Stoma
From Freepedia
- This is not about surgically created bowel openings; see stoma (medicine)
In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells 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.
Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. As these leaves are held horizontally, upper epidermis is directly illuminated. Less number of stomata on the upper epidermis can then prevent water loss.
Monocotyledons are different. For their leaves are held vertically, they will have the same number of stomata on the two epidermis.
If the plant has floating leaves, there will be no stomata on the lower epidermis as it can absorb gases directly from water through cuticle. If it is submerged leaf, no stomata will be present on both sides of it.
Stoma in Greek means "mouth".
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Stomata control the trade-off between carbon gain and water loss
As the key reactant in photosynthesis, carbon dioxide, is found in the atmosphere, most plants require the stoma 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 without simultaneously losing water.
CAM plants open stomata at night
One solution is to open the stomata to permit gas exchange only at night, when the leaf is cool and water vapor diffuses out of the leaf much more slowly than in the daytime. Carbon dioxide is stored overnight, and then converted to carbohydrates the following day using light energy. These plants are known as CAM plants. Although CAM plants lose very little water per unit of carbon gained, the absolute rate of carbon gain is severely limited because relatively little carbon dioxide can be stored overnight. As a result, CAM plants are fit for survival only if water is extremely scarce (for example, in the desert).
Stoma 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 cells' 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.
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 cytosols in the cells. This causes the chlorine (Cl-) and inorganic ions to exit the cells. Second, it causes the release of calcium ions (Ca2+) from the cells' vacuoles in to the cytosols, which blocks the uptake of any further K+ into the cells. The loss of these solutes causes a reduction in osmotic pressure, thus making the cell flaccid and so closing the stomatal pores.
Viewing Stoma
The easiest way to view stomata on a leaf is to take a nail varnish impression of it.
- Paint about one square centimeter of the underside of the leaf with transparent nail varnish.(or thin layer of PVA glue)
- Allow to dry out thoroughly (takes a good 30 minutes).
- Peel off and place on a microscope slide.
The stomata leave clearly visible impressions in the nail varnish. A graticule slide allows for the counting of how many stomata (per unit area) are on the leaf surface, a characteristic of physiological significance.
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.
