Saturday 21 January 2012
STOMATA
In botany, a stoma (also stomate ; plural stomata ) is a pore, found in the leaf and
stem epidermis that is used
for gas exchange . The pore is bordered by a pair of
specialized parenchyma cells known as guard cells that are responsible for regulating the
size of the opening. The term
stoma is also used collectively
to refer to an entire stomatal
complex, both the pore itself
and its accompanying guard cells.[1] Air containing carbon dioxide and oxygen enters the plant through these
openings where it is used in photosynthesis and respiration, respectively. Oxygen produced by photosynthesis in the spongy layer cells (parenchyma cells with pectin) of the leaf
interior exits through these
same openings. Also, water vapor is released into the atmosphere through these
pores in a process called transpiration. Stomata are present in the sporophyte generation of all land plant groups except liverworts . Dicotyledons usually have more stomata on
the lower epidermis than the upper epidermis. Monocotyledons , on the other hand, usually have the same
number of stomata on the
two epidermes. In plants with
floating leaves, stomata may
be found only on the upper
epidermis; submerged leaves may lack stomata entirely. The word stoma derives from Greek στόμα , "mouth".[2] Function Carbon gain and water
loss Carbon dioxide, a key reactant
in photosynthesis, is present
in the atmosphere at a
concentration of about 390
ppm (as of December 2011).
Most plants require the stomata to be open during
daytime. The problem is that
the air spaces in the leaf are
saturated with water vapour,
which exits the leaf through
the stomata (this is known as transpiration). Therefore, plants cannot gain carbon
dioxide without
simultaneously losing water vapour.[3] Alternative approaches Ordinarily, carbon dioxide is
fixed to ribulose-1,5- bisphosphate (RuBP) by the enzyme RuBisCO in mesophyll cells exposed directly to the
air spaces inside the leaf. This
exacerbates the transpiration
problem for two reasons:
first, RuBisCo has a relatively
low affinity for carbon dioxide, and second, it fixes
oxygen to RuBP, wasting
energy and carbon in a process
called photorespiration. For both of these reasons, RuBisCo
needs high carbon dioxide
concentrations, which means
wide stomatal apertures and,
as a consequence, high water
loss. Narrower stomatal apertures
can be used in conjunction
with an intermediary
molecule with a high carbon
dioxide affinity, PEPcase
(Phosphoenolpyruvate carboxylase ). Retrieving the products of carbon fixation
from PEPCase is in an energy-
intensive process, however.
As a result, the PEPCase
alternative is preferable only
where water is limiting but light is plentiful, or where
high temperatures increase
the solubility of oxygen
relative to that of carbon
dioxide, magnifying RuBisCo's
oxygenation problem. CAM plants A group of mostly desert
plants called "CAM" plants
(Crassulacean acid metabolism, after the family Crassulaceae,
which includes the species in
which the CAM process was
first discovered) open their
stomata at night (when
water evaporates more slowly from leaves for a
given degree of stomatal
opening), use PEPcarboxylase
to fix carbon dioxide and
store the products in large
vacuoles. The following day, they close their stomata and
release the carbon dioxide
fixed the previous night into
the presence of RuBisCO. This saturates RuBisCO with carbon
dioxide, allowing minimal
photorespiration. This
approach, however, is
severely limited by the
capacity to store fixed carbon in the vacuoles, so it is
preferable only when water is
severely limiting. Opening and closure For more details on this topic,
see Guard cell. Confocal microscopy image of an Arabidopsis thaliana stoma showing two guard cells exhibiting fluorescence from green fluorescent protein and native chlorophyll (red) However, most plants do not
have the aforementioned
facility and must therefore
open and close their stomata
during the daytime in
response to changing conditions, such as light
intensity, humidity, and
carbon dioxide concentration.
It is not entirely certain how
these responses work.
However, the basic mechanism involves
regulation of osmotic
pressure. When conditions are
conducive to stomatal
opening (e.g., high light
intensity and high humidity),
a proton pump drives protons (H+) from the guard cells. This means that the cells' electrical potential becomes increasingly negative. The negative
potential opens potassium
voltage-gated channels and so
an uptake of potassium ions (K+) occurs. To maintain this internal negative voltage so
that entry of potassium ions
does not stop, negative ions
balance the influx of
potassium. In some cases,
chloride ions enter, while in other plants the organic ion
malate is produced in guard
cells. This increase in solute
concentration lowers the water potential inside the cell, which results in the diffusion
of water into the cell through osmosis. This increases the cell's volume and turgor pressure. Then, because of rings of cellulose microfibrils that prevent the width of the
guard cells from swelling, and
thus only allow the extra
turgor pressure to elongate
the guard cells, whose ends
are held firmly in place by surrounding epidermal cells, the two guard cells lengthen
by bowing apart from one
another, creating an open pore
through which gas can move. [4] When the roots begin to sense
a water shortage in the soil, abscisic acid (ABA) is released. [5] ABA binds to receptor proteins in the guard cells'
plasma membrane and
cytosol, which first raises the
pH of the cytosol of the cells and cause the concentration of free Ca2+ to increase in the cytosol due to influx from
outside the cell and release of Ca2+ from internal stores such as the endoplasmic reticulum and vacuoles. [6] This causes the chloride (Cl-) and inorganic ions to exit the cells. Second,
this stops the uptake of any further K+ into the cells and, subsequently, the loss of K +. The loss of these solutes causes
an increase in water potential , which results in the diffusion
of water back out of the cell
by osmosis. This makes the cell flaccid, which results in the closing of the stomatal pores. It is interesting to note that
guard cells have more
chloroplasts than the other
epidermal cells from which
guard cells are derived. Their function is controversial. [7][8] Inferring stomatal
behavior from gas
exchange The degree of stomatal
resistance can be determined
by measuring leaf gas
exchange of a leaf. The transpiration rate is dependent on the diffusion resistance provided by the
stomatal pores, and also on
the humidity gradient between the leaf's internal air
spaces and the outside air.
Stomatal resistance (or its
inverse, stomatal
conductance) can therefore be
calculated from the transpiration rate and
humidity gradient. This allows
scientists to investigate how
stomata respond to changes in
environmental conditions,
such as light intensity and concentrations of gases such
as water vapor, carbon
dioxide, and ozone. Evaporation (E) can be calculated as;[9] E = (ei - ea) / P r where ei and ea = partial
pressures of water in the leaf
and in the ambient air; P =
atmospheric pressure; and r =
stomatal resistance. The
inverse of r is conductance to water vapor (g), so the
equation can be rearranged to; [9] E = (ei - ea) g / P and solved for g; [9] g = EP / (e i - ea) The rate of evaporation from
a leaf can be determined using
a photosynthesis system . These scientific instruments
measure the amount of water
vapour leaving the leaf and
the vapor pressure of the
ambient air. Photosynthetic
systems may calculate water use efficiency (A/E), stomatal conductance (gs), intrinsic water use efficiency (A/gs), and sub-stomatal CO2 concentration (Ci).[3] These
scientific instruments are
commonly used by plant
physiologists to measure CO 2 uptake and thus measure photosynthetic rate. [10] Evolution The fossil record has little to
say about the evolution of stomata.[11] They may have evolved by the modification
of conceptacles from plants' alga-like ancestors. [12] It is clear, however, that the
evolution of stomata must
have happened at the same
time as the waxy cuticle was evolving - these two traits
together constituted a major
advantage for primitive
terrestrial plants.
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