💧 Water by osmosis
The soil water is usually more dilute than the cytoplasm inside the root hair cell. So water moves into the cell by osmosis, through the partially permeable cell membrane.
Plant Tissues, Organs and Systems » Transpiration and Translocation
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.3.2
- How roots, stem and leaves work together to move substances around a plant
- How root hair cells, xylem and phloem are adapted to their jobs
- What transpiration is and how water travels up a plant
- What translocation is and how stomata control gas exchange and water loss
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Unlock This CourseA transport system for plants
A plant needs water and mineral ions from the soil, and it needs sugars made in its leaves. These substances have to get to every cell. The roots, stem and leaves form a plant organ system for the transport of substances around the plant.
Two tissues do the transport work: xylem and phloem. They run from the roots, up through the stem and out into the leaves.
Key terms:
- Lignin: a tough substance that strengthens the walls of xylem tubes.
- Transpiration: the loss of water vapour from the leaves of a plant, and the movement of water up the plant that goes with it.
- Transpiration stream: the continuous flow of water through the xylem, from the roots to the leaves.
- Translocation: the movement of dissolved sugars through the phloem to the rest of the plant.
Taking in water: root hair cells
Water and mineral ions enter the plant through the roots. The cells that do this job are root hair cells. Each one has a long, thin extension that pokes out into the soil.
⚡ Ions by active transport
Mineral ions are often in short supply in the soil. The root hair cell moves them in by active transport, against the concentration gradient, using energy from respiration.
The long hair gives the cell a very large surface area. That makes the uptake of water and ions efficient.
Xylem: the water pipes
Xylem tissue transports water and mineral ions from the roots to the stem and leaves. It is made of hollow tubes. Because the tubes have no cell contents in the way, water can flow through easily.
The walls of the tubes are strengthened by lignin. This stops the tubes collapsing when water is pulled through them. The xylem is adapted for the transport of water in the transpiration stream.
Transpiration
Transpiration is how water gets from the soil to the air. It happens in four steps.
- Water evaporates from the surface of the cells inside the leaf.
- The water vapour moves out of the leaf through the stomata, by diffusion.
- Water is pulled up from the xylem in the leaf to replace the water that was lost.
- This pulls more water up the xylem from the roots, and root hair cells take in more water from the soil.
The result is a continuous flow of water from the roots to the leaves. This flow is the transpiration stream. It brings water for photosynthesis, and brings mineral ions to the leaves.
Worked example
A plant takes up 12 cm3 of water in a day and loses 11 cm3 through its leaves. The water lost is 11 ÷ 12, so about 92% of the water taken in is lost by transpiration. The small amount left is used for photosynthesis and to keep the cells turgid.
Stomata and guard cells
A stoma is a tiny pore in the surface of a leaf. Each stoma has a pair of guard cells either side of it, which open and close it. Their role is to control gas exchange and water loss.
☀ Open stomata
When the guard cells open the stoma, carbon dioxide can diffuse in for photosynthesis and oxygen can diffuse out. Water vapour also escapes, so transpiration happens.
🌙 Closed stomata
When the guard cells close the stoma, very little water is lost. But gas exchange is also stopped, so the plant cannot take in carbon dioxide.
The plant has to balance two needs: letting carbon dioxide in, and keeping water in. The guard cells let the plant do both by opening and closing the stomata.
Phloem and translocation
Phloem tissue transports dissolved sugars from the leaves to the rest of the plant. The sugars are used straight away in growing parts, or taken to be stored.
Phloem is made of tubes of elongated cells. Cell sap can move from one phloem cell to the next through pores in the end walls. The movement of food molecules through phloem tissue is called translocation.
💧 Xylem
Carries water and mineral ions. Moves them from the roots up to the stem and leaves. Hollow tubes strengthened by lignin.
🍬 Phloem
Carries dissolved sugars. Moves them from the leaves to the rest of the plant. Elongated cells with pores in the end walls.
Common mistakes
Mixing up the two tissues: xylem carries water and ions, phloem carries sugars. Writing that transpiration is the loss of water from the roots: it is lost from the leaves. Saying that translocation moves water: it moves dissolved sugars.
Exam-style question
(a) Describe how a root hair cell is adapted for taking in water. [2 marks]
(b) Explain how water moves from the roots to the leaves of a plant. [3 marks]
(c) State the name of the process that moves sugars around a plant, and the tissue that does this. [2 marks]
Model answer
(a) It has a long hair that gives a large surface area (1). Water moves in by osmosis from the dilute soil water (1).
(b) Water evaporates from cells in the leaf and leaves through the stomata (1). Water is pulled up the xylem to replace it (1). This makes a continuous flow, the transpiration stream, from the roots to the leaves (1).
(c) Translocation (1) in the phloem (1).
Exam tip
In part (b), name the tissue (xylem) and the process (transpiration). Examiners look for the exact key words.