TRANSPIRATION. What is Transpiration?. An evaporation of water in the form of water vapour from the surface of the plant to the atmosphere Where does transpiration take place?. Transpiration. Transpiration mainly takes place through openings on leaves – STOMATA
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4) As water diffuses out of the cell, the water potential within the cell will decrease. Due to OSMOSIS, water from adjacent cells will be drawn into the cell to replace the water loss
These adjacent cells will in turn draw water from other neighboring cells
5) Water is drawn from the XYLEM vessels (in the veins) into the neighboring mesophyll cells
There will be a water potential difference between the xylem vessels and the mesophyll cells. As water is drawn from the xylem vessels, a suction pressure will develop and this pressure will pull water up the xylem vessels from the roots to the leaves
Investigation #1:Studying the loss of water by a plant into the atmosphere
Ans: The polythene bag prevents evaporation of
water from the soil, and also prevents
vapour released by soil microorganisms
from affecting the result of the experiment
2) What do you think will happen in the two set-ups after 2 hours?
Ans: Water would be found condensed on the
bell jar with the leafy plant inside
3) How can you show that it is water?
Ans: We can use anhydrous cobalt chloride
paper to test it. It will turn the paper from
blue to pink. Alternatively, we can use
anhydrous copper sulphate. Water will
turn it from white to blue
4) Why is it better to use forceps instead of fingers to hold cobalt chloride paper?
Ans: It is to avoid moisture on our fingers from
being absorbed by the cobalt chloride
5) How would you explain the results of this experiment?
Ans: Transpiration occurs in plants through
I have no trouble absorbing sunlight, but I just keep losing and losing water. Is there any way that I can prevent excessive water loss?
1) Waxy layer of cuticle on the leaf’s outer surface of the epidermis
Each stoma is surrounded by two guard cells which contain chloroplasts
It is kidney-shaped
The inner wall is thicker than the outer wall
1) Normal Plants
- Mainly on the lower surface of plants
2) Floating plants
- Mainly on the upper surface of plants
- Leaves may also have air sacs to keep them
afloat. These sacs can be used in gaseous
3) Submerged Aquatic Plants
- No stomata (not required since gaseous
exchange can be carried out by diffusion
though the leave surface)
- No cuticle (the primary function of cuticle is
to prevent excess water transpiration which
is not present in aquatic plants)
4) Plants in dry and hot conditions
- usually have much less stomata to reduce
the amount of water loss
Investigation #2:Investigating stomatal distribution in a leaf by using cobalt chloride paper
cobalt chloride paper
Which piece of cobalt chloride paper will turn pink first?
Ans: The piece of cobalt chloride paper attached to the lower epidermis of the leaf will turn pink first.
Investigation #3: Comparing the abundance of stomata on the upper and lower surfaces of a leaf
1) Which surface has more air bubbles coming off?
Ans: There should be more air bubbles appearing
on the lower surface of the leaf
2) Why do air bubbles appear on the leaf surfaces?
Ans: Air in the air spaces between the mesophyll
cells in leaf expands on heating and passes
out through stomata of the leaf, forming air
3) What does the result show?
Ans: The result shows that more stomata are
present on the lower epidermis of a leaf
Investigation #4: Comparing the abundance of stomata on the upper and lower surfaces of leaves by weighing
Smear with vaseline on both surfaces of the leaf
Smear with vaseline on lower surface of the leaf only
Smear with vaseline on upper surface of the leaf only
Do not apply vaseline on the leaf
Ans: It is used to block the stomata so as to
prevent water being lost by evaporation
2) Which leaf would show the greatest/least change in weight?
Ans: Leaf D > Leaf C > Leaf B > Leaf A
3) What do the results of leaves B and C indicate?
Ans: The change in weight of leaf B is less than
that in leaf C, which indicates that more
water is lost in leaf C. This suggests that
there are more stomata present in the lower
surface than the upper surface of leaves
Plants living in hot and dry environment
Some trees also shed their leaves during dry seasons (e.g. autumn) in order to reduce the rate of transpiration
Investigation #5: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a bubble potometer
graduated capillary tube
1) Why is it important to cut the leafy shoot under water?
Ans: This prevents air from entering the xylem vessels of the stem and blocking the water uptake
2) What is the relationship between transpiration rate and the distance traveled by the bubble?
Ans: The rate of movement of the bubble is proportional to the transpiration rate of the plant. Under normal conditions, the rate of water absorption of a plant is equal to the rate of transpiration
3) Compare the transpiration rate of the leafy shoot under the different conditions.
Investigation #6: Comparing the rates of transpiration of a leafy shoot under different environmental conditions using a weight potometer
1) What is the function adding a layer of oil in the set-up?
Ans: The layer of oil can prevent evaporation of water in the flask, which can affect the result
2) What is the relation between the transpiration rate and the change in weight of the plant?
Ans: Since the water loss from the set-up is due to transpiration only, the change in weight of the set-up is directly proportional to the transpiration rate of the plant
3) What limitations may lead to inaccurate results in the experiment?
Ans: The initial weight of the set-up may not be accurate because water may be present on the wall of the apparatus and also on the leafy shoot
4) Compare the weight potometer and the bubble potometer. Which one is easier to use? Which one is more accurate?
Ans: A weight potometer is easier to use and is more accurate to measure the transpiration rate of plants. It is because the weight potometer can measure the rate of transpiration directly whereas the bubble potometer can only measure the rate of water uptake of plants
(IV) Wind Speed
(I) Light Intensity
(V) Water Supply
Rate of transpiration
Light intensity (lux)
Rate of evaporation of water from mesophyll cells
Ability of air to hold water vapour
Rate of diffusion of water vapour from intercellular space in leaf to outside
Rate of transpiration
Rate of transpiration
Temperature (0 C)
A decrease in humidity makes the diffusion gradient of water vapour from the moist intercellular space of a leaf to the external atmosphere steeper, therefore the rate of diffusion of water vapour increases
Rate of Transpiration
Rate of transpiration
Relative humidity (%)
Under very windy conditions, stomata will be closed to reduce water loss
Water vapour around the leaf is swept away
Transpiration rate increases
Diffusion gradient between intercellular space in leaves and outside becomes steeper
Wind speed (km/hr)
Rate of transpiration
Lack of water (plants become dehydrated)
Soil dries, plant wilts and stomata close
Transpiration rate decreases
Enrichment Reading 9.1
Daily Changes in Transpiration Rate
Plant tissues fall into 3 fundamental categories:
＊These tissues can be found in roots, stems
1) Tracheids – long, thin cells with closed ends and are dead at maturity. Contain numerous pits through which water moves
2) Vessel elements – similar to tracheids but contain holes at each end and are joined end-to-end forming vessels. They are thick-walled and non-living (no cytoplasm and no nuclei)
1) Sieve tube members - a sieve tube, like xylem vessels, is a series of cells (sieve elements) joined end to end. The cross walls between successive sieve elements are perforated, forming sieve plates. The cell walls are thin. Although the cells are living, they lack a nucleus. Unlike xylem vessels, the cells walls are not thickened by lignin
2)Companion cells – specialized parenchyma cells that develop alongside a sieve tube member. They are elongated, thin-walled and possess a nucleus. Companion cells are linked with the sieve tubes by small canals filled with cytoplasm, which are smaller than pits. Companion cells help to regulate the metabolic activities of sieve tube elements, and help to load and unload the food for transport
Layer of thin-walled cells between xylem and phloem
The cambium produces new layers of phloem on the outside and new layers of xylem on the inside, thus increasing the diameter of the stem
1) Water and minerals absorption
Mesophyll cells in leaves (high [ ] of carbohydrates) -> sieve tubes of phloem (low [ ] of carbohydrates) -> active growing areas (e.g. root tips) /storage areas (e.g. fruits)
Support in young plants or non-woody parts of plants is contributed mainly by turgidity of the thin-walled cells (parenchyma) in the cortex and pith
Water absorbed by osmosis
Water lost by osmosis
-- outer side -> new phloem