Comparative Studies of the Hill Activity of Differentially Centrifuged Chloroplast Fractions

Abstract

A fraction with high Hill activity was differentially centrifuged between 20,000 and 50,000 x g from sonically ruptured isolated spinach chloroplasts. This fraction, CF20-50, was compared under a variety of conditions with other fractions isolated at higher and lower centrifugal forces. It was characterized physiologically and by particle size and shown to be extremely stable. It appears from centrifugal fractionation and electron microscopy to be composed of photoactive subunits. These subunits are larger than those presumed by the present quantasome model and are thought of as fundamental functional units of photosynthesis. Hypothetically, CF20-50 is a highly efficient oxygen-evolving, structured complex composed of functional subunits, which, when separated from the organized structure, are less efficient.