Download Leaf structure and gas exchange

Leaf Structure and Gas
exchange in plants
ORGANS
of
Flowering
plants
•
•
•
•
•
Leaves
Stems
Roots
Flowers
Fruits
Various Inputs and Outputs
Water and
mineral ions
Some need to be moved around the plant
PLANT REQUIREMENTS
• GASES:
– CARBON DIOXIDE: for photosynthesis
– OXYGEN: for respiration
• WATER:
– As a reactant in photosynthesis
– For support(turgor pressure)
• MINERAL IONS
– For making compounds required by the plant
e.g amino acids, chlorophyll
Getting what it needs for
photosynthesis
• Most photosynthesis occurs in the leaves
(see next slide for labels)
Leaf Cross Section:
Anatomy of a leaf
cuticle
Upper
epidermis
palisade
mesophyll
Vascular
bundle
Xylem
Phloem
spongy
mesophyll
Air space
Lower
epidermis
Guard
cells
cuticle
stoma
During the day
Rate of photosynthesis > rate of respiration
• CO2 is being used up in Photosynthesis by
mesophyll cells, lowering its concentration here
• CO2 diffuses from environment via open
stomata into airspace above the stoma then
into mesophyll cells
• Movement is PASSSIVE; along a CO2
concentration gradient
During the day
Rate of photosynthesis > rate of respiration
• MORE O2 is being produced in Photosynthesis
than is used in respiration
• O2 concentration in airspace in leaf increases
• O2 diffuses from the airspace above the stoma,
through the open stomata into the
environment
• Movement is PASSSIVE; along a O2
concentration gradient
During the day
• Water is constantly used in photosynthesis
• Water is constantly evaporating from moist cell
walls of leaf cells(transpiration)
• This creates a high water vapour concentration
in the airspaces in the leaf
• Water (as a gas) diffuses out of the leaf via
open stomata along a water concentration
gradient
• The rate of diffusion depends of the size of the
gradient.
Day time exchange from the leaf of a plant
Xylem
cuticle
Upper
epidermis
Palisade
cells
chloroplasts
Spongy
cells
stoma
lower
epidermis
stoma
Guard
cell
phloem
Liquid water
Water vapour
Carbon dioxide
oxygen
Stomata
• Opening in the epidermis
• Mainly on lower surface of leaf
Stomata
• Formed by two guard cells
• Guard cell contain chloroplast
• Guard cells are sausage shaped
Guard Cells are Special
• They have chloroplast(unlike the other
cells in the epidermis)
• They have a cell wall that is thicker on the
side adjacent to the pore than on the
other side
• The have radially orientated microfibrils
that limit expansion of these cells in girth
• Two guard cells are anchored to each
other at their end
Because of their structure when
guard cells fill with water and
become turgid they bend into a
sausage shape and a pore
forms between them
Opening and Closing of Stoma
1. Solute uptake(mostly K+)
into guard cells:
1. Solute loss from guard
cells (mostly K+):
2. Water follows by
osmosis
2. Water follows by
osmosis
3.Guard cells expand,
become turgid and bend
3. Guard cells become
flaccid and change shape
5. Stoma OPENS
4 Stoma CLOSES
H2 O
H2 O
H2 O
H2 O
K+
H2 O
H2 O
H2 O
H2 O
H2 O
H2 O
Stomata
• Usually open in morning
• Usually close at night
• Also close to limit water loss
Guard cells: located in the epidermis
Some plant species have
adaptation to arid conditions
• Thick waxy cuticle: reduces evaporation
of water across leaf surface
• Sunken stomata(stomata in pits) or
“hairy” leaves: Both strategies trap water
vapour close to the leaf surface and
reduce water vapour gradient
• Inverted stomatal rhythm: stomata that
open at night and close during the day, to
reduce evaporative water loss in the
hottest parts of the day
Some plant species have
adaptation to arid conditions
• Thick Succulent leaves that can store
water; also have a relatively small surface
area over which transpiration can occur
Agave Leaf Cross Section
Saguaro Cactus-
Gas Exchange in non woody stems
• Diffusion from environment via stomata
Gas Exchange in Woody Stems
• Gas exchange occurs through Lenticels
• Lenticels are spongy areas in corky
surface of woody stems that allow gas
exchange
Gas exchange in Roots
• Oxygen diffuses from air spaces in the soil
into root hair cells and root cells
• Carbon dioxide diffuses out in the opposite
direction