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AQA GCSE Biology · 8461
AQA 8461 · 4.2.3.2 Plant Organ Systems Check the specification (PDF) (opens in a new tab)
Explain the adaptations of root hair cells, xylem, and phloem for their functions.
Describe the processes of transpiration and translocation.
Explain the effect of environmental factors (temperature, humidity, air movement, light) on transpiration rate.
Describe the role of stomata and guard cells in gas exchange and water loss.
Calculate rates of transpiration and interpret related graphical data.
Roots, stems and leaves work together as a transport system. Leaves need water for photosynthesis, while the sugars made by photosynthesis must reach other parts of the plant. These substances travel in different tissues: xylem carries water and mineral ions from the roots towards the leaves; phloem carries dissolved sugars from the leaves to places where they are used or stored.
Water enters the roots and moves up through xylem. In a leaf, some of it evaporates from moist cell surfaces. The water vapour then diffuses out through tiny pores called stomata. This loss of water vapour from the plant is transpiration.
As water is lost from a leaf, more water is drawn up the xylem to replace it. The continuous upward flow is the transpiration stream. Evaporation at the leaf helps pull the column of water upwards from the roots; the roots do not simply push it to the top. Mineral ions travel with the water. The movement of water in this stream is passive: it does not directly require energy from the plant.
A stoma is a pore in the leaf surface, surrounded by two guard cells. When the guard cells open the pore, carbon dioxide can diffuse into the leaf for photosynthesis and oxygen can diffuse out. Water vapour can escape through the same opening. When the guard cells close the pore, less water is lost, but gas exchange is also reduced. Guard cells therefore help the plant balance its need for carbon dioxide against the risk of losing water.
Water loss through stomata helps draw water up xylem, while phloem carries sugars within the plant from leaves to growing and storage tissues.
Photosynthesis in the leaves produces sugars. Translocation is the movement of these dissolved food molecules through phloem to other parts of the plant. For example, sugars may be delivered to growing tissues for immediate use or to storage organs, where they can be stored as starch. Unlike the upward transpiration stream in xylem, translocation can carry sugars to destinations above or below the leaves. You need to know what phloem transports and where it goes, but not the detailed mechanism of transport within phloem.
Water moves from roots up the xylem to leaves. It evaporates in the leaf and diffuses out as water vapour through stomata. This loss draws up replacement water in the transpiration stream; mineral ions travel with it.
Each stoma is a pore controlled by a pair of guard cells. Opening permits carbon dioxide in and oxygen out, but also allows water vapour to escape. Closing reduces both gas exchange and water loss.
Phloem carries dissolved sugars made in leaves to other parts for immediate use or storage. Movement may be upwards or downwards, depending on the destination.
Keep the transport tissues distinct: water and mineral ions travel upwards in xylem; dissolved sugars travel through phloem to parts of the plant that need or store them.
Do not describe stomata as cells. They are pores, each controlled by surrounding guard cells.
Translocation is not necessarily downwards: sugars can move to destinations above or below the leaves.
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Transpiration
The loss of water vapour from a plant's leaves, mainly through the stomata.
Transpiration stream
The upward flow of water through xylem from the roots towards the leaves, replacing water lost by transpiration.
Translocation
The movement of dissolved food molecules, particularly sugars, through phloem to other parts of a plant.
Stomata
Tiny pores in a leaf's surface through which gases and water vapour can pass.
Guard cells
Pairs of cells around stomata that control whether the pores are open or closed.
Put your knowledge into practice — try past paper questions for Biology
Transpiration
The loss of water vapour from a plant's leaves, mainly through the stomata.
Transpiration stream
The upward flow of water through xylem from the roots towards the leaves, replacing water lost by transpiration.
Translocation
The movement of dissolved food molecules, particularly sugars, through phloem to other parts of a plant.
Stomata
Tiny pores in a leaf's surface through which gases and water vapour can pass.
Guard cells
Pairs of cells around stomata that control whether the pores are open or closed.