What is the fluid-mosaic model of the cell membrane? Describe the various components of the cell membrane that exist according to this model, commenting, especially, on the role of proteins in the membrane. Comment also on the non-static nature of the membrane: which components can move freely within the membrane, which cannot?
The fluid-mosaic model for the structure of the cell membrane holds that the membrane consists of a protein-embedded bilayer of phospholipids, where the lipids hydrophobic tails are pointed toward the center of the membrane and their hydrophilic heads toward the surfaces of the membrane. The proteins embedded within the lipid bilayer can exist in several different positions: extending through both layers, projecting out one side or the other, or totally contained within the bilayer. Those proteins that extend entirely through the lipid layers often form pores, allowing substances to pass through the membrane; the amino acid structure of the proteins gives the pores selectivity in determining what substances can pass through. Those proteins that project from the exterior of the membrane often act as enzymatic receptor sites. In addition, the fluid-mosaic model holds that the structure of the membrane is not static. But, whereas lipid molecules, which are linked together only by weak bonds, can move freely within their layers, proteins have much less freedom
