Transport in Plants

Seatwork Read pages 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 Osmosis: the natural tendency to flow from area of low solute concentration to high solute concentration.

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.

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.

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

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.

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.

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 cm.

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).

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.

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.

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

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.

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

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.)

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