Some of the earliest experiments involving the interactions of lipids and water to explain the properties of membranes were performed towards the end of the 19th century, on a kitchen table in Germany by Agnes Pockels. Agnes studied the behaviour of oil poured onto water in a flat dish, identifying the in fluence of impurities on the surface tension of fluids. She sent her results to Lord Rayleigh, who was sufficiently impressed to get them published in the scientific journal Nature in 1891. In 1932,Irving Langmuir, working in New York, won a Nobel Prize for showing that lipids spread on water produce a layer only one molecule thick and that all the molecules were orientated the same way. This happens because one end of the lipid molecule is attracted to water (it is hydrophilic), and the other end is repelled(it is hydrophobic). Each lipid molecule is shaped like an old-fashioned wooden clothes peg, with the top of the peg being the hydrophilic end, and the two legs of the peg representing the hydrophobic region. All the pegs float on the surface of water head down, legs uppermost, forming a monolayer. In 1925, Evert Gorter and James Grendel isolated membranes from red blood cells, finding that membrane was made up of two layers (a bilayer) of lipids, making a sandwich with the hydrophilic peg heads on the outsides, and the hydrophobic legs on the inside. As there is water both inside and outside the cell, this arrangement is maintained, keeping both hydrophobic surfaces together on the inside of the membrane bilayer. This arrangement was directly visualized decades later with the advent of electron microscopy, where thin sections at high magnification showed membranes as two dark lines separated by a light region between them (see, for example, Figures 4 and 14c). In 1935, James Danielli and Hugh Davson suggested that this lipid bilayer was covered on both sides by a layer of proteins, a model that lasted until 1972 when Seymour Singer and Garth Nicholson suggested that the proteins could also be threaded through the lipid layers, and project from either side of the membrane. Proteins with this confi guration are termed transmembrane proteins, and a single protein might weave its way through the lipid bilayer several times. The lipid molecules in a membrane are highly mobile, constantly moving past one another and the membrane proteins, leading to the description of the ‘fluid mosaic’ membrane. This activity of the lipids was neatly demonstrated by Michael Edidin in 1970, when he labelled the membrane lipids of two different cell types with either green or red fluorescent chemicals. This gave a ‘patchwork’ of red and green labelling from individual cells grown together in a mixed cell culture. Edidin then added viruses to the culture which caused the membranes of adjacent cells to fuse to each other, thus mixing their membrane lipids and, within an hour, all red and green patches of fluorescence were replaced by an overall orange labelling, showing complete mixing of the individually labelled lipid molecules within the fused membranes.
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