Absorption of mineral substances from soil and transport of ions through cellular membranes. An origin and a role transmembrannogo potential in transport of ions

you are: Absorption of mineral substances from soil and transport of ions through cellular membranes

As all ions are charged, speed of their diffusion and distribution in an equilibrium condition is defined not only permeability of a membrane and distinctions in concentration of ions on its both parties (chemical potential), but also and the electric potential arising between internal and; external the membrane parties. Therefore we say that movement of ions is caused by a gradient of electrochemical potential. Usually vegetative cages on an inside of the membranes have negative potential that leads to primary absorption kationov (+) in comparison with anionami (-). It is necessary to discuss this potential in more details.

If to divide a membrane solutions with different concentration freely diffundirujushchih ions between its two parties there will be the pressure named transmembrannym in potential. It is possible to measure it by means of two microelectrodes connected to the sensitive voltmeter. One electrode made of a glass capillary with diameter approximately 1 micron, is entered through a cellular wall and plazmalemmu in a cage, and another, the comparison electrode, settles down out of a cage (fig. 7.8). In cages with big central vakuolju the internal electrode usually gets not only through plazmalemmu, but also through tonoplast so measured sizes represent a potential difference between vakuolju and a cellular environment. Similar measurements show that the potential difference fluctuates from 50 to 200 mv, and cage contents are characterised by more negative charge.

Transmembrannyj potential is partially caused by selective permeability of a cellular membrane that limits speed of movement of one ion concerning another. To +, for example, can get through a membrane much faster, than С1-. If both these of an ion in a cage have higher concentration, than round it, faster net-diffusion ions To + outside on a concentration gradient in the final! A result will lead to occurrence of higher negative charge in a cage as there there will be superfluous quantity С1.

Active transport (prokachivanie ions through a membrane) represents, possibly, most important regulator transmembrannogo potential. We will discuss some details of this process later. In the beginning we will consider consequences of active transport of ions of any one type only in one direction. Such process is called as electrogene, as: conducts to accumulation of negative charges on one party of a membrane and positive - on another. One of the main ions participating in creation transmembrannogo of potential, is N +. At pumping out N + from a cage in it there is a negative potential.

Having arisen once, transmembrannyj the potential in turn can influence the subsequent movement of ions. We will assume, for example, that transmembrannyj the cage potential makes-166 mv, and internal contents of a cage

It is charged negatively. This endocellular negative potential causes diffusion in a cage of positively charged ions, such, as To +. Receipt of negatively charged ions, for example С1- will be simultaneously suppressed. Between tranomembrannym in potential and diffuzionnymi streams of any ion, for example To +, it is possible to calculate quantitative dependence under the formula named the equation of Nernsta (see more low).

Some not charged molecules, such as sucrose, get through a membrane at the same time or other ion, usually N +. This process named kotransportom or simportom, plays especially important role in regulation of movement of sucrose in floemu and from it (see gl. 8).

Receipt in cages floemy complex N + - sucrose (' loading ') and its allocation from cages floemy (' unloading '), possibly, occur by moving of molecules through a membrane to participation permeazy. Not charged molecules of sucrose ' force the way ' through permeazu ions N +, and the direction is net-diffusion defined by electrochemical gradient N +.

Uravnenie Nernsta connects electric potential in a cage with distribution of the charged ions:

Where E - transmembrannyj potential (mv), measured with use of the earthed electrode out of a cage; p - valency and an ion charge;

d - concentration (moljarnost) an ion in a cage; С0 - concentration (moljarnost) an ion out of a cage. We will assume that E =-116 mv. For monovalent kationov, for example Na + or To +, п=1 and lg С1/С0 =-116/-58=2. As

2 is lg 100, To + and Na + will be diffundirovat in a cage without an expense of metabolic energy until internal concentration of each ion will not be equal 102, or 100Hna-ruzhnaja concentration. On the other hand, diffusion monovalent aniona, for example С1 _, will lead to that its internal concentration will make only 10-2, or 0,01Х external concentration.

Typical sizes transmembrannogo potential and concentration To +, Na + and С1- inside vakuoli cages of the higher plant are shown more low. Concentration Na + in a cage is less than the size calculated on the equation of Nernsta, and concentration

K + and GI - - it is more. Obviously, Na + it is actively transported from a cage, and To + and С1- - in a cage. To + - the unique element, which actual concentration it is close to Nernsta calculated on the equation.