1. PHYSIOLOGY and the ENVIRONMENT1
PHYSIOLOGY and the ENVIRONMENT
The passage of water through a plant.
“The structure of a dicotyledonous root in relation to the uptake and movement of water across the root by apoplastic and symplastic pathways”.
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2. The need for water In plants and animals:2
In plants and animals:
medium for metabolic reactions,
Hydrolysis – braking down large molecules into small molecules by adding water,
transport of solutes e.g. mineral ions, glucose etc.,
Cooling by evaporation from surfaces,
(in plants) creates pressure inside cells to give support.
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3. Structure of a root Root – anchorage and a surface for water uptake.3
Root – anchorage and a surface for water uptake.
Surface area is increased by root hairs just behind the root tip.
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4. Structures of a root Epidermis – single layer of cells on the outside.5
Epidermis – single layer of cells on the outside.
Endodermis – single layer of cells forming a cylinder and enclosing the xylem and phloem.
The area between the xylem, phloem and endodermis is packed with thin walled unspecialised cells.
Cortex – contains large parenchyma cells. The walls of these cells are permeable to water and dissolved solutes. Also air spaces allow diffusion of oxygen across the root for respiration.
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5. Dicot. root 6

6. 7

7. 8

8. Uptake of water 10
Mainly by younger parts of the root hair region. The water potential of water in most soils is close to 0 (as the conc. of solutes is very low).
Most mineral ion are in low conc. in soil and are taken up by active transport.
With a high conc. of minerals inside root cells ψ is more negative than the surrounding soil, therefore a water potential gradient exists. Water moves by osmosis into the cells.
Once into the root cells, water and mineral ions can move across the cells of the cortex from the epidermis to the central tissues by two main pathways.
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9. APOPLAST PATHWAY Most water moves this way.11
Most water moves this way.
The water moves through the continuous system of
adjacent cell walls –
there being NO barriers to movement so water diffuses freely.
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11. SYMPLAST PATHWAY Water moves through the cytoplasm from cell to cell,
moving through the plasmodesmata – channels joining cells.

13. Apoplast and symplast cont.
The symplastic route is slower as resistance to water movement in the cytoplasm is greater (4x) than that in the cell wall.
Movement of water to the root centre by the apoplastic pathway is stopped at the endodermis which has a waterproof layer in the cell walls called the CASPARIAN STRIP. This is made of a waxy compound – suberin – impermeable to water. Water is prevented from passing around the endodermal cells through the cell walls but must pass through the plasma membrane into the cytoplasm.
The casparian strip forces water to take the symplast route through the endodermal cells.
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(diag)

14. From root to leaf
Xylem and phloem are termed vascular tissue. They are grouped together in a dicotyledonous stem into vascular bundles. They run the entire length of the plant from the roots to the veins in leaves.
phloem
xylem
(diag)
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15. 14
XYLEM
Xylem tissue contains tubes called vessels. Dead cells - vessel elements are arranged end to end.
Starting as living cells once they reach full size their walls become impregnated with lignin – impermeable to water.
The living parts die leaving the cell walls surrounding a water filled cavity (lumen).
The end walls break down and water has little resistance as it moves through the vessels.
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16. Xylem vessels with lignified walls15
Lignin is laid down as rings or spirals. Older vessels have a continuous layer of lignin except for perforations called pits. Here lignin fails to deposit and only the cellulose cell wall remains. Pits of neighbouring cells match up to allow water to pass sideways as well as up.
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17. Xylem 16

18. Transpiration 17
Root hairs are in contact with soil water, which has a high water potential (less negative), the leaves and stem are exposed to the atmosphere, which has a much lower water potential (more negative).
There is therefore a water potential gradient through the whole plant, so water is drawn through and lost.
The movement of water is passive requiring no energy from the plant.
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19. Water potential gradient from soil to air18
Water potential gradient from soil to air
Low water potential (more negative)
Water potential gradient
Air kPa
Leaf kPa
Root -100 kPa
Soil -10 kPa
High water potential (less negative)
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20. Trans. cont 19
Water vapour can be lost from 3 sites on the aerial parts of a plant.
leaves – through the cuticle and stomata.
flowers
stems in herbaceous (non-woody) plants and lenticels on woody stems.
Most water vapour loss is through the stomata. These are open during daytime allowing CO2 to enter for photosynthesis.
Evaporation occurs from the cell walls of mesophyll cells into the intercellular spaces, it then diffuses through the stomata.
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21. 20
Inside the leaf

22. Mesophyll area of leaf 21
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23. Trans. Cont. 22
Again the water moves along the water potential gradient from inside the leaf to the atmosphere.
Remember – there are usually more stomata on the lower surface of the leaf than the upper in a dicot.
Evaporation through the cuticle can be up to 10% depending on its thickness.
Loss of water by evaporation is called transpiration. The “stream” of water flowing up through the plant is called the transpiration stream.”
The vascular bundles in the leaf veins extend into the mesophyll cells. Here the xylem consists of no more than one or two vessels with little lignification so water can easily pass by apoplast and symplast pathways to the mesophyll cells.
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24. Xylem vessels decrease in leaf vein23
Xylem vessels decrease in leaf vein
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25. Stomata 24
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26. Mechanisms responsible for the movement of water in xylem25
Mechanisms responsible for the movement of water in xylem
Three possible mechanisms all of which may contribute to some extent to water movement in the xylem.
Capillarity
Adhesion of water to the side of a tube. (if a fine tube is dipped into water the water will rise up the tube). in xylem vessels, as small as 20 µm dia., water rises less than 50 mm. so capillarity alone cannot account for the movement.
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27. Movement in xylem cont. Root pressure26
Root pressure
Sap will exude from a cut shoot. This implies there is a force pushing water up the stem from the roots. This force is known as root pressure.
At the endodermis in the root, water and dissolved minerals are forced from the apoplastic to the symplastic pathway (due to the Casparian strip). Once inside living cells the ions can be actively secreted.
Ions are secreted from the endodermal cells into the cells around the xylem and these secrete them into the xylem vessels. This creates a water potential gradient and by osmosis water diffuses from the cortex, through the endodermis and into the xylem. Root pressure is caused by this accumulation of water in the xylem.
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28. Movement in xylem cont. 27
Anything that inhibits active transport, such as metabolic inhibitor, low temp., or shortage of oxygen will reduce root pressure.
A pressure of up to 150 kPa can result from root pressure but this too in insufficient account for all water movement.
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29. The cohesion –tension theory.28
Cohesion-tension theory most adequately accounts for water movement. It can be divided into 4 main stages.
1. Leaves transpire. Water evaporates from inside the leaf through stomata to the drier air.
2. Water molecules have cohesion. Hydrogen bonds form between neighbouring water molecules, causing them to stick to each other. So as water is lost by transpiration, more is pulled up the xylem to replace it.
3. This pulling action stretches the water column in the xylem so that it is under tension.
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30. Cohesion-tension theory cont.29
4. Water molecules also cling to the walls of the xylem. This is adhesion and also helps to pull the water column upwards.
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31. Cohesion-tension cont.30
So removal of water from xylem in the leaf vein creates a pulling force drawing water through the xylem from roots through the stem in continuous columns.
This works as long as the column is not broken. The properties of water and the structure of xylem maintain these columns.
Properties of xylem.
small diameter, strong rigid walls withstand tension.
An attraction between water mols. and lignin helps water adhere (stick) to the walls. The narrower the vessels the more water mols. are in contact with the walls. Narrower columns are less likely to break.
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32. Cohesion-tension cont.31
Property of water
there are strong forces between water molecules.
(cohesion is the force of attraction between like molecules)
(adhesion is the force of attraction between unlike molecules)
As water moves through the xylem the thickened lignified walls prevent the vessels from collapsing.
Cohesion-tension theory offers an explanation for the movement of water through plants including very tall trees.
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33. Further evidence. 32
Measuring the diameter of tree trunks over 24hrs shows that during daylight the diameter decreases. While at night it increases.
This is because during the day transpiration puts the xylem under tension and vessels contract. At night transpiration is reduced, the tension is released and the xylem cells increase in diameter.
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34. Summary diagram 33
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