Vakuol. Acidity, pH and bufery

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Acidity and shchelochnost. Acid name the substance, capable to give a proton (N +), and the basis - the substance, capable it to attach. If to add to any water system acid concentration of protons in system will raise; if to add the basis it will attach protons and by that to reduce their concentration. It in turn will lead to increase of concentration of ions IT because at dissotsiatsii waters between N +, It - and Н2О is supported balance.

Thus, in water systems acidity=[N +], and shchelochnost=[It-] (square brackets mean here concentration in gramme atoms or grammes-molecules on litre, i.e. moljarnost a solution). In water solutions [N +] H [It-]=10-14. As [N +] and [It-] are connected by inversely proportional dependence, we can, knowing [N +], to define [It-]. In a neutral solution (with equal concentration of ions of hydrogen and gidroksila) [N +]=[It-]=10-7 M.

rn. Acidity of cellular juice fluctuates at plants from 10-1 to 10-7 M, i.e. Can differ a million times. At such big range of distinctions for convenience it is necessary to express acidity in a logarithmic scale, through the decimal logarithm of concentration of hydrogen ions (expressed in mol/l), taken with a return sign. For this size the designation rn is accepted:

Hence, if [N +]=10-hM, rn=h. Accordingly decrease rn on one unit means tenfold increase in acidity, and increase at one unit - its tenfold reduction.

Acidity for plants is very important, and in particular is important for their growth. Optimum for activity of enzymes, the range rn is narrowest - usually enzymes are most active at values rn, close to the neutral. In cytoplasm it is supported рН~7 thanks to buffers (see more low), and also the pumps which are pumping over ions N + in vakuol or from vakuoli. Vakuol at plants serves as the tank for organic acids, therefore rn its contents fluctuates usually within 3,5-5,5; however in cages of some fruits rn vakuoljarnogo juice it is equal 1,0. Activity of ATR-GENERATING systems in hloroplastah and mitohondrijah depends on a difference rn between two parties of the membrane surrounding organellu.

When biochemists allocate any organelly or substances from vegetative cages, they always watch that ekstragirujushchaja environment was appropriate amount zabufe-rena (usually in limits rn 6-8), i.e. That containing in cellular vakuoljah acids have been neutralised, otherwise can occur denaturatsija cellular enzymes. Growth of plants in the big degree depends from rn a soil solution, mainly because rn defines availability of nutrients being in soil, and extreme values rn can cause as well toxic effects (optimum for growth it appears usually rn, close to neutral; see gl. 7 and 14).

Titruemaja acidity. True acidity of vegetative juice express through concentration of hydrogen ions, and titruemuju acidity define by quantity of the alkali spent for neutralisation of juice (on its finishing rn to 7). To predict titruemuju acidity on concentration of hydrogen ions it is impossible, because in vegetative juice weak acids at which degree dissotsiatsii is small contain. True acidity is equal titruemoj only in the event that acid is completely dissociated, i.e. If it is strong acid. By definition

At full dissotsiatsii 1 n. НС1 corresponds rn 0, as-lgl=0. (Actually, however, dissotsiatsija does not happen full and 1,0 n. НС1 corresponds rn 0,10.)

Weak acid name such acid at which only small part of molecules is dissociated. In 1 n. To acetic acid (SNzSOON) the share of the dissociated molecules makes only 0,42%.

Thus, 1 n. СН3СООН, corresponds rn 2,37, as-lg 0,0042=2,37.

However, though true acidity 1 n. НС1 in 200 with superfluous time above, than at 1 n. SNzSOON, the same quantity of alkali is spent for titration of equal volumes of these solutions. The matter is that, as alkali connects ions N + in a solution of acetic acid, the part nedissotsii-rovannyh molecules of acetic acid dissociates earlier, so the total of the dissociated form all time is supported at level of.0,42%, otherwise, quantity of ions N + in a solution continuously replenishes, while all potentially present in:нем dony N + will not be connected by alkali.

Bufery. In vegetative cages contain so-called bufery - mixes of the substances, capable to support rn at constant level. At weak acid almost all it titruemye hydrogen ions are present at a kind of potential ions N +. Therefore if to prepare a mix from such acid and its any salt even at addition of considerable quantities N + - or IT of ~-ions rn will remain environments almost constant. A similar mix name ' the buffer ' for its ability to prevent change rn. For weak acid ON (where And means any anion, for example acetate)

In an equilibrium state the relation between not dissociated molecules and ions remains to constants; at weak acid it can be, for example, equally 9НА: 1Н +: 1A-. With addition of this or that basis balance is broken, as are entered IT - the ions connecting ions N +. However the characteristic relation for balance immediately is again restored thanking dissotsiatsii some part of molecules ON which stock is very great; therefore concentration of hydrogen ions [N +] remains almost invariable.

That rn did not vary as well at addition of ions N +, the buffer should have also a reserve anionov (And-). With that end in view to it add the salt of weak acid dissociated highly; for NaA, for example, the relation of not dissociated molecules to ions can be equal 1 NaA:9Na +:: 9 And-. When acid and salt mix, new balance, but the maintenance of not dissociated molecules ON (a source of ions N + is established, capable to incorporate with IT - ions) and anionov And - (capable to incorporate to ions N +) remains high. At equal quantities of acid and salt rn it will be supported at constant level. If, for example, to add one drop concentrated НС1 to 1 l of water it will lower rn on 3,7 ed; but if the same quantity BUT to add to 1 l of the buffer consisting from 0,1 n. Weak acid and its salt addition НС1 practically at all will not render influence on rn. Thus zabufereno contents cellular vakuolej: At it there are organic acids, for example lemon or apple, and simultaneously salts of these acids.

Bufery in protoplasm. Fibers serve in protoplasm as the main buffers and amino acids because the total charge of amino acids can be both positive, and negative, depending on rn. Added ions N + join to karboksilnym to molecule groups, and added ions It - incorporate to ions N +, otshchepljajushchimisja from its amino groups. To imagine it, it is necessary to address to the general formula of amino acids represented on fig. 2.12.