Fig. 8. Proposed model for diffusion of individual molecules versus large complexes on the sperm head. In scenario (a), a single molecule (lipid or protein) is freely diffusing between all compartments. In (b), the molecule is part of a large complex that is unable to traverse the boundary between the postacrosme and equatorial segment. This creates the impression of a barrier. In (c), the complex disassembles in the postacrosomal side of the barrier, single molecules diffuse across it and reassemble on the equatorial segment. They may reform into their original complex or join existing complexes, all of which continue to diffuse randomly.