<P> As support for the lipid bilayer membrane theory grew, this alternative concept was developed which denied the importance of the lipid bilayer membrane . Procter & Wilson (1916) demonstrated that gels, which do not have a semipermeable membrane, would swell in dilute solutions . Loeb (1920) also studied gelatin extensively, with and without a membrane, showing that more of the properties attributed to the plasma membrane could be duplicated in gels without a membrane . In particular, he found that an electrical potential difference between the gelatin and the outside medium could be developed, based on the H+ concentration . </P> <P> Some criticisms of the membrane theory developed in the 1930s, based on observations such as the ability of some cells to swell and increase their surface area by a factor of 1000 . A lipid layer cannot stretch to that extent without becoming a patchwork (thereby losing its barrier properties). Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties . In 1938, Fischer and Suer proposed that water in the protoplasm is not free but in a chemically combined form, and that the protoplasm represents a combination of protein, salt and water . They demonstrated the basic similarity between swelling in living tissues and the swelling of gelatin and fibrin gels . Dimitri Nasonov (1944) viewed proteins as the central components responsible for many properties of the cell, including electrical properties . </P> <P> By the 1940s, the bulk phase theories were not as well developed as the membrane theories and were largely rejected . In 1941, Brooks & Brooks published a monograph The Permeability of Living Cells, which rejects the bulk phase theories . </P>

Who proposed that cell membranes are phospholipid bilayers