Practice question

Early electron microscopy after osmium fixation revealed trilaminar image of membranes about 7.5 nm thick: two dark lines 2 nm each representing protein stained by osmium, light core of lipid. J. David Robertson at Harvard analyzed myelin, chloroplast, and plasma membranes and in 1959 proposed Unit Membrane Model asserting all biological membranes share same basic construction of lipid bilayer sandwiched between non-lipid layers, with asymmetry that inner and outer surfaces differ chemically, explaining functional polarity. Robertson emphasized continuity of membranes through ER and nuclear envelope and suggested proteins extended as sheets. This unified view improved over Davson-Danielli paucimolecular sandwich which proposed protein layers without EM proof, and provided terminology unit membrane widely used in histology. Although later Singer and Nicolson showed proteins are globular amphipathic entities embedded within fluid lipid phase rather than extended coats, Robertson's model was pivotal for recognizing universal bilayer organization and inspiring later fluid mosaic concept, placing bilayer as central architectural principle.