the Water mode of plants. Water inflow in vakuol under the influence of osmotic forces

you are: the Water mode of plants

As the most part of water present at a cage is in vakuoli, we will begin the analysis of a problem of transport of water with consideration of that way which to a water molecule is required to be overcome to get in vakuol cages. Water should pass through two membranes (plazmalemmu and tonoplast) and through cytoplasm lying between them. We know about distinctions in ability of these three structures to pass water that is why usually all three structures are considered in common as uniform membrannyj a barrier a little.

To understand, how water passes through a membrane, we will imagine that a cage, in vakuoli which salts, sugar, amino acids and other substances contain, is placed in a vessel with the distilled water (fig. 6.1). According to the molekuljarno-kinetic theory of a molecule of all substances are in a condition of the fast chaotic movement which speed depends on energy of these molecules. Average speed of their movement is defined by temperature (and serves, in effect, as its measure). As water molecules are small and pass through cellular membranes much faster, than molecules of other substances, we can for the sake of be limited simplicity to moving consideration only water molecules. These molecules diffundirujut in all directions: in a cage and from a cage, in various cellular organelly and from them.

We know, however, that vakuol contains significant amounts of the various dissolved substances. Molecules of these dissolved substances weaken communications between molecules containing in vakuoli waters, drawing them to itself and by that reducing total water stream from a cage outside. In a sense dissolved substances reduce activity of molecules of the water which are in a cage. As consequence of it kinetic energy of water in vakuoli more low, than rather more at its finest out of a cage. Told means that outside vakuoli about any site of its membrane hits in unit of time more molecules of water and bolshee their number gets on this site inside, rather than leaves it. As a result of this fast non-uniform bilateral diffusion of molecules of water through a membrane vakuoli the volume vakuoli increases and created tourist's mountains - cage contents nestle on its wall.

Water Diffusion through a seminontight membrane is called osmosom; concentration of the dissolved substances in vakuoli serves as a measure of the maximum ability of a cage to absorb water. Power level of molecules of the given substance, reflected in the speed of their diffusion, name chemical potential of this substance. Here, however, we speak only about water and consequently we will use the special term intended for this special case: water potential. The water potential (-f) characterises ability of water diffundirovat, to evaporate or be absorbed. It has dimension of energy, delennoj on volume (that coincides with dimension of pressure), and its size express usually in atmospheres or bars (1 бар=0,987 atm). Actually we cannot measure energy of molecules of water, for example, in a laboratory glass, therefore is conditional for zero is accepted f at its finest under normal conditions (standard temperature and pressure). To measure probably only a difference energy the molecules of water which are in different conditions. The more low energy of molecules of water, the more low and water potential; as f at its finest it is accepted equal to zero, with increase in concentration of the dissolved substances-f becomes more and more negative. At osmose molecules of the dissolved substance reduce energy of molecules of water so the solution has more negative potential, than pure water.

Water Molecules move always in a direction from higher water potential to lower just as water flows downwards, passing to more and more low power level. Water potential of a solution influence besides the dissolved substances and other factors, for example pressure; therefore that to a component of water potential which is defined by presence of the dissolved substance, designate the special term - osmotic potential (fp). The osmotic potential of a solution is connected by direct dependence with concentration of the dissolved substance. With increase in this concentration the osmotic potential becomes more and more negative. If 1 mol (i.e. Number of grammes of the substance, equal to its molecular weight) any not dissociating substance, for example sucrose to dissolve in 1 l of water, i.e. To prepare moljalnyj a solution the osmotic potential of such solution will be under normal conditions equal-22,7 a bar. In less concentrated solutions osmotic potentials accordingly are less negative.

In that case when the solution is separated from at its finest by a seminontight membrane, water arrives in a solution and thereof there is a pressure (osmotic pressure), equal on size, but opposite on a sign to initial osmotic potential. The solution possesses potential for which account there is such pressure, and he can be found out, if, for example, this solution to place in the device named osmometrom (fig. 6.2). Numerically osmotic potential is equal to that pressure which is necessary for putting to a solution in osmometre to prevent receipt in it waters.

The Cage containing in space limited to a cellular wall a solution surrounded with a membrane (i.e. vakuol), represents, as a matter of fact, osmometr.

If such cage to ship in pure water in it water will start to arrive. In absence protivodavlenija a cellular wall water inflow in a cage is defined by water potential of a cage (fkl), during the initial moment of time equal to osmotic potential (fp) a solution filling vakuol. However with water penetration in vakuol its volume increases, water dilutes cellular juice and a cellular wall starts to be under pressure. Till what time water will arrive in vakuol? If this receipt depended only on osmotic potential theoretically it could proceed indefinitely. However actually with volume increase vakuoli cytoplasm nestles on a cellular wall and there is tourist's mountains th a pressure, and together with it and equal to it on size protivodavlenie a cellular wall on cellular contents. As pressure potential (fd) understand usually protivodavlenie a cellular wall, but this term can designate and tourist's mountain pressure (equal to the first on size, but opposite to it on a sign). When fd will reach enough big size, the further inflow of water in vakuol stops. Dynamic balance at which total water stream is equal to zero is established, i.e. The quantity of water in vakuoli does not change, though molecules of water and continue to move quickly through a membrane in both directions. Thus the positive potential of pressure is counterbalanced completely by negative osmotic potential and a cage ceases to absorb water; its water potential is equal in such condition to zero.

During any given moment of time the water potential of a cage is defined by a difference between potential of pressure and osmotic potential. If to give the chance to a cage to reach in the distilled water maximum round of mountains and it will absorb water until fd it will not be made even with i.e. fkl does not become equal to zero. After that it more cannot absorb water any more from any solution. It cannot and take away it from other cage. If nearby there will be two cages with different fkl water through a cellular wall will pass from a cage with higher (less negative) fkl in a cage with lower (more negative) fkl.

We Will consider now water inflow in vakuol and from vakuoli a vegetative cage in situ (i.e. In a plant). Vakuol and a protolayer surrounding it are concluded in a cellular wall through which water diffundiruet it is free. (The cellular wall in this sense reminds a filtering paper which too consists of cellulose.) The cellular wall is sated by water until humidity of soil is sufficient, and transpiratsija is not too intensive. In these conditions f in the field of a cellular wall above, than in vakuoli, and total water stream is directed inside, in vakuol. Other picture is observed at a moisture lack: during such periods in cellular walls water deficiency can be felt and, hence, f in this area it will appear more low, than in vakuoli. As a result water will flow from vakuoli (total water stream is directed outside). Cages start to lose tourist's mountains; owing to decrease in tourist's mountain pressure they become languid and soft. If because of very big loss of water tourist's mountain pressure reduces to zero, the sheet will wither absolutely; the further loss of water will lead to rupture of protolayers and to  destruction of cages though as we will see more low, the plant can avoid sharp loss of water, quickly having closed the ustitsa in reply to a moisture lack. Subsequently, if cages remained intaktnymi, they can absorb again water and restore the of tourist's mountains; it will occur or at reception by a plant of enough of water, or at night when transpiratsija also evaporation practically stops is quite compensated by absorption of water from soil.

F - water potential; it is equal to zero for at its finest; it is equal

To zero or it is negative for cages (fkl). fp - osmotic potential; it is always negative, pressure fd-potential; it is usually positive in live cages (i.e. In the cages which contents are under pressure), but it is negative in cages ksilemy (in which the water tension is created). fkl there is a total result of action fl and fd.

фкл=фл+фд.

At a full tourist's grief fl =-fdd and fkl=0. At initial plazmolize fd=0 and фкл=ф.

In artificial conditions it is possible to observe and stronger reduction of protolayers. If we place a fabric slice in a solution more concentrated, than vakuoljarnyj juice outflow of water from cages will proceed until protolayers will not separate from cellular walls and will not be compressed in a lump in the middle of a cage. The external solution easily passes through a cellular wall which practically does not obstruct to water traffic, and fills space between a cellular wall and the reduced protolayer. A cage in such condition name plazmolizirovannoj (fig. 6.3). If plazmoliz not too strong and not too long the cage after it will transfer to water, restores usual tourist's mountains. In the vegetative cages lacking water in the air environment, plazmoliza as that does not occur, as there is no free solution which could fill space between a protolayer and a cellular wall; In the nature excessive water deficiency, most likely, leads to rupture and  destruction of cages.

Knowing f for the given cage, it is possible to predict, as she will behave in relation to this or that solution, i.e.

Whether it will absorb water from it or, on the contrary, to give it water. For experimental definition fkl separate cages or fabric slices immerse abreast solutions of any substance, for example sucrose, with gradually increasing concentration. The water potential of a solution in which weight (or the volume) cages will not increase and will not decrease, and will give us size fkl (fig. 6.4). The Same method, i.e. Immersing of cages abreast solutions with gradually increasing concentration, it is possible to define and fp as the osmotic potential of that external solution in which begins plazmoliz (tourist's mountains is equal to zero, i.e. fd=0), it is equal to internal osmotic potential. Initial plazmoliz is that condition at which the protolayer simply adjoins a cellular wall and here and there starts to lag behind it slightly. Therefore fl define, looking through under a microscope cuts of the fabrics placed in a mentioned number of solutions, and marking a solution, which osmotic potential has appeared just sufficient to cause plazmoliz half of cages. This method of definition of osmotic concentration of cellular juice, old and a little primitive, nevertheless, apparently, it is possible to consider as the best. In the majority of other methods cellular juice, and this procedure, possibly is required to squeeze out, changes contents vakuoli. Pressure potential fd it is possible to measure in large cages nitchatoj seaweed Nitella, entering into them the special micromanometer intended for such measurements (fig. 6.5). For the higher plants to measure fd it is more difficult, and consequently it usually define as a difference between fkl and fp (fig. 6.4). It is possible to measure cellular water potential In the whole runaways by means of described below the device.