What did transmission electron microscopy reveal about the core of the plasma membrane in the 1950s?

1. Advances in microscopy in the 1950s, notably transmission electron microscopy, sharpened the view of the membrane core. 2. Researchers saw that the core consisted of a double layer rather than a single layer. 3. That observation is what the 1935 sandwich model could not account for, and it drove the search for a better model. 4. Sugar chains and membrane fluidity were established by other lines of evidence, and proteins are in fact a principal component. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 5 "Structure and Function of Plasma Membranes", section 5.1 Components and Structure_