Identify and describe Why Plants Need a Transport System, Translocation and Phloem Tubes and What Happens to Transported Sugars in the context of plant organ system.
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GCSE Biology Revision
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GCSE Biology revision
Plant organ system
Plant tissues, organs and systems
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Explain the relationships between Why Plants Need a Transport System, Translocation and Phloem Tubes and What Happens to Transported Sugars and how they contribute to plant organ system.
Apply and analyse knowledge of Why Plants Need a Transport System, Translocation and Phloem Tubes and What Happens to Transported Sugars to interpret unfamiliar information about plant organ system.
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Key knowledge
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Why Plants Need a Transport System
- Plants produce sugars through photosynthesis in their leaves, but these sugars must be transported to the rest of the plant for energy use or storage
- Plants also need to transport water and mineral ions absorbed by the roots up to the leaves where water is used in photosynthesis
Translocation and Phloem Tubes
- Translocation is the process by which sugars are transported around the plant
- Phloem cells are arranged end to end to form long columns called phloem tubes, which contain small pores between adjacent cells
- These pores allow the movement of sap, a liquid mixture of water and dissolved sugars, through multiple cells over long distances
- Unlike xylem, phloem can transport substances in both directions ā up or down the plant
What Happens to Transported Sugars
- Once sugars reach their destination cells, they can be used directly as a source of energy
- Alternatively, sugars can be stored for later use as an energy source
Xylem Tubes and Their Structure
- Xylem tubes are made up of dead xylem cells with no end walls between them, forming one long hollow tube
- Xylem tubes are strengthened with a material called lignin, giving them structural support
- The role of xylem is to transport water and mineral ions from the roots, up the stem, to the leaves
The Transpiration Stream
- Transpiration is the evaporation of water from the leaves of a plant, primarily through the stomata
- Water molecules act like a continuous chain ā as water evaporates from the leaf, it pulls the next water molecule up through the xylem to replace it
- This continuous movement of water up through the plant is called the transpiration stream
- The stomata must remain open to allow carbon dioxide in for photosynthesis, which means water loss through transpiration is unavoidable
Factors Affecting the Rate of Transpiration: Light & Temperature
- Higher light intensity increases the rate of transpiration because more stomata open to allow carbon dioxide in for photosynthesis, letting more water evaporate
- At night, stomata close as photosynthesis stops, so there is very little transpiration
- Higher temperatures increase the rate of transpiration because water particles have more kinetic energy and are more likely to evaporate and diffuse out through the stomata
Factors Affecting the Rate of Transpiration: Air Flow & Humidity
- Water diffuses out of the leaf down a concentration gradient, as the inside of the leaf has a higher concentration of water than the outside air
- High air flow (wind) blows water molecules away from the leaf surface, maintaining a steep concentration gradient and increasing the rate of transpiration
- High humidity means the surrounding air already contains a large amount of water vapour, which reduces the concentration gradient and decreases the rate of transpiration
Comparing Phloem and Xylem
- Xylem cells are dead and form hollow tubes strengthened by lignin, transporting water and minerals in one direction only ā upwards from roots to leaves
- Phloem cells are living and transport sugars dissolved in sap, and can move substances both up and down the plant
Key Vocabulary Summary
- Translocation refers to the transport of sugars through phloem tubes around the plant
- Transpiration refers to the evaporation of water from the leaves through the stomata
- The transpiration stream describes the continuous flow of water molecules up through the xylem, driven by evaporation at the leaf surface