Hydrogen ion concentration in subcellular particles: Physical meaning and methods of determination

Various methods of measuring the pH within particles of small size are reviewed. In relation to the chloroplasts the authors analyse the possible reasons for the inadequacy of the measurement of transmembrane pH difference (Δ pH) from the distribution of the pH indicators within and outside the vesicles. From analysis of the published data they show that the divergences appearing on measuring Δ pH by different methods may be due not only to the binding of the pH indicators to the membrane but also the heterogeneity (morphological and functional) of the system. The question of the physical meaning of the concept of pH within subcellular particles of small size is examined.