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Anonymous asked: What evidence is there for (plasma) membrane fluidity?
Like most ideas in cell biology, the fluid mosaic model of the cell membrane evolved from many previous models as new evidence and techniques came to light. Initially the bilayer model was proposed by Gorder and Grendel based on membrane from red blood cells and measuring how their area was spread out. Later, freeze-fracturing the cells and using electron microscopy brought a new generation of models.
Science publishedthe model in question 18 February 1972.
The Fluid Mosaic Model of the Structure of Cell Membranes by Singer and Nicolson
“In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent.”
The researches use a variety of evidence to support their argument including isotope labeling and antibody labeling. Modern experiments can demonstrate this phenomenon using GFP and photobleaching (ref).