var _gaq _gaq = (Function () {var ga = document.createElement ('script'); ga.type = 'text / javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol? 'https: / / ssl': 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName ('script') [0]; s.parentNode.insertBefore (ga, s); })(); Water is a unique and essential to all living organisms, has excellent solvent, is used as an intermediate in many chemical reactions of great importance. The water obviously plays an important role in plants where it is transported along with many substances nutritive, di conseguenza essa è utile per la traslocazione e il trasporto a lunga distanza di soluti; e quando diciamo trasporto di soluti non intendiamo semplice passaggio di sostanze tra membrane cellulari adiacenti, ma ci riferiamo a veri e propri trasporti, flussi d'acqua che si verificano fra tessuti e organi molto distanti tra di loro. Vi siete mai chiesti come facciano le piante ad assorbire dal terreno l'acqua di cui hanno bisogno? E soprattutto vi siete mai chiesti come fa poi quest'acqua a salire fino alla parte aerea (in alcuni casi anche per decine e decine di metri) della pianta sfidando la forza di gravità?
As the water moves within the xylem?
logic suggests two possibilities, or is pushed from the bottom up or pulled down. The first possibility is not to say that right now! (See below root pressure). fact, according to studies carried out so far has been amply demonstrated that the water is pulled up through the body of the plant.
The driving force behind the water upwards is the same process that determines the evaporation from the leaf tissues and is known as transpiration .
This has affected many generations of botanists, and after years of study there it was realized that this event (forgive
unscientific but the definition is to give a good idea ) causes a real effect tube, a backwash of water to the upper parts of the plant.
unscientific but the definition is to give a good idea ) causes a real effect tube, a backwash of water to the upper parts of the plant.
that in physical terms, we call voltage
. All cells need water. Consequently to be working properly they should be strongly hydrated, so we always find around the matrix of the water film. In particular, the cell wall provides a surface permetttere big enough to water molecules to be retained for accession, in fact (as shown by the image) in the case of mesophyll cells
leaf that are in the immediate vicinity of the foliar gaps where air is present, observe a film of water that surrounds them. This film of water surrounding the mesophyll cells is that through the process of perspiration is emitted into the atmosphere. 
When water evaporates from the surface of the cell walls bordering the intercellular spaces inside the leaf during transpiration, it is replaced by water coming from inside the cell. Quest 'water diffuses across the plasma membrane, which is readily permeable to water but not to the solutes of the cell. The result is that, as the concentration of solutes inside the cell tends to increase by decreasing the water potential the cell, which then becomes increasingly negative. Consequently, it establishes a potential gradient between this water cell and the adjacent more waterlogged. These cells in turn will attract water from other cells and those from others, a real chain of events that will reach the xylem vessel to load which will put a real "aspiration" or tension of water. this tension because of the extraordinary cohesion between water molecules will be broadcast throughout the xylem to the roots, since the xylem of the stem is in close contact with the root. The result is a kind of straw that will pull effect on the water. So the loss of water makes the water potential of root negative and increases its ability to absorb water from the soil. Therefore, the decreased water potential caused by transpiration but also by the use of water by the cells will create a gradient of water potential of leaves at the soil solution bathing the roots. this provides the driving force for water movement in soil-plant atmosphere continuum. This theory is also known as the cohesion tension theory as the chemical and physical properties of water play an important role since it is the cohesion of water molecules that can be subjected to tension. The theory could also be called the theory of the adhesion-cohesion-tension as the bonding of water molecules in the xylem vessels or tracheids is equally important. We must also remember that water molecules interact via hydrogen bonds. When a water molecule leaves the atmosphere does not create a hole of water, but is pulled behind another water molecule. This continuous call is propagated back to the vascular bundles, creating a suction that draws real water. xylem vessels also must be able to withstand the suction as the negative hydrostatic pressure is very high to raise a certain amount of water from the bottom up. The xylem is particularly suitable for this purpose since it is reinforced by a molecular complex known as lignin.
When water evaporates from the surface of the cell walls bordering the intercellular spaces inside the leaf during transpiration, it is replaced by water coming from inside the cell. Quest 'water diffuses across the plasma membrane, which is readily permeable to water but not to the solutes of the cell. The result is that, as the concentration of solutes inside the cell tends to increase by decreasing the water potential the cell, which then becomes increasingly negative. Consequently, it establishes a potential gradient between this water cell and the adjacent more waterlogged. These cells in turn will attract water from other cells and those from others, a real chain of events that will reach the xylem vessel to load which will put a real "aspiration" or tension of water. this tension because of the extraordinary cohesion between water molecules will be broadcast throughout the xylem to the roots, since the xylem of the stem is in close contact with the root. The result is a kind of straw that will pull effect on the water. So the loss of water makes the water potential of root negative and increases its ability to absorb water from the soil. Therefore, the decreased water potential caused by transpiration but also by the use of water by the cells will create a gradient of water potential of leaves at the soil solution bathing the roots. this provides the driving force for water movement in soil-plant atmosphere continuum. This theory is also known as the cohesion tension theory as the chemical and physical properties of water play an important role since it is the cohesion of water molecules that can be subjected to tension. The theory could also be called the theory of the adhesion-cohesion-tension as the bonding of water molecules in the xylem vessels or tracheids is equally important. We must also remember that water molecules interact via hydrogen bonds. When a water molecule leaves the atmosphere does not create a hole of water, but is pulled behind another water molecule. This continuous call is propagated back to the vascular bundles, creating a suction that draws real water. xylem vessels also must be able to withstand the suction as the negative hydrostatic pressure is very high to raise a certain amount of water from the bottom up. The xylem is particularly suitable for this purpose since it is reinforced by a molecular complex known as lignin. What is root pressure?
We have seen that the transport of water through the roots to the aerial part of the plant is due to a difference in water potential between the soil solution and xylem sap in this. When at night the process of transpiration is absent or almost no water potential gradient is maintained through the input of solutes by active transport in the xylem, which causes a lowering of water potential in the xylem that draw water from the surrounding cells by osmosis. This phenomenon causes the formation of a positive hydrostatic pressure that pushes water from the bottom up. But we must stress that the root pressure is not generated in all plants, even in some it is almost absent, and that during the day, when the movement of solutes in the xylem is not particularly fast, it is strong enough to push the water from the bottom upwards, especially in tall trees, then the driving force behind the water to the aerial parts of plants is always sweating. It follows that in plants where the root pressure occurs, it must be seen as a side effect due to the high presence of ions in xylem, and only indirectly as a mechanism of water transport.
0 comments:
Post a Comment