Download Water transport

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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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The need for water
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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Structure of a root
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Root – anchorage and a surface for water uptake.
Surface area is increased by root hairs just behind the
root tip.
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Structures of a root
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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Dicot. root
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Uptake of water
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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APOPLAST PATHWAY
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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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
SYMPLAST PATHWAY
Water moves through the
cytoplasm from cell to cell,
moving through the plasmodesmata – channels joining
cells.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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.
(diag)
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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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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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Xylem vessels with lignified walls
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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Xylem
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Transpiration
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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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Water potential gradient from soil to air
Low water
potential
(more
negative)
Air -30 000kPa
Root -100 kPa
Water potential gradient
Leaf -1000 kPa
Soil -10 kPa
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High water
potential
(less
negative)
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Trans. cont
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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Inside the leaf
Mesophyll area of leaf
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Trans. Cont.
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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
next
mesophyll cells.
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Xylem vessels decrease in leaf vein
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Stomata
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Mechanisms responsible for the
movement of water in xylem
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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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Movement in xylem cont.
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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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Movement in xylem cont.
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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The cohesion –tension theory.
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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Cohesion-tension theory cont.
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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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Cohesion-tension cont.
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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Cohesion-tension cont.
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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Further evidence.
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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Summary diagram
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end