Movement and redistribution of nutrients. The mechanism floemnogo transport

you are: Movement and redistribution of nutrients

For the long period of studying floemnogo transport the set of theories about its mechanism has been put forward. The majority of them can be rejected, as they have been based on the experiences causing as it has been shown later, occurrence of artefacts. As to other theories the data discussed in them it is possible well to explain action of other mechanisms, instead of what were offered by their authors. Only one theory, in the general opinion, Will be co-ordinated almost with all known facts, and the remained small divergences, obviously, will be explained further by insignificant updating of the basic theory. Therefore for simplicity we will describe only the theory of an overflowing of solutions (or the theory of a current under the pressure of Mjunha), offered by originally German physiologist Charles Mjunhom.

According to this theory, the overflowing of solutions on floeme is completely based on simple principles osmosa that is better it is possible to show, having considered purely physical analogue system. Two rigid spheres designed from a membrane with selective permeability, immerse in water and connect among themselves an impenetrable tube (fig. 8.5.

Originally one sphere is filled with the concentrated solution of sucrose (and another - diluted (). As a result osmosa water starts to arrive in both spheres and in system there is a pressure. As pressure increases in more concentrated solution And, it will be transferred on a tube to less concentrated solution (). If the pressure transferred from And, exceeds the pressure which has arisen in In water instead of arriving in In, will be compelled to leave it. As water in this case arrives in And and leaves from In, the overflowing of a solution of sucrose will occur from And to V.Ono will proceed until concentration of solutions And and In not vyravnjajutsja. All system will be In this condition under equilibrium pressure and directed net-stream a solution will stop. However if we add sucrose to And and to delete it from In the overflowing of a solution of sucrose from And to In will proceed. If thus the water removed from In, can back arrive through an external solution in And there will be continuously operating closed system. To show it, solutions, associates And and In, are connected in drawing by a tube.

In a plant the sucrose formed at photosynthesis, is actively pumped up in sitovidnye tubes of small sheet veins during the process named loading floemy. This process leads to decrease in water potential sitovidnyh tubes, and water starts to arrive in them a way osmosa. As a result there is a pressure that compels a solution to move-to to consuming cages on a pressure gradient. From a place of consumption sucrose actively leaves that causes increase of water potential sitovidnyh tubes. Then water diffundiruet from sitovidnyh tubes on a gradient of the water potential caused mainly by presence in them of pressure. It quickly comes back on ksileme to a sucrose source.

This theory Will well be co-ordinated with the cores known about floeme the data: floema is under positive pressure; between the donor and an acceptor exist both pressure gradients, and osmotic gradients; contents sitovidnyh tubes flow on them a continuous stream; opened sitovidnye a time favours to such overflowing of a solution. There is one problem, consisting that, as it is known, floemnyj transport demands more active metabolism, than what is necessary for simple maintenance plazmalemmy sitovidnyh tubes. this dilemma can be solved, if: to take into consideration that fact that the metabolism is required for three separate processes: loadings floemy in leaves sucrose, sucrose carrying over to places of consumption and its deduction in a membrane sitovidnyh tubes. The different parity of these three processes defines a direction of movement of contents sitovidnyh the tubes located vertical numbers adjoining to each other.

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