Molecular assemblies are held together by relatively strong intermolecular forces. 

In the case of water the dominant interactions are hydrogen bonding and dipole-dipole interactions.  In ice the hydrogen bonding results in a rigid stucture where each water has the same orientation relative to its neighbors.  In melting ice, many of the hydrogen bonds are ruptured and less order is apparent.  Notice that ice has large holes between the molecules; thus ice has a larger volume than the same amount of liquid water.

In the case of lipid bilayers (cell membranes are an example) one of the dominant interactions is the van der Waals attractions between the long alkyl tails of the lipids.  The structure of the lipid bilayer also depends on the polar interactions between the water at each side of the bilayer and the polar head groups.