gas exchange in leaves aqa book p182-3. homework green and red q p.179 green and red p.183
TRANSCRIPT
Objectives
• How do plants exchange gases?• What is the structure of
dicotyledonous plant leaf?• How is the leaf adapted for efficient
gas exchange
Cell structure of a leafThe palisade cells are in theuppermost layers of the leaf
epidermis
palisade cell ( photosynthesis)
vessel (carries water)
stoma (admits air)
What is the structure of dicotyledonous plant leaf and how do plants exchange
gases?1. Label your diagram with colour to
show the passage of gases from page 183
2. By which process does carbon dioxide move into a leaf?
3. List the features by which a leaf is adapted for efficient gas exchange.
4. The leaves are sometimes called the lungs of a plant. Give some reasons why this is true/false
Leaves like lungs?
• Yes• Site of gas exchange• Leaf flat, air spaces between spongy cells
so large surface area for gas exchange• Permeable so moist
• No• Leaves do not ventilate• No blood supply • Concentration gradient not maintained in
the same way
Stomata
• Minute pores usually on lower epidermis.
• Why needed?• Why on lower leaf?• Name a plant where they are on the
upper epidermis…
Why are stomata on the
underside of leaves?• Less solar
radiation• Less wind• Therefore
less water loss
How do guard cells work?
• When the cell is turgid the uneven cellulose cell wall causes it to become curved so it opens the pore
• When the cell is flaccid the cell is not curved and the pore closes • Draw fig3 a and b page 183• Add to top - they close in dark/wilting and cells will be flaccid• Add to bottom – open in light for gas exchange cells turgid
How do guard cells work?
Light causes guard cell
chloroplasts to photosynthesise
Photosynthesis produces glucose
Lowers water potential so
water moves in by osmosis
Guard cells become turgid so opens stoma
This is not the whole story…
Gas Exchange in The Leaf
The spongy mesophyll layer of the leaf, with its large air spaces and
thin-walled cells is the principal gas exchange surface within the leaf
The spongy mesophyll layer is in close contact with numerous pores or stomata, across which
gases enter and leave the leaf along steep concentration gradients
airspace
spongy mesophylllayer
waxy cuticleupper epidermis
palisade mesophylllayer
lower epidermis
guard cells
stoma
xylem vessel
Leaf blades display a large surface area and are very thin; the depth of the leaf is extremely small, providing a system that offers short diffusion paths for the
exchange of gases; leaves are held outwards from the parent plant
allowing for air movements to maintain the gaseous diffusion gradients between
the inside and outside of the leaves
The cells of the spongy mesophyll layer are loosely packed, creating
numerous air spaces that provide a large surface area for gas exchange
Gases dissolve in the film of water that covers the
saturated walls of the mesophyll cells, and are exchanged between the
cells and the intercellular air spaces
Gases diffuse into and out of the intercellular
spaces through numerous tiny pores
called stomata
Oxygen and carbon dioxide diffuse into and out of the leaf
through the stomata along their concentration gradients
The concentration gradients for these gases is dependent upon their concentration within the
intercellular spaces of the mesophyll layer which, in turn, is dependent on the rates of cellular
respiration and photosynthesis
Gas exchange at night is concerned only with the process of respiration
As the mesophyll cells respire, oxygen is taken up into the cells
from the intercellular spaces, and carbon dioxide is released
Oxygen concentrations, within the intercellular spaces,
fall below that of the air surrounding the leaf thereby
creating a concentration gradient that maintains a
continual flow of oxygen into the leaf throughout the night
Carbon dioxide concentrations, within the intercellular spaces,
increase as respiration proceeds; carbon dioxide
diffuses out of the leaf along its concentration gradient
Gas exchange during the day is concerned with both photosynthesis and respiration
Under ideal conditions, the rate of photosynthesis exceeds the rate of respiration for most of
the daylight hoursCarbon dioxide gas, produced by the respiring cells, is used in the process of photosynthesis,
and the oxygen produced during photosynthesis is used in the process of respiration
As photosynthetic rates generally exceed respiratory rates, then the
demand for carbon dioxide gas by photosynthesising cells is greater than that supplied by respiration;
carbon dioxide levels in the intercellular spaces is lower than
that in the mesophyll cells and the gas diffuses into the cells
Gas exchange during the day is concerned with both photosynthesis and respiration
The continuous removal of carbon dioxide, from the
intercellular spaces into the photosynthesising cells,
maintains a concentration gradient that results in the
diffusion of this gas from the surrounding air into the leaf
In a similar way, the oxygen generated by photosynthesis
exceeds the respiratory demand of the mesophyll cells; the
oxygen concentration in the intercellular spaces exceeds
that of the surrounding air, and oxygen diffuses out of the leaf
along its concentration gradient
• Large surface area to volume ratio of the leaf blades, together with the large surface area provided by the numerous air spaces within the mesophyll layer, maximise diffusion rates
• Steep concentration gradients for oxygen and carbon dioxide are maintained by the diffusion of these gases into and out of the mesophyll cells as photosynthesis and respiration proceed; the large numbers of stomata provide a system that links the internal air spaces with the external atmosphere, allowing for gas exchange with the environment
• Leaves are held out into the atmosphere by their supporting stalks, allowing for air movements to increase diffusion gradients across the stomata
• The short diffusion path is the result of the thin, permeable cell walls of the spongy mesophyll cells together with the short depth of the mesophyll layer in contact with the external environment