| << Chapter < Page | Chapter >> Page > |
Although the bilayer nature of the cell membrane was described in the mid-1920's , it was not until 1972 that the currently accepted model of the plasma membrane, the fluid mosaic model , was formally outlined by S. J. Singer and Garth L. Nicolson in the journal Science .
Singer’s work on membrane structure originated in the 1950’s when he, along with other protein chemists, demonstrated that many water-soluble proteins like those found in cytoplasm could unexpectedly dissolve in nonaqueous, non-polar solvents. Furthermore, the shape a protein assumed differed in hydrophobic and hydrophilic environments (Singer, 1992).
From an historical perspective these results are significant because they led Singer to wonder about the structure of the proteins revealed to be closely associated with lipid-rich, and therefore nonaqueous, cell membranes in the 1930’s (Eichman, 2007). As he later wrote,
Although we had not experimented with membrane proteins and knew very little about membranes at the time,
as almost an aside we speculated [in a 1962 publication]that because “the cellular environment of many
proteins contains high concentrations of lipid components in a wide variety of cellular membranes, thegross conformations of these proteins in situ may be determined by this association with a nonaqueous
environment.” This notion set off a train of ideas and experiments that eventually led us to the fluidmosaic 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).
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 aremaximized 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.
Notification Switch
Would you like to follow the 'Discovering the structure of the plasma membrane' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|