13.9 Transpiration and
factors affecting it
Learning outcomes
Students should be able to understand the following:
 Why transpiration occurs
 What factors affect the rate of transpiration
 How to measure the transpiration rate
Candidates should be able to:
 Explain transpiration and the effects of light,
temperature, humidity and air movement
Why do plants need transpiration?
Lots of water is lost from the leaves of a
plant because:
(a)Leaves have a large surface area
(b)Leaves have stomata
Therefore 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
Giant Redwood trees
can lose 700 litres of
water per day
Is the rate of transpiration constant?
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.
The speed at which a plant loses water is
called the rate of transpiration. This varies
depending on the plant’s environment.
What environmental factors will
affect the rate of transpiration?
 humidity (amount of moisture in the air)
 light intensity
 temperature
 air movement (wind).
Why does transpiration vary?
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.
Class Practical
Estimating the rate of transpiration from a plant cutting
• Set up the cuttings and record the starting volume and
mass for each one
• Make a second set of readings and answer questions in
the next lesson
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.
Extension and Homework
1. AQA AS Biology textbook pg 198
Summary questions 1-3
2. Read ‘Measurement of water uptake using a
potometer’ on page 198-199 then answer
application questions 1-5
2. Complete the exam style question about
transpiration rate in plants
Mark scheme - exam style question
Answers – transpiration rate questions
1.(a)(i) distance moved by bubble in specified time period;
diameter / radius of capillary (lumen);
(ii) surface area/ mass of leaves/ plant;
(b)
rapid loss then decreases;
2
1
1
loss in mass as water loss is not replaced;
initially stomata are open / later the stomata close;
higher water potential of leaf cells compared with atmosphere /
diffusion of water from leaf to atmosphere;
water potential of leaf reduced / diffusion gradient decreased;
max 3
[7]
Learning outcomes
Students should be able to understand the following:
 Why transpiration occurs
 What factors affect the rate of transpiration
 How to measure the transpiration rate
Candidates should be able to:
 Explain transpiration and the effects of light,
temperature, humidity and air movement