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Exchange and Transport

Exchange and Transport. 13.8 Movement of water up stems. Learning outcomes. Students should be able to understand the following: What transpiration is The process by which water moves through a leaf How water moves up the xylem Candidates should be able to:

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Exchange and Transport

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  1. Exchange and Transport 13.8 Movement of water up stems

  2. Learning outcomes Students should be able to understand the following: • What transpiration is • The process by which water moves through a leaf • How water moves up the xylem Candidates should be able to: • Describe the roles of root pressure and cohesion tension in moving water through the xylem

  3. How do tall trees get enough water? Redwoods are the tallest species of tree in the world. The tallest redwood ever measured was 120 metres tall. That is six times the height of the Angel of the North! How does a plant this size get water from its roots to the branches at the top? The water will travel up the stem, or trunk, of the tree. What experiment could show that water travels up stems?

  4. Movement of water up a stem

  5. Transpiration • The loss of water from the leaves of a plant is called transpiration, and the resulting flow of water through the plant is called the transpirationstream. The transpiration stream is important because: • it carries water for photosynthesis to the palisade cells in the leaves • the water carries essential mineral salts in solution • evaporation from the leaves has a cooling effect

  6. Water movement out of stomata

  7. Opening and closing of stoma

  8. Transport of water up the xylem There are 3 pathways: • Root pressure (-minor) • Capillarity (-minor) • Cohesiontension (major)

  9. Root pressureis caused by the mineral ions which are actively transported into xylem vessels in the root by endodermal cells. This makes the water potential of the xylem more negative and causes water to enter the xylem by osmosis. Capillarityis responsible for some water creeping up the xylem vessels simply because they are very narrow.

  10. Cohesion-tension theory • The mass flow of water through the xylem relies on 2 important properties of water: • Cohesion – the water molecules tend to stick together • Adhesion – the water molecules also tend to stick to the inside of the xylem vessels.

  11. Cohesion-tension theory

  12. Putting it all together • Transport in xylem • transpiration – pathways Private life of plants - xylem, phloem.wmv

  13. Extension and Homework AQA AS Biology textbook pg 195 Summary question 1 Application questions 1-5 2. Complete the exam style question about transpiration and cohesion-tension

  14. Mark scheme - exam style question (a) Suitable accepted evidence, 1 mark for evidence and 1 mark for explanation EITHER (only) upward pressure could force liquid water out of leaves;OR Sap exuding from a cut, rooted stem; (only) upward force could make this happen; 2 (b) (Note: max. two for any component) (i) Evaporation from leaves during daytime only/mainly; tension/negative pressure (on water) in xylem creates inward pull (on walls of xylem vessel); xylem vessels become narrower; due to adhesion of water molecules (to walls of xylem vessels); (ii) root pressure gives outward force/push on walls of xylem vessels; tree would become wider/stay same diameter; xylem vessels become wider/stay same diameter; max 3 [5]

  15. Learning outcomes Students should be able to understand the following: • What transpiration is • The process by which water moves through a leaf • How water moves up the xylem Candidates should be able to: • Describe the roles of root pressure and cohesion tension in moving water through the xylem

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