How do plants move water and food around, and how is water lost through the leaves?
Describe the roles of xylem and phloem, explain the uptake and transport of water and transpiration, and state the factors affecting the rate of transpiration
A focused answer to the O-Level Combined Science outcome on transport in plants. Water uptake by root hair cells, xylem and phloem and what each carries, the transpiration stream, and the factors affecting transpiration rate.
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What this dot point is asking
SEAB wants you to describe how water and food are transported in a plant: the jobs of xylem and phloem, how water is taken up by the roots and pulled up through the plant, what transpiration is, and the factors that change its rate. The xylem-versus-phloem comparison and the transpiration factors are common exam marks.
The answer
Two transport tissues
Plants have two transport tissues:
- Xylem carries water and dissolved mineral ions from the roots up to the leaves. The movement is one-way, upwards.
- Phloem carries dissolved food (mainly sugars made in the leaves) to all parts of the plant. The movement can be in either direction (to growing tips and storage organs).
Water uptake by the roots
Water is absorbed from the soil by root hair cells. Each has a long, thin extension giving a large surface area, and water enters by osmosis because the cell sap has a lower water concentration than the soil water. Mineral ions are absorbed by active transport (against their concentration gradient).
The transpiration stream
Water travels up the xylem from roots to leaves in a continuous column, the transpiration stream. It is pulled up because water evaporates from the leaf cells and is lost through the stomata; this loss creates a "pull" that draws more water up the xylem behind it.
Transpiration
Transpiration is the loss of water vapour from a plant, mainly through the stomata in the leaves. Water evaporates from the moist surfaces of the leaf cells and the vapour diffuses out through the open stomata. Transpiration pulls water and minerals up the plant and helps cool the leaves.
Factors affecting the rate of transpiration
The rate increases with:
- higher temperature (faster evaporation),
- more wind / air movement (blows away the vapour, keeping a steep gradient),
- lower humidity (drier air keeps a steep gradient),
- higher light intensity (the stomata open in light for photosynthesis, letting more water out).
Examples in context
Example 1. Why cut flowers wilt without water. A cut flower keeps transpiring, losing water from its leaves, but cannot replace it once removed from the plant. Standing it in water lets the stem draw water up the xylem again, which is why fresh water keeps flowers upright.
Example 2. Plants in dry climates. Desert plants have adaptations such as few, sunken stomata and a thick waxy leaf surface to reduce transpiration, because water is scarce. This shows how the factors affecting transpiration shape the way plants survive in different environments.
Try this
Q1. State what xylem transports and what phloem transports. [2 marks]
- Cue. Xylem transports water and dissolved mineral ions; phloem transports dissolved food (sugars).
Q2. Explain how water enters a root hair cell from the soil. [2 marks]
- Cue. By osmosis: the soil water has a higher water concentration than the cell sap, so water moves into the cell across the partially permeable membrane.
Q3. State two factors that increase the rate of transpiration. [2 marks]
- Cue. Any two of: higher temperature, more wind (air movement), lower humidity, brighter light.
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.
Original4 marksCompare the functions of xylem and phloem in a plant, and for each tissue name what it transports and the direction of movement.Show worked answer →
Xylem transports water and dissolved mineral ions. The movement is one-way, upwards, from the roots to the leaves (and stem).
Phloem transports dissolved food, mainly sugars (sucrose) made in the leaves. The movement can be in either direction, from the leaves (the source) to other parts of the plant such as growing tips and storage organs (the sinks).
Markers reward xylem carrying water and mineral ions upwards from roots to leaves, and phloem carrying dissolved sugars from the leaves to the rest of the plant (transport in both directions).
Original4 marksTranspiration is the loss of water vapour from a plant. State where most transpiration occurs and name two factors that increase the rate of transpiration, explaining how each one has its effect.Show worked answer →
Most transpiration occurs through the stomata, the small pores mainly on the underside of the leaves (water evaporates from the moist cell surfaces inside the leaf and diffuses out through the stomata).
Two factors that increase the rate (any two):
Higher temperature: increases the rate of evaporation of water from the leaf cells, so more water vapour diffuses out.
More wind (air movement): blows away the water vapour from around the leaf, keeping a steep concentration gradient so water diffuses out faster.
Lower humidity: drier air outside maintains a steeper concentration gradient, so water diffuses out faster.
Markers reward stomata as the main site, and two correct factors each linked to the effect (faster evaporation, or a steeper concentration gradient for diffusion out).
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