Transport in Plants

Edexcel AS-level BiologyTopic 17 of 1721 lessons

Revision notes for Transport in Plants in Edexcel Biology (8BI0). Read the start of any lesson here, then sign in free for the whole lesson, the R.E.C.I.P.E. recall steps and the quiz.

1. Xylem and PhloemPlants have two transport tissues, xylem and phloem, together called vascular tissue.
2. XylemXylem tissue contains several cell types: xylem vessels, which carry water, tracheids, fibres, which give support, and parenchyma, which stores food.
3. Evidence for the Movement of Water Through XylemSeveral simple experiments show that water moves up the plant through the xylem, and that it is pulled up under tension.
4. PhloemPhloem transports assimilates such as sucrose and amino acids.
5. Root Hair CellsWater and mineral ions enter the plant through root hair cells.
6. Symplast PathwayAfter entering the root hair, water must cross the cortex of the root to reach the xylem in the centre.
7. Apoplast PathwayThe second route for water across the root cortex is the apoplast pathway.
8. TranspirationTranspiration is the loss of water vapour from the aerial parts of a plant, mainly from the leaves.
9. Cohesion-Tension TheoryThe cohesion-tension theory explains how water moves up the xylem from the roots to the leaves, even in tall trees.
10. Adhesion and CapillarityAs well as being attracted to each other (cohesion), water molecules are attracted to other substances.
11. Modelling TranspirationIt is difficult to watch water moving inside a living plant, so simple physical models are used to test whether the cohesion-tension theory is…
12. Measuring the Rate of Water UptakeThe rate of water uptake by a leafy shoot can be measured with a potometer.
13. Factors Affecting TranspirationThe rate of transpiration depends on how quickly water vapour can evaporate inside the leaf and diffuse out through the stomata.
14. Root PressureRoot pressure is a pressure that pushes water up the xylem from the roots.
15. Translocation in the PhloemTranslocation is the movement of organic substances (assimilates), such as sucrose and amino acids, through the phloem.
16. Phloem LoadingGlucose is manufactured in the chloroplasts by photosynthesis.
17. The Symplast PathwaySucrose made in the mesophyll cells of a leaf must be loaded into the sieve tubes.
18. The Apoplast PathwayIn apoplastic loading, sucrose leaves the symplast and travels through the apoplast, the cell walls and spaces between cells, before being actively…
19. Phloem UnloadingPhloem unloading is the removal of sucrose and other assimilates from the sieve tubes at a sink, such as a growing root tip, a developing fruit or a…
20. Mass Flow HypothesisThe mass flow hypothesis (proposed by Ernst Münch) explains how phloem sap moves from source to sink.
21. Pressure Flow HypothesisThe pressure flow hypothesis is another name for the mass flow model of translocation.

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