Chapter 9 Transport in Plants Lesson 3 - The 3 mechanisms in water transport_Advantages and disadvantages ofwilting

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  • Capillarity works because water molecules are attracted to charged surfaces. The smaller the bore of the tube, and the greater the charge on its wall, the higher the water will move. Xylem vessels are very narrow bore tubes with highly charged surfaces (cellulose is highly charged). For mosses, this is fine, but the process can only account for a short height of water movement. Root pressure. We have already seen that water potential can move water to great heights. The water seen on leaves in the morning (morning dew) is supposed to occur by water taken up as a result of active ion uptake at night. During the night, the plant keeps its stomata closed so there is little transpiration, and the water osmotically taken up as a result of ion uptake exceeds traspiration and is released onto the surface of the leaves in a process called guttation. There are two main problems with this mechanism. The first is that the solutions get diluted as they take up water. This requires enormous energy inputs to maintain a concentrated salt solution in the xylem. To take up the volumes of water lost by plants, it has been calculated that the roots would have to produce so much energy that the water would boil. The second problem is even more serious. These salts cannot be recycled and would just accumulate in the leaves as the water was transpired causing immense osmotic and toxicity problems. Transpiration pull. Even in Ireland, the atmosphere usually contains a lower concentration of water than the plant. Thus is has a much more negative water potential. This causes a loss of water from the leaves which is transmitted through the plant to the root.
  • Just as in our original example of water potential, the water moves from the intercellular spaces to the atmosphere. Water is lost from cells to the intercellular spaces, and these cells then being at a lower (more negative) water potential than their neighbours will abstract water from them. This continues across the leaf until it reaches the bundle sheath cells. These abstract water from the xylem. Water is highly cohesive. The water molecules are bonded to each other by hydrogen bonds, and are thus very cohesive. Experiments to demonstrate the cohesive strength of water have generally put the strength of bonding of water molecules at several hundred atmospheres. (This only works in the absence of contaminating gasses which will come out of solution under vacuum, hence columns of water do get broken in the plant under normal physiological conditions). The entire column of water therefore is dragged up, and the reduction in pressure potential in roots literally sucks water in. Again this is propagated across the pith of the root up to the endodermis.

Transcript

  • 1. How do plants overcome gravity in the transport of materials? Copyright © 2010 All Rights Reserved
  • 2. 3 Mechanisms of water movement
    • Root pressure
    • Capillary action
    • Transpiration pull
    Copyright © 2010 All Rights Reserved
  • 3. 1) Root pressure Active transport of ions into xylem vessels Lowers water potential in xylem vessels Osmosis Copyright © 2010 All Rights Reserved
  • 4. 2) Capillary action
    • 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
    Concept is used in blood donation Copyright © 2010 All Rights Reserved
  • 5. Capillary action (continued…) Copyright © 2010 All Rights Reserved
  • 6. 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
  • 7. 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
  • 8. 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 4.2 mm small capillary 0.7 mm smallest capillary Copyright © 2010 All Rights Reserved
  • 9.
    • 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 column
    Copyright © 2010 All Rights Reserved
  • 10. Copyright © 2010 All Rights Reserved
  • 11. 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:
    • Cool the plant; prevent scorching
    • Facilitates the lifting of water and mineral salts from roots to leaves
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  • 12. 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
  • 13. 3) Transpiration pull Copyright © 2010 All Rights Reserved
  • 14. Transpiration pull (continued…) Evaporation of water 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 concentrated cell sap = lower water potential Draw water from cells deeper inside the leaf by osmosis Cells remove water from veins (xylem vessels)
  • 15. Summary Cohesion and adhesion Transpiration pull Copyright © 2010 All Rights Reserved
  • 16. Activity In groups of four, discuss what could be five factors that affect transpiration rate? Copyright © 2010 All Rights Reserved
  • 17. Factors affecting transpiration rate
    • Humidity of air
    • Temperature of air
    • Strong wind
    • Light
    • Size and shape of leaves (surface area)
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  • 18. Copyright © 2010 All Rights Reserved
  • 19. Potometer - instrument measuring rate at which plants absorb water Pg. 187 Investigation 9.10 Copyright © 2010 All Rights Reserved In the presence of sunlight
  • 20. Note: Rate of absorption is proportional to the Rate of transpiration Copyright © 2010 All Rights Reserved Shoot in humid air Shoot in wind Shoot in sunlight Average time/ min 3 rd reading/ min 2 nd reading/ min 1 st reading/ min Rate of transpiration (cm 3 /minute) Time taken for meniscus to move from B to A (volume of water absorbed between B and A) = _______ cm 3
  • 21. What happens when there is excessive transpiration? Copyright © 2010 All Rights Reserved
  • 22. Wilting occurs! Copyright © 2010 All Rights Reserved
  • 23. 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, CO 2 entering leaves is reduced
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  • 24. What is the difference between epidermis, epithelium and endodermis?
    • 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.
    • 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.
    • Endodermis - In a plant stem or root, a cylinder of cells that separates the outer cortex from the central core. The endodermis controls flow of water and minerals within the plant. In most plants, this tissue is restricted to the roots
    Copyright © 2010 All Rights Reserved