104
H. Pfanz
are two examples. Nevertheless, as long as the perturbation does not exceed
a certain threshold, cellular pH-stat mechanisms are able to cope with excess
protons or hydroxyl ions. To maintain cellular pHs within a distinct range,
several biochemical and biophysical pathways are possible (see Raven 1986,
1988; Davies 1973, 1986). In the repertoire of the cellular pH-stat mechanisms there is the metabolic destruction of organic acids, the reduction of
nitrate, or the pumping of protons from one compartment to another,
strategies necessary to reduce a possible proton burden.
In this chapter the principles of pH, the methods of pH determination,
and the influence of pH on cellular reactions will be described. The proton
concentrations of extracellular and several intracellular leaf cell compartments and the possible changes of pH during development and aging, or due
to the influence of air pollutants, are discussed. The pH dependency on
lignifying and IAA-oxidizing peroxidases will be shown as an example of
apoplastic reactions. Being the most important symplastic reaction, the
effects of pH on photosynthesis are examined.
5.2 Definitions
5.2.1 The pH Concept
PH value (potentia Hydrogenii) is a measure of the actual concentration
(activity) of protons (or hydronium ions H30+) in a solution. It is defined as
(1)
and thus as the negative decadic logarithm of the H+ concentration (activity).
Per definition em , a solution is called neutral when its pH value is aproximately 7. Solutions above pH 7 (7-14) or below pH 7 (7-0) are called
alkaline (basic) or acidic, respectively (theoretically, pH values below zero
or above 14 are possible). In other words, if the proton concentration
increases, the pH decreases, and vice versa. A change in the proton concentration by a factor of 10 is equal to a pH change of one unit.
5.2.2 The Buffer Concept
A buffer may be defined as a solution which resists pH changes (within a
certain limit) despite the addition of acids or bases. This "resistance" to
changes in pH is due to the presence of compounds in the solution able to
neutralize the surplus H+ or OH-. The principle of buffering can be seen in
Eqs. (2) and (3).
DH + NaOH - - DNa + H20
(2)
H. Pfanz
are two examples. Nevertheless, as long as the perturbation does not exceed
a certain threshold, cellular pH-stat mechanisms are able to cope with excess
protons or hydroxyl ions. To maintain cellular pHs within a distinct range,
several biochemical and biophysical pathways are possible (see Raven 1986,
1988; Davies 1973, 1986). In the repertoire of the cellular pH-stat mechanisms there is the metabolic destruction of organic acids, the reduction of
nitrate, or the pumping of protons from one compartment to another,
strategies necessary to reduce a possible proton burden.
In this chapter the principles of pH, the methods of pH determination,
and the influence of pH on cellular reactions will be described. The proton
concentrations of extracellular and several intracellular leaf cell compartments and the possible changes of pH during development and aging, or due
to the influence of air pollutants, are discussed. The pH dependency on
lignifying and IAA-oxidizing peroxidases will be shown as an example of
apoplastic reactions. Being the most important symplastic reaction, the
effects of pH on photosynthesis are examined.
5.2 Definitions
5.2.1 The pH Concept
PH value (potentia Hydrogenii) is a measure of the actual concentration
(activity) of protons (or hydronium ions H30+) in a solution. It is defined as
(1)
and thus as the negative decadic logarithm of the H+ concentration (activity).
Per definition em , a solution is called neutral when its pH value is aproximately 7. Solutions above pH 7 (7-14) or below pH 7 (7-0) are called
alkaline (basic) or acidic, respectively (theoretically, pH values below zero
or above 14 are possible). In other words, if the proton concentration
increases, the pH decreases, and vice versa. A change in the proton concentration by a factor of 10 is equal to a pH change of one unit.
5.2.2 The Buffer Concept
A buffer may be defined as a solution which resists pH changes (within a
certain limit) despite the addition of acids or bases. This "resistance" to
changes in pH is due to the presence of compounds in the solution able to
neutralize the surplus H+ or OH-. The principle of buffering can be seen in
Eqs. (2) and (3).
DH + NaOH - - DNa + H20
(2)