Download Transpiration

Transpiration
Transpiration is the loss of water by evaporation from the leaves through the stomata.
The source of water for the plants is soil water. It is taken up by root hair cells by osmosis.
Once in the root hair cell, water moves by osmosis from cell to cell through the endodermis
into the Xylem. The water is then pulled under pressure through the transpiration stream up
the Xylem (a long hollow tube) to the leaves.
The water moves by osmosis into the palisade cells and will saturate the air spaces in
the leaves. Water is lost from air spaces within stomata to outside.
Function of water in leaves
1.
2.
Photosynthesis
To maintain turgor shape.
A potometer measures water loss in leaves. There are two types. Weight potometer and
bubble potometer. A weight potometer measures the amount of water lost by a plant
through transpiration.
Experiment:
1.
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Place plant with roots and leaves into a conical flask.
Add water and mark level of water.
Add a layer of oil to surface of water to prevent evaporation of water.
Weigh
“A” g
Wait for 48 hours
Reweigh “B” g
Loss in mass: “A” – “B” = “C” g
Note volume of water added to restore water volume to previous level – “D”
C should equal D……….BUT SOMETIMES….
1.
Loss in mass is greater than volume of water added.
This could mean that the plant could be dying or losing leaves.
2.
Loss in mass is less than volume of water added.
This could mean that the plant is growing and photosynthesising.
Bubble potometer
Measures how fast a plant loses water through transpiration over a short period of time –
under different environmental conditions.
Precautions while setting up!
1.
2.
Cut shoot under water, to avoid air bubbles in Xylem.
Make a diagonal cut in stem: longer surface area for uptake of water.
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Seal the join with Vaseline to avoid air bubbles in the system
Dry/blot leaves to create a gradient in favour of water evaporation.
Reset scale to zero before starting.
Factors affecting transpiration
Factor
Effect
Explanation
Light
Dark
Increased Transpiration
Decreased Transpiration
Stomata are open
Stomata are closed
High temperature
Increased Transpiration
Low temperature
Decreased Transpiration
High wind
Increased Transpiration
Low wind
Decreased Transpiration
Water molecules moved away from stomata,
which increases gradient in favour of
diffusion of water from leaves.
Less gradient formed
Decreased Transpiration
Moisture in air reduces gradient
Increased Transpiration
Greater gradient in favour of evaporation
Defoliation
Decreased Transpiration
Reduced surface area/fewer stomata from
which water can evaporate
Availability of
water
Reduced water loss
At times of drought, plants can close their
stomata to prevent water loss.
High humidity
(moist air)
Low humidity
(dry air)
Any given volume of air requires more
water to saturate at high temperature (i.e.
greater evaporation rate)
Less evaporation
Experiments to detect water loss from leaves
1.
Cobalt chloride paper: sensitive to moisture.
Blue
(Dry)
Pink
(Moist)
Result: Lower surface goes pink quicker as it has more stomata. Exceptions are glass
and plants on water such as lily.
2.
To prove that the greater part of transpiration occurs on the lower surface:
Hang leaves by their stalks from a line. Ensure the stalks are covered with Vaseline
to prevent water loss.
A
B
C
Treatment
D
Result
A No Vaseline applied (to either surface)
Very wilted – water lost from both surfaces
B Vaseline applied to upper surface only
Wilted – water lost from lower surface
C Vaseline applied to lower surface only
Some wilting – water loss from upper
surface – not as much as in B
D
Vaseline applied to both surfaces
No wilting – no water loss – leaves fresh
Conclusion
1.
2.
3.
Vaseline seals water (known from beginning)
Some water is lost from the upper surface
More water is lost from the lower surface – as there are more stomata there
Transpiration Stream
This is a force. The evaporation of water from the leaves causes a type of suction,
which pulls (this is the force!) water up the stream from the roots. The water travels in the
xylem vessels of the vascular bundles.
Water will move out of the xylem by osmosis to ensure the cell vacuoles are full of
water (turgid). In the leaf, the air spaces are saturated with water. This allows cell
membranes to be moist, to allow rapid diffusion. There are several factors assisting the
water from the root to the leaf:
1.
2.
3.
4.
Xylem vessels are very narrow
Cohesion – water molecules stick together
Root pressure – forces water up the xylem
Transpiration – evaporation from leaves
Importance of transpiration
1.
2.
3.
4.
5.
Supplies water to leaf for photosynthesis
Supplies mineral ions in solution to leaf (e.g. magnesium for chlorophyll)
Cooling for leaf
For turgor shape
Moist cell membranes (gases only pass through solution)
Adaptation of plants to reduce water loss (normally in arid conditions)
1.
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Rolled up leaves to reduce surface area for water loss
Leaves reduced to spines (e.g. cactus or conifer trees)
Sunken stomata to trap water as they are unexposed to wind and sun
Hairs around stomata (to trap moisture)
Waxy cuticle (e.g. holly) – waterproof and reflects light
Swollen stems to store water
Stomata
Stomata – “hole” where gases (and
water vapour) can diffuse in and out of
Two turgid guard cells –
normally these are very
close to each other
Day
Chloroplasts absorb light for
photosynthesis
Cell sap gets stronger
(i.e. lower water potential)
Night
No light – no photosynthesis
Cell sap gets weaker
(i.e. higher water potential)
No water enters
Guard cells are flaccid
Stomata are closed
Water enters guard cell via
osmosis
Guard cell swells and becomes
turgid
This pulls the guard cells apart
The stomata are open