Particles within membranes: a freeze-etch view.

Particles are commonly present on the membrane faces revealed by freeze-etching. The number, distribution and size of these particles vary considerably both between different membranes and, in many cases, between the 2 fracture faces found in individual membranes. Many of the larger particles appear to be too large to fit totally within smooth-surfaced membranes, so raising the question of how particles, especially the larger ones, are contained within membranes. This could be accomplished by a local reorganization of the membrane9s internal structure such that small particles would be totally enclosed within smooth-surfaced membranes, while large particles would protrude from the membrane surface. Alternatively, all sizes of particles could be contained within membranes by a bulging of the 2 component lamellae such that protuberances, having a larger diameter than the underlying particles, would arise on the membrane surface. Evidence is presented to show that in the case of specialized particles, which are located in rows around the base of flagella in the mollusc Cominella maculosa , protuberances are present on the membrane surface. However, it is possible that particles could be accommodated within membranes from other tissues by a different mechanism, and only further work will decide whether or not the present findings can be applied to these other membrane surfaces.