determined by this association with a nonaqueous environment. This notion set off a train of ideas and experiments that eventually led us to the fluid mosaic model. (Singer, 1992, p.3)At the time Singer's train set off, the standard model for membrane structure, the Davson-Danielli-Roberston (DDR) model, was a bilayer of lipids sequestered between two monolayers of unfolded protein (Figure 1). Each protein layer faced an aqueous environment, cytoplasm or interstitial fluid, depending upon whether the membrane enclosed an organelle or the cell itself (Figure 1; Singer, 1992).Figure 1: Original figure from Singer (1992) illustrating the Davson-Danielli-Robertson model of the plasma membrane. Notice that the lipid bilayer is isolated from the surrounding aqueous environment by two layers of unfolded membrane protein (p). Each membrane forming lipid is composed of a polar head group (h) and fatty acyl tail (f). Text added. Figure 1 (Davson Danielli Robertson model of membrane.jpg)When Singer and colleagues applied their understanding of the influence of solvent environment on protein conformation specifically to the problem of membrane proteins, they realized the DDR model was energetically untenable. As he and Nicolson (1972) later wrote,The latter [DDR] model is thermodynamically unstable because not only are thenon-polar amino acid residues of the membrane proteins in this model perforce [by circumstance] largely exposed to water but the ionic and polar groups of the lipid are sequestered by a layer of protein from contact with water. Therefore, neither hydrophobic nor hydrophilic interactions are maximized in the classical [DDR] model. (Singer and Nicolson, 1972, p.721)That is, the DDR model was energetically unfeasible because the constitutive molecules could not stably persist in aqueous cytoplasm in the physical conformation proposed. Just as oil and water will spontaneously separate when left to stand after shaking, the hydrophilic and hydrophobic components of a single polypeptide or an entire cell will spontaneously organize so that hydrophilic elements are in contact with the aqueous environment and the hydrophobic elements are sequestered, isolated from contact with polar components. Thus, they reasoned that membrane proteins in a cell will assume globular (folded) conformations, due to hydrophobic and hydrophilic amino acid residues interacting with each other and the solvent environment, not the unfolded structures suggested by the DDR model. Similarly, membrane proteins will not be positioned to prevent contact between the polar head groups of membrane lipids and the aqueous cytoplasm. So, if membrane proteins are globular and not layered on top of the membrane, where are they? How are they associated with the membrane? The mosaic element of Singer and Nicolson's (1972) fluid mosaic model answered these questions. According to this model, membrane proteins come in two forms: peripheral proteins, which are dissolved in the cytoplasm and relatively loosely associated with the surface of the membrane, and integral proteins, which are integrated into the lipid matrix itself, to create a protein-phospholipid mosaic (Figure 2; Singer and Nicolson, 1972). Figure 2: Original figure from Singer and Nicolson (1972) depicting membrane cross section with integral proteins in the phospholipid bilayer mosaic. Phospholipids are depicted as spheres with tails, proteins as embedded shaded, globular objects. Peripheral proteins,