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Transport in Plants

Transport in Plants. Seatwork. Read pages 531-534. SR #1-7 Remainder of the lesson covers section 9.5. Review of Diffusion. Diffusion: natural tendency for particles to move from areas of high concentration to low concentration (concentration gradient). Review of Osmosis.

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Transport in Plants

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  1. Transport in Plants

  2. Seatwork Read pages 531-534. SR #1-7 Remainder of the lesson covers section 9.5.

  3. Review of Diffusion Diffusion: natural tendency for particles to move from areas of high concentration to low concentration (concentration gradient).

  4. Review of Osmosis Osmosis: the natural tendency to flow from area of low solute concentration to high solute concentration.

  5. Transport in Xylem Root hairs and epidermis: absorb water by osmosis. Endodermis: filters water and minerals. Stem: aqueous solution moves by diffusion & active transport. Leaf: vessels branch and rebranch into numerous veins. At the end of vein, water and minerals diffuse into the cells of the leaf. 99% water is lost through transpiration: water lost due to evaporation.

  6. However... • Plants do not have muscles to push substances up against gravity. • Plants do not have valves to keep substances from flowing the other way.

  7. 3 Theories of Translocation in Plants • Root Pressure • Capillary Action • Cohesion-tension

  8. 1) Root Pressure • Water builds up in xylem of roots either by: • Cells actively pump water into xylem • Cells actively pump ions into xylem, creating a concentration gradient  osmosis. • Accumulation of water in xylem builds pressure and forces water upward.

  9. Problems with the Root Pressure Theory • for tall tree to raise water 100m, need difference in pressure in roots and leaves of 1000 kPa. • Pressure gradient has never been demonstrated in real life.

  10. 2) Capillary Action • Relies on adhesive properties of water • Adhesion: attraction of water to other polar molecules. • The cause of a meniscus  clings onto side of capillary. Drawback: can only explain movement of water of 60-90 cm.

  11. 3) Cohesion-tension • Also called transpiration pull. • Most widely accepted explanation of how water moves up a tall plant. • As each water molecule evaporates from stomata, another molecule is right behind it. • Pulls up second molecule due to cohesion: attraction of water molecules to each other. • Loss of water from stomata lowers water pressure  water will move by osmosis up and out. • Limitation: we still do not know how water begins to move up a maple tree in the spring, before the leaves are out (therefore, no transpiration).

  12. Mineral Transport • Minerals exist in soil as ions. • Minerals would need to enter the root by diffusion if passive transport. • However, concentration of minerals in root exceeds that of minerals in soil. • Therefore, against concentration gradient. • Minerals taken into roots by active transport through living cells, and then take a free ride with water in the xylem.

  13. Transport in Phloem • Also only theories available for explanation. • Must take in following 5 observations: • Phloem cells must be living • Materials can move through phloem in more than one direction. • (shows with radioactive carbon and phosphorous) • Phloem may transport large amounts of material quite rapidly within a plant. Ex// pumpkin can gain 5500 kg in one month. • Oxygen deficiency and low temperatures both inhibit but do not stop phloem transport. • Different plants may move different substances at different times.

  14. Mass-flow Theory • Most broadly accepted theory of phloem transport. • Combination of osmosis and pressure dynamics.

  15. If sucrose added to membrane X, water flows into tube. • Sucrose solution flows from membrane X to membrane Y. • Pressure builds up in tube, and water forced out. • Solution at two membranes at equal concentraion, and flow will stop. • If a way to continuously add more sucrose to membrane X and withdraw from Y, flow would be continuous.

  16. Mass Flow in Plants: • Scientists believe that there is another variable involved: living tissues of phloem cells must control some movement of organic materials.

  17. CLASSWORK/HOMEWORK Page 326, #1, 2, 3, 4, 5, 7, 8 Page 328, #17 (Note: Independent variable  variable that does not depend on other variable(s). Dependent variable  variable that depends on the other variable(s). Control  sample trial to compare other trials to.)

  18. Classwork/Homework (2) Read pages 531-534. SR # 1-7.

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