots to the leaves.
Q2. Define transpiration. [2 marks]
- Cue. The loss of water vapour from a plant, mainly through the stomata of the leaves, by evaporation and diffusion.
Q3. Explain why transpiration is faster at a higher temperature. [2 marks]
- Cue. Water evaporates faster from the leaf cells and warm air holds more vapour, steepening the gradient, so water vapour diffuses out more quickly.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marks(a) State what is transported by the xylem and the direction of movement. (b) State what is transported by the phloem. (c) Define transpiration.Show worked answer →
(a) The xylem transports water and dissolved mineral ions, upward from the roots to the stem and leaves.
(b) The phloem transports food, mainly dissolved sugars (sucrose) made in the leaves, to other parts of the plant (such as growing tips, roots and storage organs); this movement can be in either direction.
(c) Transpiration is the loss of water vapour from the leaves (and other surfaces) of a plant, mainly through the stomata, by evaporation and diffusion.
Markers reward water and mineral ions moving up in the xylem, sugars carried in the phloem, and a correct definition of transpiration as the loss of water vapour, mainly through the stomata.
Original4 marksExplain how an increase in temperature and an increase in wind speed each affect the rate of transpiration, and explain why.Show worked answer →
An increase in temperature increases the rate of transpiration, because the water molecules gain energy and evaporate faster from the leaf surfaces, and warm air can hold more water vapour, steepening the diffusion gradient out of the leaf.
An increase in wind speed increases the rate of transpiration, because moving air carries away the water vapour from around the stomata, keeping the concentration of water vapour outside low. This maintains a steep diffusion gradient, so water vapour diffuses out faster.
Markers reward higher temperature speeding evaporation, and wind removing water vapour to keep the gradient steep, both increasing transpiration.
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