According to the Second Law of Thermodynamics, an overall increase of entropy contributes to the driving force for any physicochemical process, but entropy has seldom been investigated in biological membrane systems. Photosynthetic membranes of plants are flattened sacs organized in stacks called grana, interconnected by non-stacked membrane sacs. Here, for the first time, we apply Isothermal Titration Calorimetry to investigate the Mg2+-induced stacking of photosynthetic membranes isolated from spinach leaves. After subtracting a large endothermic interaction of MgCl2 with membranes, unrelated to stacking, we demonstrate that the enthalpy change (heat change at constant pressure) is zero or marginally positive or negative. This first direct experimental evidence strongly suggests that an entropy increase significantly drives membrane stacking in this ordered biological structure. Possible mechanisms for the entropy increase include: (i) the attraction between discrete oppositely-charged areas, releasing counterions; (ii) the release of loosely-bound water molecules from the inter-membrane gap; (iii) the increased orientational freedom of previously-aligned water dipoles; and (iv) the lateral rearrangement of membrane components.
Reference:H Jia, J R Liggins and W S Chow (2013) Entropy and biological membranes: Experimentally-investigated stacking of plant photosynthetic membranes. Scientific Reports 4: 4142.