STPM Biology Chapter 7
Content
THE STOMATA
Leaves
The exchange of oxygen and carbon dioxide in the leaf occurs
through pores called stomata (singular = stoma).
Normally stomata open when the light strikes the leaf in the
morning and close during the night.
The immediate cause is a change in the turgor of the guard cells.
The inner wall of each guard cell is thick and elastic. When turgor
develops within the two guard cells flanking each stoma, the thin
outer walls bulge out and force the inner walls into a crescent
shape. This opens the stoma. When the guard cells lose turgor,
the elastic inner walls regain their original shape and the stoma
closes.
The table shows the osmotic pressure measured at different times
of day in typical guard cells. The osmotic pressure within the other
cells of the lower epidermis remained constant at 150 lb/in2. When
the osmotic pressure of the guard cells became greater than that
of the surrounding cells, the stomata opened. In the evening, when
The immediate cause is a change in the turgor of the guard cells. The inner
wall of each guard cell is thick and elastic. When turgor develops within the
two guard cells flanking each stoma, the thin outer walls bulge out and force
the inner walls into a crescent shape. This opens the stoma. When the guard
cells lose turgor, the elastic inner walls regain their original shape and the
stoma closes.
The table shows the osmotic pressure measured at different times of day in
typical guard cells. The osmotic pressure within the other cells of the lower
epidermis remained constant at 150 lb/in2. When the osmotic pressure of the
guard cells became greater than that of the surrounding cells, the stomata
opened. In the evening, when the osmotic pressure of the guard cells
dropped to nearly that of the surrounding cells, the stomata closed.
STOMATA ;
Is a pore on the epidermis of the leaves and
stems of plants which can open and close
FUNCTIONS :
1. Enable gases exchange between leave cells
and atmospheric
2. Regulate and control water loss from plants
THE STRUCTURE OF STOMATA
STOMATA
The stomata,
Is a tiny openings
in the epidermis of
a plant leaf,
are surrounded
by a pairs of guard
cells.
The guard cells
open and close the
stoma by changing
shape
THE STRUCTURE OF STOMATA
THE STRUCTURE OF A STOMA TA:
( Stoma are found on epidermal layer of leaves, stems and part of flowers of flowering
plant )
Stomata is bounded by a pair of guard cells which are
kidney shape in dicot and dumbell-shaped in monocot
leaves.
Guard cells (gc) is a specialised epidermal cell which
contain chloroplasts for photosynthesis.
The cellulose cell walls of gc that surround the stoma
(inner cell wall), are thicker and less elastic than those in
contact with epidermis ( outer cell wall).
The different elasticity of the cellulose cell wall is
contributed by the microfibril cellulose that surround
the wall of the cell.
The two guard cells are fused at their end allowing
the cell to bend as they expand.
The diff. causes the outer wall elongates more
than the inner wall when the cell is in turgor
pressure.
The presence of chloroplast enables the gc to
photosynthesise,
For gaseous exchange
-During the day, CO2 diffuse into the leaves for
photosinthesis and O2 produces from
photusinthesis diffuses out.
-During night, O2 diffuse into the leaves for
respiration and CO2 diffuse out.
Stomata regulate the rate of photosynthesis based
on large number of internal and external signals.
Allow transpiration to cooling leaves
-During transpiration, water vapour diffuse out
through stomata due to water vapour
concentration gradient.(water vapour
concentration higher in spaces of leaves and
lower in atmosphere)
-Diffusion of water vapour create a transpiration pull
in the xylem so that water in transported up to the
leaves for photosynthesis.
Reduce exessive loss of water when pore close
-Done physically as when water is lost from the
guard cells, the cells become flaccid.The stoma
is automatically closed
There are two hypothesis
1. The starch-sugar hypothesis
2. The potassium ion accumulation hypothesis
STARCH –SUGAR HYPOTHESIS
At a day time,
the guard cell undergoes photosynthesis due to
the presence of chloroplast in the gc
sugars are formed
the accumulation of sugar, increase the
osmotic concentration of gc, water potential in gc
becomes lower than the water potential in
neighbouring epidemal cells
Difference in water potential causes the
movement of water molecules from epidermal cells
into guard cells by osmosis.
The entrance of water, causes the gc to
become swell ,and turgid
Turgor pressure increases, the thinner
outer cellulose cell walls elongate more
than the inner thicker walls
The gc bend outward and stoma opens.
At night,
Guard cells do not go through photosynthesis
There is no sugar produced in guard cells , sugar
converted to starch, reduces osmotic
concentration in guard cells
Water potential in guard cells become higher
than the w.p in epidermis cells
Water potential in guard cells become higher than
the w.p in epidermis cells
Water molecule diffuses out of gc into
neighbouring epidermis cells
Gc becomes flaccid because of no turgor
pressure.
Closing the stoma
POTASSIUM IONS ( K+) ACCUMULATION
HYPOTHESIS
( more recent hypothesis)
Potassium ions from neighbouring epidermis cells
actively transported to the guard cells
Energy needed is supplied by ATP generates
during phosphorylation of the light reaction of
photosynthesis
ATP ( from photophosphorylation) is use to drive
the proton-pump in the cell membrane of gc,
activate by the light
……..POTASSIUM IONS ( K+) ACCUMULATION HYPOTHESIS
( more recent hypothesis)
As protons (H+) are pumped out of the gc , the inner
gc becomes more negatively charged. This activate
potassium channels to pumped in potassium ions, K+
into gc from epidermis cells. Cl– ions then enter to
balance the charge
The accumulation of K+/ ( malate ions in some
plants ) causes the water potential of gc more negative
than that in epidermis cells.
Water from epidermis cells enters by osmosis into
the guard cells
The gc becomes turgid ,
Turgor pressure increased , thinner outer
cellulose cell walls expand more than the
inner wall
Guard cells bend outward and the stoma
opens
Proton pump
Potassium channel
K
H+
K+ channels activated, K+
ions pumped in
H+ ion
H+ pumped out of the guard cells,
lowest the electrochemical gradient
Stoma opens
Leaves
The exchange of oxygen and carbon dioxide in the leaf occurs
through pores called stomata (singular = stoma).
Normally stomata open when the light strikes the leaf in the
morning and close during the night.
The immediate cause is a change in the turgor of the guard cells.
The inner wall of each guard cell is thick and elastic. When turgor
develops within the two guard cells flanking each stoma, the thin
outer walls bulge out and force the inner walls into a crescent
shape. This opens the stoma. When the guard cells lose turgor,
the elastic inner walls regain their original shape and the stoma
closes.
The table shows the osmotic pressure measured at different times
of day in typical guard cells. The osmotic pressure within the other
cells of the lower epidermis remained constant at 150 lb/in2. When
the osmotic pressure of the guard cells became greater than that
of the surrounding cells, the stomata opened. In the evening, when
The immediate cause is a change in the turgor of the guard cells. The inner
wall of each guard cell is thick and elastic. When turgor develops within the
two guard cells flanking each stoma, the thin outer walls bulge out and force
the inner walls into a crescent shape. This opens the stoma. When the guard
cells lose turgor, the elastic inner walls regain their original shape and the
stoma closes.
The table shows the osmotic pressure measured at different times of day in
typical guard cells. The osmotic pressure within the other cells of the lower
epidermis remained constant at 150 lb/in2. When the osmotic pressure of the
guard cells became greater than that of the surrounding cells, the stomata
opened. In the evening, when the osmotic pressure of the guard cells
dropped to nearly that of the surrounding cells, the stomata closed.
STOMATA ;
Is a pore on the epidermis of the leaves and
stems of plants which can open and close
FUNCTIONS :
1. Enable gases exchange between leave cells
and atmospheric
2. Regulate and control water loss from plants
THE STRUCTURE OF STOMATA
STOMATA
The stomata,
Is a tiny openings
in the epidermis of
a plant leaf,
are surrounded
by a pairs of guard
cells.
The guard cells
open and close the
stoma by changing
shape
THE STRUCTURE OF STOMATA
THE STRUCTURE OF A STOMA TA:
( Stoma are found on epidermal layer of leaves, stems and part of flowers of flowering
plant )
Stomata is bounded by a pair of guard cells which are
kidney shape in dicot and dumbell-shaped in monocot
leaves.
Guard cells (gc) is a specialised epidermal cell which
contain chloroplasts for photosynthesis.
The cellulose cell walls of gc that surround the stoma
(inner cell wall), are thicker and less elastic than those in
contact with epidermis ( outer cell wall).
The different elasticity of the cellulose cell wall is
contributed by the microfibril cellulose that surround
the wall of the cell.
The two guard cells are fused at their end allowing
the cell to bend as they expand.
The diff. causes the outer wall elongates more
than the inner wall when the cell is in turgor
pressure.
The presence of chloroplast enables the gc to
photosynthesise,
For gaseous exchange
-During the day, CO2 diffuse into the leaves for
photosinthesis and O2 produces from
photusinthesis diffuses out.
-During night, O2 diffuse into the leaves for
respiration and CO2 diffuse out.
Stomata regulate the rate of photosynthesis based
on large number of internal and external signals.
Allow transpiration to cooling leaves
-During transpiration, water vapour diffuse out
through stomata due to water vapour
concentration gradient.(water vapour
concentration higher in spaces of leaves and
lower in atmosphere)
-Diffusion of water vapour create a transpiration pull
in the xylem so that water in transported up to the
leaves for photosynthesis.
Reduce exessive loss of water when pore close
-Done physically as when water is lost from the
guard cells, the cells become flaccid.The stoma
is automatically closed
There are two hypothesis
1. The starch-sugar hypothesis
2. The potassium ion accumulation hypothesis
STARCH –SUGAR HYPOTHESIS
At a day time,
the guard cell undergoes photosynthesis due to
the presence of chloroplast in the gc
sugars are formed
the accumulation of sugar, increase the
osmotic concentration of gc, water potential in gc
becomes lower than the water potential in
neighbouring epidemal cells
Difference in water potential causes the
movement of water molecules from epidermal cells
into guard cells by osmosis.
The entrance of water, causes the gc to
become swell ,and turgid
Turgor pressure increases, the thinner
outer cellulose cell walls elongate more
than the inner thicker walls
The gc bend outward and stoma opens.
At night,
Guard cells do not go through photosynthesis
There is no sugar produced in guard cells , sugar
converted to starch, reduces osmotic
concentration in guard cells
Water potential in guard cells become higher
than the w.p in epidermis cells
Water potential in guard cells become higher than
the w.p in epidermis cells
Water molecule diffuses out of gc into
neighbouring epidermis cells
Gc becomes flaccid because of no turgor
pressure.
Closing the stoma
POTASSIUM IONS ( K+) ACCUMULATION
HYPOTHESIS
( more recent hypothesis)
Potassium ions from neighbouring epidermis cells
actively transported to the guard cells
Energy needed is supplied by ATP generates
during phosphorylation of the light reaction of
photosynthesis
ATP ( from photophosphorylation) is use to drive
the proton-pump in the cell membrane of gc,
activate by the light
……..POTASSIUM IONS ( K+) ACCUMULATION HYPOTHESIS
( more recent hypothesis)
As protons (H+) are pumped out of the gc , the inner
gc becomes more negatively charged. This activate
potassium channels to pumped in potassium ions, K+
into gc from epidermis cells. Cl– ions then enter to
balance the charge
The accumulation of K+/ ( malate ions in some
plants ) causes the water potential of gc more negative
than that in epidermis cells.
Water from epidermis cells enters by osmosis into
the guard cells
The gc becomes turgid ,
Turgor pressure increased , thinner outer
cellulose cell walls expand more than the
inner wall
Guard cells bend outward and the stoma
opens
Proton pump
Potassium channel
K
H+
K+ channels activated, K+
ions pumped in
H+ ion
H+ pumped out of the guard cells,
lowest the electrochemical gradient
Stoma opens
