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Text 3 The membrane model, which had been devoloped by Danielli & Davson, had been accepted by most scientist for many years. But in 1972 Singer & Nicolson proposed their own model, which they called the “fluid mosaic model.“ The proteins play an important role in their model. In their article they say: Membrane Proteins […] It seems both reasonable and important to discriminate between two categories of proteins bound to membranes, which we have termed peripheral and integral1 proteins. […] A peripheral protein is held to the membrane only by rather weak noncovalent (perhaps mainly electrostatic) interactions and is not strongly associated with membrane lipid. […] An integral protein molecule with the appropriate size and structure […] may transverse the entire membrane; that is, they have regions in contact with the aqueous solvent on both sides of the membrane. […] The [...] protein molecules are postulated to be amphipathic2 as are the phospholipids. That is, they are structurally asymmetric, with one highly polar end and one nonpolar end. 'The highly polar region [...] is in contact with the hydrophilic phase in the intact membrane; the nonpolar region is [...] embedded in the hydrophobic interior of the membrane. [...] It is clear from these considerations that different proteins, if they have the appropriate amino acid sequence3 to adopt an amphipatic structure, can be integral proteins of membranes; in this manner, the heterogeneity of the proteins of most functional membranes can be rationalized. Please summarize in your own words what Singer & Nicolson say about membrane proteins. 1. 2. 3. integral (= integrated into) – eingebaut, integriert amphipathic = amphiphilic 3 Proteins are makromolecules that consist of many different amino acids. The chemical properties (like e.g. polarity) and the shape of a protein are determined by its amino acids sequence. 1 2