SUMMARY / RELATED TOPICS

Stoma

In botany, a stoma called a stomate, is a pore, found in the epidermis of leaves and other organs, that facilitates 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 stomatal opening; the term is used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, referred to as the stomatal aperture. Air enters the plant through these openings by gaseous diffusion and contains carbon dioxide, used in photosynthesis and oxygen, used in respiration. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Water vapor diffuses through the stomata into the atmosphere in a process called transpiration. Stomata are present in the sporophyte generation of all land plant groups except liverworts. In vascular plants the number and distribution of stomata varies widely. Dicotyledons have more stomata on the lower surface of the leaves than the upper surface.

Monocotyledons such as onion and maize may have about the same number of stomata on both leaf surfaces. In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface. Leaves with stomata on both the upper and lower leaf are called. Size varies across species, with end-to-end lengths ranging from 10 to 80 µm and width ranging from a few to 50 µm. Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 400 ppm. Most plants require the stomata to be open during daytime; the air spaces in the leaf are saturated with water vapour, which exits the leaf through the stomata in a process known as transpiration. Therefore, plants cannot gain carbon dioxide without losing water vapour. Ordinarily, carbon dioxide is fixed to ribulose-1,5-bisphosphate 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 low affinity for carbon dioxide, 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. Retrieving the products of carbon fixation from PEPCase is 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. A group of desert plants called "CAM" plants open their stomata at night, 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. This approach, however, is limited by the capacity to store fixed carbon in the vacuoles, so it is preferable only when water is limited. 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 and carbon dioxide concentration, it is not certain how these responses work. However, the basic mechanism involves regulation of osmotic pressure; when conditions are conducive to stomatal opening, a proton pump drives protons from the guard cells. This means that the cells' electrical potential becomes negative; the negative potential opens potassium voltage-gated channels and so an uptake of potassium ions 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 turgor pressure; because of rings of cellulose microfibrils that prevent the width of the guard cells from swelling, thus only allow the extra turgor pressure to elongate the guard cells, whose ends are held 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. When the roots begin to sense a water shortage in the soil, abscisic acid is released. 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; this caus

Kalaw

Kalaw is a hill town in the Shan State of Myanmar. It is located in Kalaw Township in Taunggyi District; the town was popular with the British during colonial rule. Kalaw is the main setting of the novel "The Art of Hearing Heartbeats" by Jan-Philipp Sendker; the hill station is located at an elevation of 50 km from the Inle lake. Kalaw is famous for trekking. Many trekking trails ranging from nearby places to Inle Pindaya; the town still feels like a high-altitude holiday resort – the air is cool, the atmosphere is calm and the tree-lined streets still contain a smattering of colonial-era architecture – while the surrounding hills are fine for easy day or overnight treks to Danu, Palaung, Pa-O and Taung Yo villages. Smith Dun Satellite map at Maplandia.com

Eprosartan

Eprosartan is an angiotensin II receptor antagonist used for the treatment of high blood pressure. It is marketed in the United States as Teveten by Abbvie, the spin-off of the pharmaceutical discovery division of Abbott Laboratories; the compound came into the Abbott Laboratories cardiovascular pipeline with its acquisition of Kos Pharmaceuticals in 2006, which had licensed it, along with "a range of hypertensive treatments", from the Biovail Corporation. Eprosartan is sometimes paired with hydrochlorothiazide, whereupon it is marketed in the US as Teveten HCT and elsewhere as Teveten Plus; the drug acts on the renin–angiotensin system to decrease total peripheral resistance in two ways. First, it blocks the binding of angiotensin II to AT1 receptors in vascular smooth muscle, causing vascular dilatation. Second, it inhibits sympathetic norepinephrine production, further reducing blood pressure; as with other angiotensin II receptor antagonists, eprosartan is better tolerated than enalapril among the elderly.

Discovery and development of angiotensin receptor blockers