H. Davson and J. Danielli, in 1935, proposed a membrane structure that was refined by Robertson into the "unit-membrane" model, which has served as the basis for membrane structure theories for many years. The unit-membrane model is diagrammed above: a lipid bilayer core coated on either side with proteins. The inside of the membrane in this model is strongly hydrophobic and the outside is polar.

Lipid molecules are held together only by hydrophobic interactions, and lipid and protein molecules are held by hydrophobic forces, plus possible charge attractions between lipid heads and side groups on the proteins. No covalent bonds are assumed to exist between molecules, since membranes can be taken apart gently by solvents and then reconstituted in an apparently intact form. The dimensions of the Davson-Danielli unit membrane correspond to those observed in electron micrographs.