The Transpiration Stream

The Transpiration Stream
How does water move into, through
and out of a plant?
– evaporation
of water from
the leaves.
stream –
movement of
through the
Root Structure
through a
root tip
Zone of cell
Zone of cell
Zone of cell
Root cap
Root Structure
Reminder: What is water potential?
Water tends to move from areas of high water concentration
to areas of low water concentration. This is osmosis.
Water also tends to move
from areas of high
hydrostatic pressure to
areas of low hydrostatic
pressure. It is also affected
by gravity and electrostatic
forces, such as those that
cause surface tension.
The collective term for the tendency of water to move due
to any of these effects is water potential.
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What is transpiration?
Transpiration is the loss of water from the leaves of a plant.
Most of this occurs from the underside of a leaf, where there
are many stomata in the epidermis.
Most plants control their water
intake by opening and closing
their stomata. This happens
when water levels change in
the guard cells around each
stoma. This occurs either
passively by osmosis, or by
active transport of solutes.
Transpiration rates also vary naturally in response to
environmental factors such as temperature and humidity.
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Root hair cells
What is the function of the root hairs?
How are the cells adapted to perform their function
How does water move across the
Mineral ions are actively transported
into the root hair cells.
This decreases the water potential of
these cells.
Water is drawn in by osmosis, along
the water potential gradient.
Why is the water content of soils
How does water move across the
Water moves across the cortex
along a water potential
This occurs via the apoplast
and symplast pathways (and
the vacuolar pathway).
The transpiration stream
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How does water move across the
When water reaches the
endodermis, there is a
The cell walls of the
endodermis contain suberin, a
waterproof material that forms
the Casparian strip.
The Casparian Strip
Which pathway
taken by water
is prevented by
the Casparian
(apoplast or
Into the xylem…
Using only the symplastic
pathway, water moves
across the endodermis and
into the cells of the stele.
Mineral ions are actively
transported into the xylem,
decreasing the water
Water enters the xylem by
The properties of water
On pages 72-73 read about the
properties of water and think about why
these properties might be important to
Use the information to complete your
The Xylem
Xylem walls contain a
waterproof substance
called lignin.
How does water move up the xylem?
A combination
of root
pressure and
cohesiontension of the
What is root pressure?
Water can be
transported up the
xylem by a positive
hydrostatic pressure
from below. This is
known as root
Mineral ions are actively transported into the roots of the
plant, causing water to enter by osmosis. This increases the
hydrostatic pressure in the root, forcing water up the stem.
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Cohesion–tension theory
Water is a polar molecule, meaning that
its positive and negative charges are not
evenly distributed. The oxygen atom has
a slight negative charge, while the two
hydrogen atoms are slightly positive.
This means that, in the xylem, water
molecules spontaneously arrange so that
positive and negatively charged poles lie
next to each other.
This causes the molecules to cohere, or
stick together, so that as some leave a
plant by transpiration, a tension is
created between the molecules, so others
are pulled up behind them.
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A side note…
Water also moves up
the xylem due to the
adhesive forces
between the water
molecules and the walls
of the xylem.
This is known as capillarity, or capillary action.
The narrower the tube, the greater the effect.
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In the leaves…
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The transpiration stream
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Opening/ Closing Stomata
• Stomata can be closed to prevent water loss.
• Guard cells curve apart when turgid.
• When flaccid, the edges of the cells lie
close together.
• However, this drastically slows transpiration
and means that no carbon dioxide can enter
the leaf (= no photosynthesis).
Draw a flow chart to explain
the different stages in the
transpiration stream.
You must include the
following words: heat
energy, stomata, diffusion,
water potential, cohesion,
adhesion, xylem vessels.
Extra challenge: Why do
stomata close at night?
Factors Affecting Water
• Diffusion rate is affected by:
• Surface area (the higher the surface
area, the greater the rate of diffusion).
• Difference in concentration (the greater
the concentration gradient, the higher
the rate of diffusion).
• Length of the diffusion path (the greater
the length, the lower the rate of
Preventing Excess Water
• Cuticle (waxy layer) on leaf which is
impermeable to water.
• Most stomata found on underside of leaf as
it is cooler.
• Thick leaves = reduced water loss.
• Spines/ hairs increasing boundary layer
(undisturbed layer of air).
• Stomata closed at certain times of the day.
• Stomata may be sunken and found in pits.
Study the graph in Fig. 2. It shows
how four different factors affect
transpiration rate.
a) Which factor appears to have the greatest effect on
transpiration rate?
b) For each factor, briefly explain why you would expect
it to affect transpiration rate as it does.
a) Which factor appears to have the greatest effect on transpiration rate?
Because it:
Increases the rate of evaporation
Increases the rate of diffusion
b) For each factor, briefly explain why you
would expect it to affect transpiration rate
as it does
Relative humidity:
High RH
more water vapour in the air outside the leaf - reduces
the diffusion gradient - reduces transpiration.
Low RH
less water vapour – steeper diffusion gradient increases transpiration
Stomatal opening:
more water vapour can diffuse out –
transpiration increase
less water vapour can diffuse out –
transpiration decreases
Wind speed:
Wind blows away any water vapour which has
diffused out of the leaf – increases the
diffusion gradient – so as wind speed
increases transpiration increases
An Experiment – Factors affecting the
rate of transpiration
Collect 4 similar leaves. Add vaseline:
Leaf A – No vaseline
Leaf B – Vaseline on the top surface
Leaf C – Vaseline on the bottom surface
Leaf D – Vaseline on the top and bottom surface
Weigh all the leaves then hang them up by the
Bring your practical book next lesson!
Property of water
to plants
Property of water
to plants
Model answer
1: Leaf absorbs
heat energy from
the sun. Water
evaporates from
the cell walls of
the leaf cells.
2: Water vapour diffuses
through the air spaces in
the leaf and out through
the stomata, down the
diffusion gradient. This
water loss from leaves by
evaporation is called
5: Movement of water upwards
lowers water content of the
xylem in the root so water
constantly diffuses into it from
the root tissues and ultimately
from the soil
Cells surrounding the xylem also
use active transport to ‘pump’
mineral ions into the xylem to
lower the water potential so
water will also enter the xylem
by osmosis. The movement of
3: Water leaving the cells in the leaf
lowers the water potential so creates
a gradient of water potential that
draws water from the xylem in the
leaf veins.
Water either diffuses from cell to cell
via the plasmodesmata or diffuses in
the cellulose cell walls. [It can also
move from cell to cell by crossing the
cell membranes by osmosis]. This
water from the xylem replaces the
water lost from the cells
Due to the cohesive forces between
the water molecules the flow of water
from the xylem at the end of the leaf
veins to the cells ‘pulls’ water along the
4: This ‘pull’ is transmitted all the way
veins from the petiole to replace it.
down the xylem so a continuous column
of water is pulled up to the leaves
from the roots. The column prevented
from falling backwards due to gravity
•upward pull
•adhesion forces between water and
‘Pull’ plus adhesion to wall puts water