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Gas Exchange
In plants
• All organisms need to ‘breathe.’
• PLANTS ARE NO DIFFERENT but they
‘breathe’ or respire at night and
photosynthesise during the day.
• Some of the oxygen produced in
photosynthesis is used in cellular
respiration, the rest is diffused out.
• Therefore in plants oxygen and carbon
dioxide are both waste products AND
nutrients
Where does exchange occur in
plants….
• Small plants have thin leaves - one cell thick so gas
exchange can occur directly with the external
environment by diffusion.
• More complex plants – vascular plants – they exchange
gases through small openings in the epidermis, or outer
layer of their leaves, stems and roots, called STOMATA.
• The stomata regulate the rate of gas exchange.
• If the stomata are closed the gas exchange virtually
stops altogether.
• Plant cells are packed loosely – allows rapid diffusion
through the extracellular spaces
More about stomata..
• Tiny pores in the epidermis.
• Bordered by guard cells, and as the name
implies they guard the ‘hole’ or are
involved in opening and closing the pore.
• The size and number of stomata vary
according to the plant species.
• Generally found on the underside of the
leaf
Take a careful look on pg 145
• Guard cells – these are joined in pairs at the ends.
• Have thicker walls
• Have inelastic fibres around the cell walls to prevent them from
expanding and opening when they need to be closed.
• Water passes into the guard cells and they become turgid or “full
and firm” buckle and open the stomata:
~ this means to open, there is lots of light, water and low internal
carbon dioxide levels
~ At night then, they usually close as the need for such rapid and
volumous gas exchange is reduced.
• Aquatic plants differ in that oxygen often diffuses from Arial plants of
the plant- mangroves
TRANSPORT SYSTEM IN
PLANTS
• Plants need water, light and carbon
dioxide to photosynthesise.
• We have already said simple plants take
all this in by simple diffusion.
• More complex plants have a transport
system – both for water they need and the
sugars they produce as a result of
photosynthesis
Page 161
• Xylem – transports water and nutrients
• Phloem – transports the sugars
• Both called vascular tissue
Take own notes in a little more detail on
pages 162/163 (simple, in dot points)
Roots
• In a plant these are essential for the
survival for the following reasons:
~ they provide stability
~ Increase the amount of nutrients etc which
can be uptaken by the increased surface
area with many fine root hairs
~ can access ground water if levels are low
externally
Transpiration
• The movement or loss of water from the
leaves
• No energy required
• Evaporation – driven by the sun
• Rate is dependent on temperature, wind,
available surface area and humidity – we
look at this more in Arid zones and
ecosystems
• Goodluck for your exams Year 11
~ Ms Hillebrand 