5 ApopJastic and SympJastic Proton
Concentrations and Their Significance
for Metabolism
H. Pfanz
5.1 Introduction
Proton concentration is a major factor in restricting life within definite
boundaries. In general, life occurs in a neutral or slightly acidic environment,
but fungi, bacteria, animals, and higher plants have nevertheless managed to
conquer terrestrial and aqueous niches at extreme pH values. The pH
extremes for cellular growth are around pH 1 and pH 11 (Souza et al.
1974; Langworthy 1978). Life therefore exists within the enormous H+
concentration range of lO lD . During evolution, higher plants have adapted
to the different soil conditions with which their root systems have been
confronted. Acidophilic, neutrophilic, and acidophobic plant types have
evolved. The pH of the soil water determines the availability of nutrients or
heavy metals and thus also determines soil toxicity (Larcher 1980). Some
important cultural plants tolerate only a very narrow soil pH range (e.g.,
Medicago sativa), whereas others are very tolerant (e.g., Secale cereale).
The pH sensitivity of these plants seems to be either due to direct H+ effects
on the roots, to mycorrhizal or rhizobial symbionts (Schubert 1987), or to
indirect effects like the release of toxic heavy metals or aluminium from the
ion-exchanging compounds in the soil solution (Ulrich 1981; Rehfuess 1981).
Also within plant organs and cells, the hydrogen ion concentration is
probably the most fundamental factor for a functional metabolism. Nearly
every reaction in a living cell is affected - or even regulated - by pH.
The hydrogen ion concentration determines not only the ionic state and
consequently the availability of several organic and inorganic metabolites,
but also the stability and function of biological macromolecules and membranes. Especially proteins are very sensitive to modifications of hydrogen
ion concentration. Many key enzymes - namely those of the chloroplast
stroma - are directly regulated by pH (Woodrow et al. 1984). The reason
for the strong pH sensitivity of enzymes is either the different ionic state
of the substrate or of the active site of the enzyme itself, or, in some
(irreversible) cases, the pH stability of the protein.
Naturally, there are many possible disturbances leading to an increase or
decrease in cellular pH. The potential alkalization of the cytoplasm due to
nitrate reduction and the possible acidification of cells during anaerobiosis
Concentrations and Their Significance
for Metabolism
H. Pfanz
5.1 Introduction
Proton concentration is a major factor in restricting life within definite
boundaries. In general, life occurs in a neutral or slightly acidic environment,
but fungi, bacteria, animals, and higher plants have nevertheless managed to
conquer terrestrial and aqueous niches at extreme pH values. The pH
extremes for cellular growth are around pH 1 and pH 11 (Souza et al.
1974; Langworthy 1978). Life therefore exists within the enormous H+
concentration range of lO lD . During evolution, higher plants have adapted
to the different soil conditions with which their root systems have been
confronted. Acidophilic, neutrophilic, and acidophobic plant types have
evolved. The pH of the soil water determines the availability of nutrients or
heavy metals and thus also determines soil toxicity (Larcher 1980). Some
important cultural plants tolerate only a very narrow soil pH range (e.g.,
Medicago sativa), whereas others are very tolerant (e.g., Secale cereale).
The pH sensitivity of these plants seems to be either due to direct H+ effects
on the roots, to mycorrhizal or rhizobial symbionts (Schubert 1987), or to
indirect effects like the release of toxic heavy metals or aluminium from the
ion-exchanging compounds in the soil solution (Ulrich 1981; Rehfuess 1981).
Also within plant organs and cells, the hydrogen ion concentration is
probably the most fundamental factor for a functional metabolism. Nearly
every reaction in a living cell is affected - or even regulated - by pH.
The hydrogen ion concentration determines not only the ionic state and
consequently the availability of several organic and inorganic metabolites,
but also the stability and function of biological macromolecules and membranes. Especially proteins are very sensitive to modifications of hydrogen
ion concentration. Many key enzymes - namely those of the chloroplast
stroma - are directly regulated by pH (Woodrow et al. 1984). The reason
for the strong pH sensitivity of enzymes is either the different ionic state
of the substrate or of the active site of the enzyme itself, or, in some
(irreversible) cases, the pH stability of the protein.
Naturally, there are many possible disturbances leading to an increase or
decrease in cellular pH. The potential alkalization of the cytoplasm due to
nitrate reduction and the possible acidification of cells during anaerobiosis