Syllabus & Teacher Guidelines
3.4.3 The Exchange System in Plants
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Examination of the structure of the leaf in relation to gaseous
exchange of oxygen and carbon dioxide through the stomata.
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Reference to the presence and role of lenticels in stem
structures.
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ACTIVITIES
* Examine microscopically a T.S. of a leaf blade. Note the
intercellular air spaces allowing free diffusion of carbon
dioxide and oxygen.
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* Examine stomata distribution on a leaf blade.
3.4.3 Structure of an Exchange System
in Flowering Plants
Objectives – What you will need to
know from this section
 Explain the role of the leaf (stomata) & stem (lenticel) to
gas exchange.
3.4.3
The Structure of an Exchange System
in Flowering Plants
 Plants absorb water through
their roots, but lose water
from their leaves by
evaporation (transpiration).
 Over 90% of the water that
enters a leaf goes right on
through it and evaporates
into the surrounding air.
 The evaporation of water
from leaves [and stems] is
called transpiration.
 The loss of water is reduced by a waxy cuticle and by
the closing of the stomata [pores] on the lower surface of
the leaf.
LEAF TS
 These pores stay open during the day so that CO2 can
enter, for photosynthesis. But, water can escape at this
time too. Stomata can stay closed at night, when there is
no light for photosynthesis. This allows plants to
conserve water.
LEARNING CHECK
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Plants absorb water from where?
Plants can lose water from where?
What is evaporation?
What is transpiration?
How do plants try and reduce water loss?
What are stomata?
 Leaves are often adapted to try and reduce this loss of
water—they have thick cuticles; thin,spiny leaves; etc.
 Since plants do not move, their oxygen can be supplied by
simple diffusion from the soil or air.
What about trees and woody plants ?
 How do gases [oxygen and
carbon dioxide] get through
the waterproof, corky
surface of bark?
 They do so through
lenticels, which are areas
where the packing of bark
cells is loosened up a bit.
 Lenticels often look like tiny
raised blisters on a branch
or twig.
Winter twig – showing lenticels
LEARNING CHECK
• What does the term “adapted” mean
• How are plants adapted to living in dry, hot
climates?
• What is diffusion?
• What are lenticels?
• How are lenticels different from stomata?