Transport in Plants (part 3): The three mechanisms in water transport & wilting

2) Capillary action Concept is used in blood donation • The water molecules in blood plasma stick to the side of the vessel and slowly “climbs up” • However this mechanism does not account for the greater distance that water can travel in trees Copyright © 2010 All Rights Reserved

Capillary action is due to the properties of water Adhesion of the water for the glass allows the water to move up the wall of the cylinder just a little above the level of the water in the center of the cylinder Cohesion of water molecules cause the gradual curve observed Copyright © 2010 All Rights Reserved

Adhesion and Cohesion Adhesion = attraction between unlike molecules e.g. water molecule + wall of cylinder Cohesion = attraction between like molecules e.g. between water molecules Why? • The oxygen end of water has a negative charge and the hydrogen end has a positive charge • The hydrogens of one water molecule are attracted to the oxygen from other water molecules • This attractive force is what gives water its cohesive and adhesive properties

Capillary tubing • The height at which the water will rise in a small cylinder, or capillary tube, is dependent on the diameter of the tube • The height of rise in the straws is less than what would be expected in a glass capillary because of lower adhesion between water and plastic 10 mm straw 0.7 mm smallest capillary 4.2 mm small capillary Copyright © 2010 All Rights Reserved

• The water is adhering to the beads, but cohesion keeps a strand of water between the beads • As the gap widens, the strand of water gets narrower until it eventually breaks because cohesion of the water is not strong enough to pull the water together over wide distances • This is essentially why water moves up further in smaller capillary tubes • There is less mass of water in a small capillary and the cohesion of water can hold a longer water Copyright © 2010 All Rights Reserved

Transpiration Definition of transpiration: The loss of water vapour from the aerial parts of the plant, especially through the stomata of the leaves Functions of transpiration: 1. Cool the plant; prevent scorching 2. Facilitates the lifting of water and mineral salts from roots to leaves Copyright © 2010 All Rights Reserved

Transpiration Intercellular spaces in spongy mesophyll Water used to form thin film of moisture over surfaces of spongy mesophyll Evaporation of water into intercellular spaces Diffuses through stomata to drier air outside leaf Copyright © 2010 All Rights Reserved

Transpiration pull (continued…) concentrated cell sap = lower water potential Evaporation of water Cells remove water from veins (xylem vessels) Draw water from cells deeper inside the leaf by osmosis The evaporation of water from leaves result in a suction force which pulls water up the xylem vessels Suction force due to transpiration is transpirational pull Transpirational stream = stream of water going up the plant Copyright © 2010 All Rights Reserved

Time taken for meniscus to move from B to A (volume of water absorbed between B and A) = _______ cm3 1streading/ min 2nd reading/ min 3rdreading/ min Average time/ min Rate of transpiration (cm3/minute) Shoot in sunlight Shoot in wind Shoot in humid air Note: Rate of absorption is proportional to the Rate of transpiration Copyright © 2010 All Rights Reserved

Is wilting good or bad? Good Bad • reduces transpiration rate as the leaves fold up, reducing area exposed to the sunlight • guard cells become flaccid and the stomata close • the rate of photosynthesis is reduced (water becomes the limiting factor) • As the stomata are closed, CO2entering leaves is reduced Copyright © 2010 All Rights Reserved

What is the difference between epidermis, epithelium and endodermis? 1.Epidermis - a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and mineral salts. 2.Epithelium - is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. What’s the difference between epidermis and epithelium? Epidermis is one type of epithelium. Epithelium is a general name for cells that make up various linings throughout the body. 3.Endodermis - In a plant stem or root, a cylinder of cells Copyright © 2010 All Rights Reserved

Transport in Plants (part 3): The three mechanisms in water transport & wilting

Transport in Plants (part 3): The three mechanisms in water transport & wilting

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