Settler
Explain how transpiration is a consequence
of gas exchange
Module 3
Exchange and transport
9.3 Transpiration
(2 lessons)
Starter
• A3 memory game of a labelled potometer
Learning Objectives
•
Define the term
transpiration
Success Criteria
• Define the term transpiration
(Grade D – C)
• Describe the factors that affect
transpiration rates and how a
potometer is used to measure
these
(Grade C –B)
• Describe how a potometer,
with the aid of diagrams, is • Explain how practical
investigations are used to
used to estimate
estimate transpiration rates
transpiration rates
(Grade B – A)
• Describe the factors that
effect transpiration rate
Transpiration
• Transpiration is the
loss of water vapour
from the upper parts of the
plant by evaporation.
Transpiration involves 3 processes
1) Osmosis from the xylem to mesophyll cells
2) Evaporation from the surface of the mesophyll cells
to the intercellular spaces
3) Diffusion of water vapour from the intercellular
spaces out through the stomata
•Define the term transpiration
(Grade D – C)
Why do plants need transpiration?
A transpiration pull effect is needed to
transport large volumes of water up the
stems of large plants/trees to the leaves.
As well as providing water for
photosynthesis, dissolved ions, sugars and
hormones are also moved about in the
transpiration stream.
Transpiration is a bit like a straw, pulling
water up the plant. Sometimes the pulling
force will be stronger and the plant will
lose more water.
Giant Redwood trees
can lose 700 litres of
water per day
Rate of transpiration
This is the amount of water vapour that a plant
loses from it’s leaves and stems per unit time.
It depends on several variables:
• Size of the plant
• Thickness of the waxy cuticle
• How widely space the stomata are
• Whether the stomata are open or closed.
These are all biotic factors
Rate of transpiration
The rate of transpiration also varies depending on the
plant’s environment.
These are abiotic factors
 humidity (amount of moisture in the air)
 light intensity
 temperature
 air movement (wind).
Why does transpiration vary?
Factors affecting transpiration
Factor
How factor affects
transpiration:
Increase in
transpiration
caused by:
Decrease in
transpiration
caused by:
Light
Stomata open in light and close
in dark
Higher light
intensity
Lower light
intensity
Temperature
Alters the kinetic energy of the
water molecules and the
relative humidity of the air
Higher
temperature
Lower
temperature
Humidity
Affects the water potential
gradient between the air
spaces in the leaf and the
atmosphere
Higher
humidity
Lower humidity
Air movement
Changes the water potential
gradient by altering the rate at
which moist air is removed
from around the leaf
More air
movement
Less air
movement
Measuring water loss
• Enclose the pot of a potted
plant in a plastic bag and put
the plant on a top-pan
balance. The mass gradually
falls as water evaporates off
the surface s of the plant.
Measuring uptake of water
Transpiration can be measured using a potometer.
A cut plant stem is sealed into the
potometer using a rubber bung.
An air bubble is introduced
to the capillary tube.
The distance the bubble
travels shows how much
water the stem has taken up.
This gives an indirect measurement of the rate of
transpiration (i.e. it measure water uptake).
Potometer experiment
Problem is that potometer is only an estimate
• Measures water uptake ;
• Not all water (taken up) is lost . 99% water
taken up is lost
• Some water used is used in photosynthesis /
making cells turgid
Potometer
What is a potometer?
• A piece of apparatus used to measure the rate of transpiration
(water loss from a plant).
Why are the effects of photosynthesis and respiration usually
ignored when taking potometer readings?
• Because the volume of water involved in metabolic processes
is insignificant compared to the large volume constantly
flowing through the plant in the transpiration stream.
If you were comparing transpiration rates of several leaves, what
leaf feature should be measured to obtain a fair comparison?
• Surface area of the leaves.
Potometer
What precautions must be followed when setting up a
potometer?
• Caution should be taken when preparing holes in rubber
bungs, pushing the glass tube, the cutting and the syringe into
the bung.
• It is crucial to avoid air bubbles in the assembled potometer.
• The plant stem must be cut under water to prevent airlocks
forming in the xylem.
• All fittings must be tight so that the only way the apparatus
can lose water is by the plant transpiring.
• Try to avoid getting the leaves of the shoot wet when
assembling the potometer in the sink, as this significantly
slows down subsequent transpiration.
plenary
• Jan 10 Question 4 - homework
•Describe the factors that affect
transpiration rates and how a potometer is
used to measure these
(Grade C –B)