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 explainmovement of water of 60-90 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 531-534. SR # 1-7.