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GAS
EXCHANGE IN VASCULAR
PLANTS
Biology ATAR Unit 2
Chapter 12
Pages 307-309
Keywords
Passive diffusion
 Stomata
 Guard cells
 Turgid
 Flaccid
 Dissolution of gases
 Evaporation
 Cellular respiration
 Photosynthesis
 Chloroplasts

Making connections



Plants ‘breathe’
through small pores in
their leaves. These
are called stomata.
Gases (O2 & CO2)
diffuse in and out of
the stomata
This is an efficient
passive process
because leaves are
very thin, thus
maximising the
surface area to
volume ratio
GAS
EXCHANGE
Gas exchange in plants occurs through passive
diffusion
The leaves of plants facilitate diffusion because:
 The leaves are flat and cells are arranged in thin
layers, thus maximising the surface area to volume
ratio
 The cells are loosely packed, allowing for rapid
diffusion through intracellular spaces which are
filled with air

STOMATA
Gas exchange occurs
through tiny pores
called stomata
(stoma singular)
 Each stoma is
surrounded by 2 guard
cells
 These guard cells
control the opening and
closing of the stoma
 Gases dissolve in
solution when they
come into contact with
the moist cell surface.

GUARD CELLS
Guard cells:
 Are joined at the ends
 Have a thicker cell wall
on the side of the
stoma
 Have bands of inelastic
fibre around the cell
wall
 Contain chloroplasts to
make glucose for the
energy needed to drive
the H+/Cl- and K+/Na+
pumps
GUARD




CELLS
Guard cells pump in K+ ions to increase the
osmotic gradient across the cell membrane
During the day, photosynthesis also produces a
build up of glucose in the guard cells
Water moves into the cell
The guard cells swell lengthwise (become turgid)
and the stoma opens
MICRO-ENVIRONMENT




The micro-environment
surrounding stomata is
warm and moist.
This facilitates the
dissolution of gases,
enabling the molecules to
diffuse into the cell.
It also means that it is a
site of water evaporation.
The plant must therefore
maintain a balance
between this water loss
and its need of CO2 for
photosynthesis.
OPENING
AND CLOSING OF THE STOMATA
DAY TIME
 Stomata open during
the day, when the
plant needs CO2 for
photosynthesis
 The rate of
photosynthesis is
greater than the rate
of cellular respiration
 CO2 and O2 are
exchanged
independently of one
another.
NIGHT TIME
 Stomata are closed
at night to prevent
unnecessary water
loss through
evaporation.
 The rate of cellular
respiration is
greater than the
rate of
photosynthesis
ADAPTATIONS
TO ENVIRONMENTAL
CONDITIONS



In terrestrial plants,
stomata are mostly
located on the
underside of the leaf.
This reduces the rate of
evaporation by
protecting them from
the hot sun.
In aquatic plants
however, the stomata
are found on the upper
surface of the leaf.



In hot, arid climates,
plants have fewer and
smaller stomata. This
reduces the rate of water
loss.
Leaf size is generally also
reduced.
In cacti, the stomata are
located on the stem of the
plant.
ADAPTATIONS
TO ENVIRONMENTAL
CONDITIONS

Mangroves have specialised roots that grow
upwards from waterlogged soil to obtain
oxygen for the submerged root system.
RATE
OF GAS EXCHANGE
The overall
exchange of gases
depends on
environmental
conditions which
influence:
1. The rate of
photosynthesis
2. The rate of
cellular
respiration
The rate of
photosynthesis and
cellular respiration
(or the rate of a
plant’s
metabolism) can
be measured by its
use and/or
production of
gases.