1) Water moves through the xylem vessels from roots to leaves where it is lost through transpiration, the evaporation of water from mesophyll cells and diffusion out of stomata. 2) Transpiration creates a suction that pulls water up the xylem vessels through cohesion and adhesion of water molecules. 3) The rate of transpiration is affected by environmental factors like light, temperature, and humidity, with higher light, temperature, and lower humidity increasing the rate.

1. Transport in
Plants
TOPIC 3: TRANSPIRATION

2. Learning
Outcomes
State that water is transported from the roots to
leaves through the xylem vessels
Define transpiration
Explain how water vapour loss is related to the
large surface area of cell surfaces, interconnecting
air spaces and stomata
Explain the mechanism by which water moves
upwards in the xylem
Explain how and why wilting occurs
Explain & investigate the factors affecting
transpiration rate
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02
03
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3. QUICK RECALL!
• Which vessel in the vascular bundle is involved in
transport of water through the plant? In which
direction(s) does the water flow in this vessel?
• During which time of day is the stomata typically
open, why?
TRY TO ANSWER THE FOLLOWING QUESTIONS

4. Transpiration
• Transpiration is the evaporation of water from the surfaces of the
mesophyll cells into the air spaces followed by the diffusion of this
water vapour out of the stomata into the atmosphere.
How transpiration occurs
• Mesophyll cells have damp cell walls. This water evaporates into
airspaces between the cells. Then, the water vapour diffuses out into
the atmosphere through the stomata. (Because there is more water
vapour inside the airspace than the outside air
• Water lost from the spongy mesophyll cells is replaced by water
coming in from the xylem.
• This loss of water from the top of the plant leads to a lower pressure
at the top as opposed to the bottom, causing water to come up the
xylem

5. Transpiration
• Water being pulled up the xylem in the stem from the roots to the
leaves is referred to as the transpiration pull.
• The movement of water up the xylem is referred to as the
transpiration stream
• The movement of water molecules up the xylem is aided by two
properties:
⚬ Cohesion – the tendency of water molecules to attract each other
(sticking together)
⚬ Adhesion – tendency of water molecules to stick to the inside of the
xylem vessel

6. • Wilting occurs when water loss is greater than water uptake – leading to
loss of turgidity of cells in stem and leaves.
• ·More transpiration takes place during day as opposed to night
⚬ More stomata are open to take in carbondioxide for photosynthesis
during the day, close during night to reduce volume of water lost by
transpiration
⚬ Stomata may also close during hot, dry conditions during the day
when water lost by transpiration is not getting replaced by water
from the soil.
⚬ If plant still does not get enough water, cells lose so much water
they are no longer turgid (they become flaccid),
￭ plant becomes soft, stem is no longer upright and leaves droop.
• Wilting causes leaves to move downward,a way from direct rays of the
sun so they do not get as hot, temperature decreases ad they can
absorb more water than is lost by transpiration – then they recover.
Wilting

7. Factors Affecting Rate of Transpiration
1) Light
• As light intensity increases, rate of transpiration increases up to a maximum
that is determined by other conditions such as humidity and temperature.
⚬ as light intensity increases, stomata open wider -> more transpiration
takes place
2) Humidity
• Humidity is a measure of the quantity of water vapour in the air. Humidity
increases, rate of transpiration decreases. Humidity decreases, rate of
transpiration increases.
⚬ When the concentration of water vapour in the outside air is low,
humidity is low, this allows for more water vapour to diffuse out of the
plant -> more transpiration.

8. Factors Affecting Rate of Transpiration
3) Temperature
• Rate of transpiration increases as air temperature increases
⚬ As temperature increases, water molecules on cell surfaces
gain more kinetic energy, evaporate into air spaces, then
diffuse out through stomata.
Wind can also increase the rate of transpiration as it will sweep away the
molecules of water vapour near the leaf, maintaining a steep concentration
gradient.

9. Apparatus used: Potometer
Assumption made: Rate of transpiration is equal to water uptake by the plant.
⚬ Potometer does not directly measure rate of transpiration
⚬ Most of the water taken up by the plant is lost (about 98%)
￭ the rest is used in photosynthesis and in keeping cells turgid
Measuring the rate of transpiration

10. • The potometer is filled by submerging it in water.
• It is set up under water to make sure no unwanted air bubbles are present
• You need to make sure the stem in the stopper is air-tight
• One air bubble is allowed to form in the capillary tube.
⚬ The distance moved by the bubble is a measure of the rate of water uptake.
⚬ Need to make note of time as well. (rate = distance/time)
• Water in resevoir is used to reset the bubble back to its starting position
Measuring the rate of transpiration

11. QUESTION
• State and explain how each of these
conditions affect the rate of transpiration:
A) Increase in light intensity
B) Decrease in temperature
C) Increase in Humidity

12. Important points
• Transpiration: the evaporation of water from the
surfaces of the mesophyll cells into the air spaces
followed by the diffusion of this water vapour out of the
stomata into the atmosphere.
• Transpiration stream: The movement of water up the
xylem
⚬ Adhesion
⚬ Cohesion
⚬ Suction created by loss of water at the top - (low
pressure at top, high pressure at the bottom due to
evaporation)

13. Important points
• Wilting occurs when water loss is greater than water uptake
– leading to loss of turgidity of cells in stem and leaves