Transcript Gas Exchange in Plants

Gas Exchange in Plants and
Xerophytic Adaptations
- Be able to describe how a plant is adapted for
efficient gas exchange with minimal water loss.
Plants – a Reminder
Write down the equation for respiration…
Write down the equation for photosynthesis…
Now try and label the cells on your sheet.
Gas Exchange in Leaves
• Plants, like animals, respire aerobically (all day
and all night!).
What gas do they use?
What gas do they produce?
• In the light plants also photosynthesise.
What gas do they use?
What gas do they produce?
Reminder
Photosynthesis:
Photosynthesis vs. Respiration
Plants need oxygen for respiration all of the
time.
Plants need carbon dioxide for photosynthesis
during the day.
During the day, the rate of photosynthesis is
around 6 times faster than that of respiration.
Explain why.
Photosynthesis vs. Respiration
Some of the oxygen produced in photosynthesis
is used in respiration.
Some of the carbon dioxide produced in
respiration is used in photosynthesis.
During the day, plants need much more carbon
dioxide than respiration can provide; carbon
dioxide diffuses into the leaf through the
stomata.
Key terms…
Dicotyledonous plants – flowering plants which
have two cotyledons (embryonic leaves) in the
seed.
Their leaves have branched veins and their
leaves and stems contain vascular bundles of
xylem and phloem arranged in circles.
Examples: roses, sunflowers, peas
Let’s examine a leaf
Use transparent nail varnish to paint a 1cm2
square on the upper and lower surface of a leaf.
When dry, use sellotape to remove the nail
varnish and stick each piece of tape to a slide.
Examine under the microscope and draw a
diagram for each one, describing any differences
you see.
View of the epidermis of a cactus
Stoma
Epidermal cells
Guard
cells
Structure of a leaf
Label the diagrams on your sheet.
Try to add the function of each
tissue in the leaf.
Label your own
The gas exchange surface
• Surfaces of mesophyll cells in contact with air
spaces in the leaf.
• Large surface area - cells in contact with air
spaces.
• Short diffusion distance - leaf is thin and
gases only need to diffuse through one cell
membrane and wall.
Problem!
What substance could be represented by the arrows?
The problem with plants…
• Losing water by evaporation via the gas
exchange surface.
• Water evaporates from the wet cell walls in
contact with air spaces in the leaf and it is lost
by transpiration.
FYI: Transpiration
Transpiration
– evaporation
of water from
the leaves.
Transpiration
stream –
movement of
water
through the
plant.
Reducing Water Loss
What factors might affect the rate of
transpiration?
•
•
•
•
Humidity
Temperature
Wind
Stomatal opening/closing
Stomata
• Carbon dioxide and oxygen diffuse in and out
of leaves through small openings called
stomata.
How stomata open and close
• Most stomata are underneath the leaf
• A stoma is surrounded by 2 guard cells
• Guard cells open and close stomata
– this controls the diffusion of gases and water
vapour.
A side note…
Guard Cells
• Have chloroplasts
• Cell walls are especially rigid.
• When cells absorb water it expands and
becomes turgid.
• Their rigid inner wall resists expansion and
they become more curved.
• When they lose water and become flaccid and
collapse closing the stomata.
Adaptations for Dry Conditions
Xerophytes are plants that
can live in dry conditions
without losing too much
water.
They are adapted to take up
as much water as possible
when it is available, and to
reduce the loss of water
through transpiration.
Conserving water
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© Boardworks Ltd 2008
Xerophytes
Marram Grass
Thick cuticle
with no
stomata on
upper surface
– why?
Also:
• Rolled
leaves
• Hairy
• Sunken
stomata
These
features
trap a layer
of humid
air near the
stomata.
Cactus
• Stem stores water
(succulent)
• Spines reduce SA, protect
against predators, trap
humid air near stomata
• Long roots spread both
deep (to take in water
from deep underground)
and wide (to take in water
from rain before it soaks
far into the ground).
Typical Xerophytic Features
Feature
Thick waxy cuticle
Small leaves/spines
Reason
Reduces evaporation from
epidermis
Smaller surface area for
evaporation
Few stomata
Stomata sunk in pits in
epidermis
Less transpiration
Layer of humid air trapped near
stomata to reduce transpiration
Hairs around stomata or over
whole leaf surface
Layer of humid air trapped near
stomata to reduce transpiration
Exam practice…
Leafy Game
• Look at the simplified diagram of a cross section
through a leaf
• You’ll then see a picture with something missing
see if you can spot what it is.
First one to shout the correct word
gets to leave!
Whole diagram
• Air Spaces
• No stomata
• Waxy Cuticle
• No palisade mesophyll