Stoma of a leaf under a microscope.

In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the underside of a plant leaf, and used for gas exchange. The pore is formed by a pair of specialized parenchyma cells known as guard cells which are responsible for regulating the size of the opening. Air containing carbon dioxide enters the plant through these openings where it gets used in photosynthesis and respiration. Oxygen produced by photosynthesis in the chlorenchyma cells (parenchyma cells with chloroplasts) 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. Image File history File linksMetadata Size of this preview: 800 × 600 pixelsFull resolution (1600 × 1200 pixel, file size: 589 KB, MIME type: image/jpeg) Stomata Photo credit: Peter Halasz. ... Image File history File linksMetadata Size of this preview: 800 × 600 pixelsFull resolution (1600 × 1200 pixel, file size: 589 KB, MIME type: image/jpeg) Stomata Photo credit: Peter Halasz. ... Pinguicula grandiflora Botany is the scientific study of plantlife. ... Divisions Green algae Chlorophyta Charophyta Land plants (embryophytes) Non-vascular plants (bryophytes) Marchantiophyta—liverworts Anthocerotophyta—hornworts Bryophyta—mosses Vascular plants (tracheophytes) †Rhyniophyta—rhyniophytes †Zosterophyllophyta—zosterophylls Lycopodiophyta—clubmosses †Trimerophytophyta—trimerophytes Pteridophyta—ferns and horsetails Seed plants (spermatophytes) †Pteridospermatophyta—seed ferns Pinophyta—conifers Cycadophyta—cycads Ginkgophyta—ginkgo Gnetophyta—gnetae Magnoliophyta—flowering plants... “Foliage” redirects here. ... This article or section does not cite its references or sources. ... Parenchyma is a term used to describe a bulk of a substance. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... The leaf is the primary site of photosynthesis in plants. ... Cellular respiration is a process that describes the metabolic reactions and process that take place in a cell to obtain biochemical energy from fuel molecules. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Chlorenchyma cells in plant anatomy are parenchyma cells that contain chloroplasts. ... It has been suggested that multiple sections of steam be merged into this article or section. ... Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. ...

Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. As these leaves are held horizontally, upper epidermis is directly illuminated. Locating fewer stomata on the upper epidermis can then prevent excess water loss. Orders see text Dicotyledons or dicots are flowering plants whose seed contains two embryonic leaves or cotyledons. ... The epidermis is the outer multi-layered group of cells covering the leaf and young tissues of a plant. ...

Monocotyledons are different. Because their leaves are held vertically, they will have the same number of stomata on the two epidermes. Orders Base Monocots: Acorus Alismatales Asparagales Dioscoreales Liliales Pandanales Family Petrosaviaceae Commelinids: Arecales Commelinales Poales Zingiberales Family Dasypogonaceae Monocotyledons or monocots are a group of flowering plants usually ranked as a class and once called the Monocotyledoneae. ...

If the plant has floating leaves, there will be no stomata on the lower epidermis and they absorb gases directly from water through the cuticle. If it is a submerged leaf, no stomata will be present on either side. Plant cuticles are a protective waxy covering produced only by the epidermal cells (Kolattukudy, 1996) of leaves, young shoots and all other aerial plant organs. ...

Stoma in Greek (στόμα) means "mouth".

Carbon gain and water loss

As the key reactant in photosynthesis carbon dioxide, is found in the atmosphere, most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapor, which exits the leaf through the stomata (this is known as transpiration). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapor. Transpiration is the evaporation of water into the atmosphere from the leaves and stems of plants. ...

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 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 photorespiration. For both of these reasons, Rubisco needs high carbon dioxide concentrations, which means high stomatal apertures and consequently high water loss. Categories: Stub | Photosynthesis ... Categories: Stub | Photosynthesis ... Ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO, is an enzyme (EC 4. ... Photorespiration refers to the alternate pathway for production of Glyceraldehyde 3-phosphate by Rubisco, the main enzyme of the Dark reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). ...

However, plants possess another enzyme that can also fix carbon dioxide: PEP carboxylase or PEPCase. 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. Phosphoenolpyruvate carboxylase (or PEPCase) is an enzyme in the family of carboxy-lyases. ...

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. Crassulacean Acid Metabolism (CAM) is a carbon fixation pathway in some photosynthetic plants. ...

Opening and closure

However, most plants do not have the above-said 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, and so an uptake of potassium ions (K⁺) occurs. This in turn increases the osmotic pressure inside the cell, drawing in water through osmosis. This increases the cell's volume and turgor pressure. Then, because the wall of the guard cell facing the stomatal pore is less elastic (more rigid) than the wall on the opposite side of the cell, the two guard cells bow apart from one another, creating an open pore through which gas can move. proton gradient: Pink represents the matrix while the red dots represent protons. ... For alternative meanings see proton (disambiguation). ... Electrical potential is the potential energy per unit charge associated with a static (time-invariant) electric field, also called the electrostatic potential or the electric potential, typically measured in volts. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Standard atomic weight 39. ... This article or section does not adequately cite its references or sources. ... Osmosis is the net movement of a solvent across a semipermeable membrane from a region of high solvent potential to an area of low solvent potential down a concentration gradient. ... This article or section does not adequately cite its references or sources. ...

When the roots begin to sense a water shortage in the soil, abscisic acid (ABA) is released. ABA binds to certain receptors in the guard cells' plasma membranes, 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. 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. When the stoma become flaccid some water is lost to the environment. This loss of water is called transpiration and is essential in the water cycle. Abscisic Acid (ABA), also known as abscisin II and dormin, is a plant hormone. ...

Interestingly, the guard cells do have chloroplasts whereas other epidermal cells (from which guard cells are derived) do not. Their function is controversial.^[1]

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, humidity, or carbon dioxide concentration.

References

1. ^ Guard Cell Photosynthesis. Retrieved on 2007-04-29.