Transport in Plants

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 of water to flow from area of low water concentration to high water concentration.

Vascular Tissue Vascular plants have specialized tissue for transporting material from one location to another. TUBES! In animals, material is transported through the ________________ system.

Vascular Bundles

Xylem Transports water and dissolved minerals Xylem: – Tracheids and/or Vessel elements: cells which grow end-to-end, but living contents eventually die, leaving non-living cell walls as the ‘tube.’ Gymnosperms: contain only tracheids Angiosperms: contain both tracheids and vessel elements.

Which is found in gymnosperms? Angiosperms?

Plants are not like animals... 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 (in leaf), another molecule is right behind it. – Pulls up second molecule due to cohesion: attraction of water molecules to each other. – Loss of water from leaf pulls up another water molecule. 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). CAREER EXPLORATION?

Phloem Translocation: the transportation of food from on region of a plant part to another region. Phloem transports plant food (glucose/sucrose). Why would glucose or sucrose be NEEDED by different tissues of the plant?

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

CLASSWORK/HOMEWORK Page 326, #1-8.

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