Grana formation: entropy-assisted local order in chloroplasts?

Grana formation is a prominent feature of the ultrastructure of chlorophyll b-containing chloroplasts, serving to fine-tune photosynthetic efficiency. This paper examines the physical forces that determine the state of minimu m free energy of the chloroplast as manifested by grana formation. In parti cular, it considers the interplay of van der Waals attraction, electrostati c repulsion and short-range hydration repulsion between thylakoid membranes . In addition to these interactions as determinants of the free energy of t he system, it is also proposed that ordering of thylakoid membranes and of intramembrane protein complexes is driven by an increase in the overall ent ropy of the system. This local order may partly come about by an increase i n entropy associated with a greater free volume for diffusion of membrane a nd stromal components.