Before discussing the fluid mosaic model, a brief introduction on biological membranes might be helpful. The
lipid bilayer is an interesting structure formed due to the
amphipathic nature of
phospholipids. Because phospholipids have long,
hydrophobic domains, they choose to sequester these from
water through the formation of large planar structures with a
nonpolar core sandwiched between the polar headgroups:
oooooooooooooooo - polar headgroup \_outer leaflet
|||||||||||||||| - nonpolar tail /
|||||||||||||||| \_inner leaflet
oooooooooooooooo /
For simplicity, the polar headgroups are each drawn with one tail, but the phopholipids each have two tails coming from the polar headgroup.
As membranes often form the surface of cells, or act as walls separating compartments within the cell, it is important for materials to pass through them.
Proteins are often found as an integral component in membranes. It was found that proteins could be embedded in the membrane and could translate laterally across the membrane surface, much like a boat on the surface of the water. Additionally, these proteins were found on one surface of the membrane or the other (e.g. the inner or outer leaflet), but very rarely was a protein observed to flip from one side of the membrane to the other.
These observations led to the postulation of the fluid mosaic model by Singer and Nicholson in 1972. This stated that globular proteins were an integral part of the membrane and some were found on one side or the other, while others penetrated all the way through:
V V
oooooooo#oooo#oooooo
||||||||#||||#||||||
||||||||#|||||||||||
oooooooo#ooooooooooo
^
Integral proteins were also believed to be amphipathic, with the hydrophobic regions buried in the membrane and the
hydrophilic regions exposed to the inside of the cell or the outside environment.
Other components of the fluid mosaic, including
sterols, also partition into the membrane depending on their surface characteristics. Nonpolar molecules such as
cholesterol insert themselves into the hydrophobic center of the membrane.
The fluid mosaic model has provided a conceptual basis on which to understand how transmembrane proteins and membrane dependent cellular processes function. The macroscopic nature of the mosaic is still being debated. Is the membrane a random soup of lipids and proteins? or are there domains of structure (known as
lipid rafts) within the membrane that impose higher order structure?