In
botanyBotany, plant science, phytology, or plant biology is a branch of biology and is the scientific study of plant life and development...
, a
stoma (also
stomate; plural
stomata) is a pore, found in the leaf and stem epidermis that is used for
gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
exchange. The pore is formed by a pair of specialized
parenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, from Greek parenkhuma, visceral flesh, from parenkhein, to pour in beside : para-, beside + en-, in + khein, to pour.- In animals :The parenchyma are the...
cells known as guard cells which are responsible for regulating the size of the opening. Air containing
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
and
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
enters the plant through these openings where it is used in
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
and
respirationCellular respiration is one of the key ways a cell gains useful energy. It is the set of the metabolic reactions and processes that take place in organisms' cells to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products...
, respectively.
OxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
produced by photosynthesis in the
spongy layerIn botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat and thin. There is continued debate about whether the flatness of leaves evolved to expose the chloroplasts to more light or to increase the absorption of carbon dioxide. In...
cells (parenchyma cells with pectin) of the leaf interior exits through these same openings. Also,
water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
is released into the atmosphere through these pores in a process called
transpirationTranspiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells. Collectively the structures are called stomata...
.
Stomata are present in the
sporophyteAll land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
generation of all land plant groups except
liverwortThe Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryopeos, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....
s. Dicotyledons usually have more stomata on the lower
epidermisThe epidermis is a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and ...
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
GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
στόμα 'mouth'.
Carbon gain and water loss
Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 384 ppm (as of March 2008). 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
transpirationTranspiration is the evaporation of water from the aerial parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stoma that are bordered by guard cells. Collectively the structures are called stomata...
). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.
Alternative approaches
Ordinarily, carbon dioxide is fixed to
ribulose-1,5-bisphosphateRibulose-1,5-bisphosphate is the molecule which carbon dioxide reacts with in photosynthetic carbon fixation. The enzyme ribulose bisphosphate carboxylase oxygenase catalyzes RuBP with carbon dioxide in order to synthesize a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol...
(BTAC) by the enzyme
RuBisCORibulose-1,5-bisphosphate carboxylase oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the...
in mesophyll cells exposed directly to the air spaces inside the leaf. This exacerbates the carbon/water tradeoff 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
photorespirationPhotorespiration is the process by which RuBP, has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen, approximately 3 carboxylations occur per oxygenation...
. For both of these reasons, Rubisco needs high carbon dioxide concentrations, which means high stomatal apertures and consequently high water loss.
However, plants possess another enzyme that can also fix carbon dioxide: PEP carboxylase or BTAC. This enzyme has high carbon dioxide affinity, so a given rate of carbon dioxide fixation can be achieved with less stomatal opening, and hence less water loss. The catch is that the products of carbon fixation by PEPCase must be converted in an energy-intensive process to continue through the carbon reactions of photosynthesis. As a result, the PEPCase alternative is only preferable where water is more limiting but light — which provides the energy in this case — is plentiful, and/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 metabolismCrassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some plants. These plants fix carbon dioxide during the night, storing it as the four carbon acid malate. The is released during the day, where it is concentrated around the enzyme...
, 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
RuBisCORibulose-1,5-bisphosphate carboxylase oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme that is used in the Calvin cycle to catalyze the first major step of carbon fixation, a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the...
. 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
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 pumpA proton pump is an integral membrane protein that is capable of moving protons across the membrane of a cell, mitochondrion, or other subcellular compartment.-Function:...
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
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
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 in turn increases the
osmotic pressureOsmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.Jacobus Henricus van 't Hoff first proposed a formula for calculating the osmotic pressure, but this was later improved upon by Harmon Northrop Morse.A related...
inside the cell, drawing in water through
osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
. This increases the cell's volume and
turgor pressureOsmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.Jacobus Henricus van 't Hoff first proposed a formula for calculating the osmotic pressure, but this was later improved upon by Harmon Northrop Morse.A related...
. 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
epidermalThe epidermis is a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and ...
cells, the two guard cells lengthen by bowing apart from one another, creating an open pore through which gas can move.
When the roots begin to sense a water shortage in the soil,
abscisic acidAbscisic acid , also known as abscisin II and dormin, is a plant hormone. It functions in many plant developmental processes, including bud dormancy.-Function:...
(ABA) is released. ABA binds to receptor proteins in the guard cells' plasma membrane and cytosol, which first raises the pH of the
cytosolThe cytosol or intracellular fluid is the liquid found inside cells. In eukaryotes this liquid is separated by cell membranes from the contents of the organelles suspended in the cytosol, such as the mitochondrial matrix inside the mitochondrion...
of the cells and cause the concentration of free Ca
2+ to increase in the cytosol due to influx from outside the cell and release of Ca
2+ from internal stores such as the endoplasmic reticulum and vacuoles. This causes the chloride (Cl
-) and inorganic ions to exit the cells. Secondly, this stops the uptake of any further K
+ into the cells and subsequentally the loss of K
+. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell flaccid and so closing the stomatal pores.
Interestingly, guard cells have more chloroplasts than the other epidermal cells from which guard cells are derived. Their function is controversial.
Inferring stomatal behavior from gas exchange
Another way to find out whether stomata are open or closed, or more accurately, how open they are, is by measuring leaf gas exchange. A leaf is enclosed in a sealed chamber and air is driven through the chamber. By measuring the concentrations of carbon dioxide and water vapor in the air before and after it flows through the chamber, one can calculate the rate of carbon gain (photosynthesis) and water loss (transpiration) by the leaf.
However, because water loss occurs by diffusion, the transpiration rate depends on two things: the gradient in humidity from the leaf's internal air spaces to the outside air, and the diffusion resistance provided by the stomatal pores. Stomatal resistance (or its inverse, stomatal conductance) can therefore be calculated from the transpiration rate and humidity gradient. (The humidity gradient is the humidity inside the leaf, determined from leaf temperature based on the assumption that the leaf's air spaces are saturated with vapor, minus the humidity of the ambient air, which is measured directly.) This allows scientists to learn how stomata respond to changes in environmental conditions, such as light intensity and concentrations of gases such as water vapor, carbon dioxide, and
ozone.
Evolution
The fossil record has little to say about the evolution of stomata. They may have evolved by the modification of conceptacles from plants' alga-like ancestors.
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.
Development
There are three major epidermal cell types which all ultimately derive from the L1 tissue layer of the shoot apical meristem, called protodermal cells:
trichomeTrichomes , are fine outgrowths or appendages on plants and certain protists. These are of diverse structure and function...
s, pavement cells and guard cells, all of which are arranged in a nonrandom fashion. An asymmetrical cell division occurs in protodermal cells resulting in one large cell that is fated to become a pavement cell and a smaller cell called a meristemoid that will eventually differentiate into the guard cells that surround a stoma. This meristemoid then divides boki tan asymmetrically one to three times before differentiating into a guard mother cell. The guard mother cell then makes one symmetrical division, which forms a pair of guard cells.
Stomata as pathogenic pathways
Stomata are an obvious hole in the leaf by which, as was presumed for a while, pathogens can enter unchallenged. However, it has been recently shown that stomata do in fact sense the presence of some, if not all, pathogens. However, with the virulent bacteria applied to
ArabidopsisArabidopsis thaliana , is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, Arabidopsis is popular as a model organism in plant biology and genetics...
plant leaves in the experiment, the bacteria released the chemical coronatine, which forced the stomata open again within a few hours.