Apoplastic & Symplastic Proton Concentrations & Their Significance for Metabolism 115
5.3.2.3 Intracellular pH Changes and Photosynthesis
Potentially acidic molecules can penetrate membranes easily when they are
in their neutral (undissociated) form [HA in Eq. (5)]. Whether they are
protonated or not is determined by the dissociation constant (Ka) and by
the pH of the solution. A useful expression relating Ka of a weak acid to
the concentration of the various forms of the acid, and to the pH, is the
Henderson-Hasselbalch equation:
[A -]
pH == pKa + log [HA]'
(5)
where A-is the dissociated and HA the protonated form of the weak acid,
and pKa the negative decadic logarithm of the dissociation constant. The pK
itself is influenced by the ionic strength (law of Debye-Hiickel; see Pfanz
and Heber 1989) and the temperature of the medium (Van't Hoff's law see
Morris 1974, Aducci et al. 1982).
The dissociation equation for a bi-protic weak acid is
H2A ~ HA - + H+ ~ A 2 - + 2H+.
(6)
For carbon dioxide or sulfur dioxide the equations are given below. The
pK values for these reactions are listed in Table 5.l.
H 2 S0 3 ~ HS0 3 - + H+ ~ sol- + 2H+
(7)
(8)
During a short-term exposure of the cells to acid stress (10-20min), the
plasmalemma seemed to be relative impermeable to the externally applied
"charged" protons (cf. Figs. 5.2 and 5.3). However, the membrane barrier
is negligible when un dissociated weak acids or bases (acting as proton or
hydroxyl ion carriers) easily penetrate the plasmalemma (Pfanz et al. 1987).
According to Eq. (6), they liberate protons (or hydroxyl ions) inside the
cytoplasm (or the vacuole; for details see Pfanz and Heber 1989). According
to the flux equation
== ,1C
R'
(9)
permeation of H+ -consuming or H+ -producing compounds is a matter of
prevailing pH gradients (under conditions where only the neutral form is
freely permeable and the dissociated forms are not transported via special
carriers). With pH values between pH 4.2 and 6.4 in the cell wall at" leaves
(Pfanz and Dietz 1987; Grignon and Sentenac 1991, Hoffmann et al. 1992),
and cytoplasmic pH values around neutrality (cf. Table 5.1), potentially
acidic molecules are trapped inside the cell (principle of the ion trap). As a
consequence of dissociation and proton liberation, pH perturbations are
likely to occur. Figure 5.4 gives an example of the inhibition of the pho-
5.3.2.3 Intracellular pH Changes and Photosynthesis
Potentially acidic molecules can penetrate membranes easily when they are
in their neutral (undissociated) form [HA in Eq. (5)]. Whether they are
protonated or not is determined by the dissociation constant (Ka) and by
the pH of the solution. A useful expression relating Ka of a weak acid to
the concentration of the various forms of the acid, and to the pH, is the
Henderson-Hasselbalch equation:
[A -]
pH == pKa + log [HA]'
(5)
where A-is the dissociated and HA the protonated form of the weak acid,
and pKa the negative decadic logarithm of the dissociation constant. The pK
itself is influenced by the ionic strength (law of Debye-Hiickel; see Pfanz
and Heber 1989) and the temperature of the medium (Van't Hoff's law see
Morris 1974, Aducci et al. 1982).
The dissociation equation for a bi-protic weak acid is
H2A ~ HA - + H+ ~ A 2 - + 2H+.
(6)
For carbon dioxide or sulfur dioxide the equations are given below. The
pK values for these reactions are listed in Table 5.l.
H 2 S0 3 ~ HS0 3 - + H+ ~ sol- + 2H+
(7)
(8)
During a short-term exposure of the cells to acid stress (10-20min), the
plasmalemma seemed to be relative impermeable to the externally applied
"charged" protons (cf. Figs. 5.2 and 5.3). However, the membrane barrier
is negligible when un dissociated weak acids or bases (acting as proton or
hydroxyl ion carriers) easily penetrate the plasmalemma (Pfanz et al. 1987).
According to Eq. (6), they liberate protons (or hydroxyl ions) inside the
cytoplasm (or the vacuole; for details see Pfanz and Heber 1989). According
to the flux equation
R'
(9)
permeation of H+ -consuming or H+ -producing compounds is a matter of
prevailing pH gradients (under conditions where only the neutral form is
freely permeable and the dissociated forms are not transported via special
carriers). With pH values between pH 4.2 and 6.4 in the cell wall at" leaves
(Pfanz and Dietz 1987; Grignon and Sentenac 1991, Hoffmann et al. 1992),
and cytoplasmic pH values around neutrality (cf. Table 5.1), potentially
acidic molecules are trapped inside the cell (principle of the ion trap). As a
consequence of dissociation and proton liberation, pH perturbations are
likely to occur. Figure 5.4 gives an example of the inhibition of the pho-