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
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.