The stomata They are microscopic openings or pores located on the surface of plant leaves through which gas exchange occurs between them and the environment that surrounds them. They can also be in the stems, fruits and inflorescences.
Stomata are found in the aerial portions of practically all terrestrial plants, including the plants considered more "ancestral" such as mosses and ferns and, of course, the more "current" plants represented by Gymnosperms and Angiosperms ( spermatophytes).
Stomata are very abundant on the leaves, preferentially on the abaxial surface (on the underside) and, in many species of trees, they are only found on that surface..
The position of the stomata is used as a taxonomic character to identify some families of plants. Those leaves with stomata on both sides of the leaf blade are known as amphistomatic, while those with only stomata on the lower epidermis are called hypostomatic..
Both the size and the frequency of appearance of the stomata is also highly variable, depending not only on the species, but also on the foliar position and the growing conditions. In the same species there may be marked differences related to the genetic component of each individual.
The opening and closing of each stoma in a leaf is a process that depends on different stimuli, both internal and external, and that plays a fundamental role in maintaining the homeostasis of the plant body.
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The main function of stomata has to do with their participation in the diffusive movement of gases such as oxygen (O2), carbon dioxide (CO2) or water vapor (H2Og) between the internal and external surfaces of plant tissues. , especially of the leaves and stems.
In this sense, we could say that stomata are "analogous" to the nose and mouth of human beings, which we use to breathe, directing the air towards the lung compartment so that gas exchange occurs with the blood..
Stomata also participate in the elimination of excess water in plant tissues, thus maintaining the water balance of plants.
These generally remain open during the day, allowing the entry of COtwo necessary for the photosynthetic process, and closed at night, avoiding the loss of water and other gases during carbon fixation.
When the water supply is limited or there are strong currents or blizzards, the stomata are kept closed, thus preventing desiccation or dehydration of the plants.
A stoma is made up of a cavity or pore called an ostiole, which is surrounded by two elongated cells called occlusive cells or guard cells, connected to each other at their ends and which can have different shapes depending on the type of plant considered..
In grasses, for example, the occlusive cells are shaped like a "dumbbell", while in most plants their shape is described as "kidney-shaped".
In addition to guard cells, stomata are associated with attached or accessory cells and the set of guard cells and attached cells is called the stomatal apparatus. Surrounding the stomatal apparatus are adjacent epidermal cells.
In many species the stomata have additional “protective” structures, in others the pore is partially “closed” by waxes, which increase the resistance of the pore to gas diffusion..
Occlusive cells or guard cells are characterized by having a cell wall composed of cellulose microfibrils arranged in such a way that the inner wall, closer to the ostiolus, is less elastic in the longitudinal direction than the outer wall (which also occurs thanks to the thickening additional of this).
They are active cells from the photosynthetic point of view, so they have a large number of chloroplasts inside.
They are characterized by not being connected by plasmodesmata with adjacent cells and because they can rapidly alter their turgor and / or volume.
Accessory cells, on the other hand, are also photosynthetic and act as a kind of "barrier" between the occlusive cells and the epidermal cells around the stomatal apparatus. Its function is to protect epidermal cells from the expansion of guard cells..
The stomata open or close in response to the turgor changes experienced by the occlusive cells or guard cells, so they are like valves that open or close depending on how "full" the latter are..
If the guard cells are turgid, then the stomata open, on the contrary, they close when the cells "empty" or "contract".
Guard or occlusive cells are "filled" or "emptied" for different reasons, generally related to modifications in their water potential, which is what determines the entry or exit of water from a given compartment..
The stomata open thanks to the guard cells receiving a signal or stimulus that tells them that they "must" allow the entry of solutes, which subsequently causes the rapid entry of large amounts of water, simply due to differences in water potential and osmotic.
Among the solutes that enter these cells, potassium (K +) and chlorine (Cl-) ions stand out. There is also malate, but this is endogenously synthesized by the occlusive cells after the stimulus that triggers the stomatal opening..
It is important to mention that ionic transport through the plasma membrane of occlusive cells occurs through specific voltage-dependent channels, which are activated as a function of a voltage difference generated by ATPase pumps that are responsible for expelling hydrogen atoms ( H +).
As expected, the closure of the stomata, that is, the "emptying" of the occlusive cells, is achieved thanks to the reverse transport of the ions that entered before, that is, the exit of chlorine, potassium and malate..
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