Transpiration notes

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  • Created by: Sophie
  • Created on: 15-05-14 19:15
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Transpirat ion
- Transpiration is the loss of water vapour (by evaporation from the
stomata) through the aerial parts of plants
- Water pulled through stem by capillary action
- Water lost through stomata, causing suction more taken in through
roots continuous flow of water = transpiration stream
- Transpiration stream important because
1) Supplies water so photosynthesis can be carried out + to keep plant
2) Carries mineral ions dissolved in water
3) Allows evaporation from leaf surface cools leaf
- If water lost quicker from leaves than can be taken in by roots, cells
- In transpiration stream water goes from root hair cells cortex xylem stem spongy
mesophyll layer stomata
- Stomata are usually located on underside of leaves because if exposed to direct sunlight, too
much water lost
-How stomata work:
a)When guard cells bend outwards, stomata
is opened lots of water is lost
b)If water leaves guard cells cells flaccid
guard cells close
c)Very little water lost when stomata
d)Stomata closed at night since no sunlight
for photosynthesis and not necessary to
cool leaves
Petroleum jelly where on Weight after 1 day
Experiments leaf
- To show more water lost from
Both sides Lost 26.9% of mass
lower surface of leaf (see table)
- To show rate of transpiration: Upper surface Lost 49.7% of mass
potometer experiments
- To show effect of leaf surface Lower surface Lost 35.7% of mass
area on transpiration rate:
Nowhere Lost 77.4% of mass
potometer experiment but
remove ¼ of the leaves every 5 minutes. Put shoot in side arm of potometer so that if bubble
reaches reservoir, will float up harmlessly and not go up xylem. User capillary tubing and not
wide tubing as easier to see change + more accurate.
- To find out what proportion of water taken up by shoot is transpired: Weight shoot and
measuring cylinder (containing water covered by oil). Weight again and note volume of water
lost. Less mass lost than volume ( in our experiment, but actually about 98%)
- To find which side of leaves has more stomata: nail varnish peeled off with sellotape
inspected under microscope
- N.B. Respiration produces water and decreases mass of plant. Photosynthesis uses water but
increases mass of plant. When comparing transpiration rates of leaves, measure stomatal
densities/surface areas for fair comparison.

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Transpirat ion
How shoot takes in water:
-Loss of water vapour from leaves (due to evaporation)
-Water pulled up xylem in continuous column as water molecules
are cohesive
-Water moves into xylem from roots to replace it
Connects cut shoot to Connected to capillary tubing
potometer. Greased + when tap opened. Used to
wired to prevent air move air bubbles back along
leaking into tube so they dont enter plant.
apparatus.…read more

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Transpirat ion
Vascular bundles
- Vessels (tubes) which transport water, minerals, sugars and hormones through plant
- Xylem- transports water + minerals from roots upwards
- Phloem- transports sugars (sucrose) + hormones from leaves up or down plant
In stem
In roots In leaf
Remember: XP (like windows)
X = Xylem = X marks the
spot (its in the centre)
P = Phloem = Periphery
Cells here contain
more chloroplasts
than cells in spongy
layer because more
sunlight at top of leaf…read more


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