Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
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mesophyll
cell
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Water vapour diffuses from an air space through an
open stoma, a process called transpiration. It is carried
away from the leaf surface by air movements. This reduces
water potential inside the leaf.
xylem
vessel
air space
in spongy
mesophyll
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Water evaporates from a mesophyll cell wall into the air space.
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Figure 7.16 Water movement through a leaf. Water is, in effect, being pulled through the plant as a result of transpiration and
evaporation. Movement of water through the plant is therefore known as the transpiration stream.
Factors affecting transpiration
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Water moves through the mesophyll cell wall or out of the
mesophyll cytoplasm into the cell wall.
Water leaves a xylem vessel through a non-lignified area such as
a pit. It may enter the cytoplasm or cell wall of a mesophyll cell.
Water moves up the xylem vessels to replace the
water lost from the leaf.
Humidity. If the water potential gradient between the
air spaces in the leaf and the air outside becomes steeper,
the rate of transpiration will increase. In conditions
of low humidity, the gradient is steep, so transpiration
takes place more quickly than in high humidity.
Wind speed and temperature. Transpiration may
also be increased by an increase in wind speed or rise
in temperature.
Light intensity. In most plants, stomata open during
the day and close at night. Most transpiration takes
place through the stomata (although a little water
vapour can escape through the epidermis if the
cuticle is thin), so the rate of transpiration is almost
zero at night. Stomata must be open during the day
to allow carbon dioxide to diffuse into the leaf for
photosynthesis. This inevitably increases the rate of
transpiration. Closing at night, when photosynthesis is
impossible, reduces unnecessary water loss.
Very dry conditions. In especially dry conditions,
when the water potential gradient between the internal
air spaces and the external air is steep, a plant may
have to compromise by partially or completely closing
its stomata to prevent its leaves drying out, even if this
means reducing the rate of photosynthesis.
lower epidermis
stoma
guard cells
In hot conditions, transpiration plays an important role in
cooling the leaves. As water evaporates from the cell walls
inside the leaf, it absorbs heat energy from these cells, thus
reducing their temperature.
If the rate at which water vapour is lost by transpiration
exceeds the rate at which a plant can take up water from
the soil, then the amount of water in its cells decreases.
The cells become less turgid (page 85) and the plant wilts
as the soft parts such as leaves lose the support provided
by turgid cells (Figure 7.17). In this situation the plant will
also close its stomata.
QUESTIONS
7.3 Suggest how the following factors may cause the rate
of transpiration to increase:
a an increase in wind speed
b a rise in temperature.
7.4 How does the transpiration cooling mechanism
compare with the main cooling mechanism
of mammals?
7.5 Transpiration has sometimes been described as a
‘necessary evil’. Explain how this statement might
be justified.
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