Practice question

Freeze-fracture technique, developed by Branton and refined by Pinto da Silva, revolutionized visualization of membrane interior by physically splitting frozen lipid bilayers. Live cells are rapidly frozen in liquid propane at -180C, fractured with cold knife under high vacuum, fracture plane preferentially passes through hydrophobic interior of membranes where van der Waals forces are weakest, splitting bilayer into exoplasmic and protoplasmic leaflets exposing intramembrane particles representing integral proteins including band 3 anion exchanger, aquaporins, connexin gap junction hexamers, and rhodopsin. Platinum-carbon replica shadowing captures topography, original biological material dissolved with acid, replica examined by transmission EM revealing random or crystalline particle distribution, quantifiable density changes upon hormonal stimulation. While mitochondria cristae, nuclear pores, and even ribosome organization can be studied, classic application was plasma membrane demonstrating proteins embedded within lipid matrix, supporting Singer-Nicolson fluid mosaic, quantifying tight junction strand complexity, and visualizing exocytosis fusion rosettes, establishing direct structural evidence for mosaic protein arrangement.