Biology Presentation
Okan Atilan
Water Uptake in Plants
Root Hair Cells
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They cover the part of the root that absorb
water.
They increases the surface area and hence,
speeds up water uptake.
When water is absorbed, it has to through
the cortex and the endodermis, to reach the
xylem.
Water Potential
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Water moves from areas of higher water
potential to areas of lower water potential.
The soil around roots has a high water
potential because the amount of water in soil
is high
Leaves have a low water potential because
of constant evaporation of water by
transpiration
Water Potential Gradient
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The difference between water potentials
create a water potential gradient
Water moves from roots to leaves, down the
water potential gradient
Mineral ions are transported into the root hair
cell by active transport
This lowers the water potential so more
water moves in by osmosis.
Apoplast and Symplast Pathway
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Water can travel from roots to xylem in two
different paths
Apoplast pathway- goes through non-living
cell walls. Water simply diffuse through them.
Symplast Pathway- goes through living
cytoplasm of the cells. Plasmodesmata
connects the cytoplasm of the neighboring
cells.
Apoplast and Symplast Pathway
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Mainly apoplast pathway is used because of its low
resistance
Water can only travel up to endodermis cells by
apoplast pathway, because Casparian strip blocks
this pathway in endodermis
Water has to travel in symplast pathway after it
reaches to Casparian strip
The advantage is, water pass through a cell
membrane so the substances in it can be controlled
After passing endodermis, water moves into the
xylem.
Transport of Water and Xylem
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There are 3 ways in which water moves in xylem
against the force of gravity
1.
Cohesion-tension theory
Water evaporates from the leaves by
transpiration. This creates a tension which
pulls more water into leaf. Due to hydrogen
bonds, water molecules stick together
(cohesion). So when the water molecules at
the top are pulled, the bottom ones follow
and the whole column of water moves
upward
2. Root Pressure:
The mineral ions are forced by
endodermis into xylem. They
transfer to xylem by active
transport. A water potential
gradient develops so more water
is pushed into the xylem. This
creates a pressure at the bottom
end of the xylem and the
molecules at the top are pushed.
This pressure is weak and it is
not strong enough to move water
to the top of big plants by itself.
But, in young plants, root
pressure has a great impact on
water transportation from roots to
leaves.
3. Capillarity
It occurs when water moves up in a thin tube. In
plants it occur because of the molecular attraction
(adhesion) between water and the inside of the
xylem tube.
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All three methods are used to move water up through
the xylem. None of them provide sufficient power to
move water on their own.
Xylem
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Xylem support the plant. In stems, xylem is found
near the outside and prevents bending of the stem.
Water can travel through the cells is the xylem
freely because these cells do not have end plates.
Transpiration
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It is the loss of water as the form of water
vapour from the leaves through stomata to
the atmosphere.
Transpiration occurs because of the stomata
found on the leaves which allow gas
exchange between cells in the leaves and
atmosphere in respiration and
photosynthesis.
Transpiration
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Water evaporates from cell walls and
accumulates in the spaces found between
the cells in the leaf.
When stomata are open, water vapour
diffuses out of the stomata.
This is because of the diffusion gradient
found between the inside of the and the
atmosphere. There is much more water
vapour in the atmosphere.
Potometer
Potometer
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It is used to measure the rate of transpiration.
It measures the amount of water taken up by
the plant.
It is assumed that the uptake of water is
directly related to water loss by transpiration.
Factors Affecting the Rate of
Transpiration
There are four main factors which affect
the rate of transpiration;
1. Light intensity- Light itself does not
affect evaporation, but in daylight the
stomata are open. This allows the water
vapour to diffuse out. In the dark,
transpiration is reduced because the
stomata are closed.
2. Temperature- When the temperature
increases, the molecules gain more
energy and start to diffuse out of the
leaves faster.
3. Humidity- If the air is humid, it can
accept little more from the plants. The
diffusion gradient between the air and
leaf reduced and hence the rate of
transpiration.
4. Wind- In moving air, the water vapour
will be swept away from the leaf as
fast as it diffuses out. A diffusion
Xerophytic Plants
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Plants that are adapted to live in dry
conditions are called xerophytes.
They are very efficient at water uptake and
they can store water.
They have special adaptations to reduce
transpiration.
Cacti
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They are adapted to desert conditions.
Their leaf adaptation include, presence of a thick
waxy cuticle, few stomata, stomata that are opened
at night and closed by day, leaf surface covered with
hair.
They have a very extensive root system.
Water is stored in leaves or stems.
They have spines which protect the plant from
predators.