Associate w/ membrane in various ways depending on function: transmembrane, integral, peripheral. By mass proteins represent 50% lipids 50% lipids small thus 1 protein/ 50 lipid molecules. Amts and types of proteins highly variable. Give ea type of membrane characteristic functional properties. Proteins Carry Out Specific Functions of Membrane. Function- protection, cell recognition, transmission of electrical impulses. gangliosides= most complex, sialic acid containing oligosaccharides, net negative chg, most abundant in pm of nerve cells. Found exclusively on noncytoplasmic monolayer. Sugar containing lipid molecules w/ most extreme assemmetry in distribution. Impt to function (ie, apoptosis and translocation of phosphatidylserine). Phospholipid distribution: phosphatidylcholine and sphingomyelin confined to outer monolayer phosphatidylethanolamine and phosphatidylserine are on inner monolayer. able to accommodate membrane proteins concentrating for transport or to enable proteins to function together. Long saturated FA chain of sphingolipids = attractive forces that hold adjacent molecules together. Microdomains enriched in sphingolipids, cholesterol and membrane proteins. The Lipid Bilayer Major Membrane Phospholipids No cholesterol in procaryotes mechanical stabililty imparted by cell wall. 
1 cholesterol/phospholipid in eucaryotes.Fluidity and cholesterol content Cholesterol fluidity.Short hydrocarbon chain lengths fluidity.The Lipid Bilayer Fluidity and length and saturation of FA hydrocarbon chains Phase transition= the temperature at which there is a change of state from liquid to solid.Fluidity depends on composition (phospholipid and cholesterol) and temperature.Rapid lateral diffusion of phospholipids 10-8cm/sec.The Lipid Bilayer Fluidity of Lipid Bilayer
/cell-membrane--artwork-547999983-5a942e7a875db900363afbf1.jpg "download cell membrane structure")
Spontaneously aggregate to bury hydrophobic tail.Phospholipids= most abundant lipid in membrane.50% of total membrane mass, 5 x 106/um2.Proteins mediate all other functions of membrane.lipid bilayer serves as basic fluid structure impermeable barrier.Thin film of lipid and protein held together by noncovalent interactions.Contain proteins act as sensors of external signals allow cell to adapt to changing environment.Plasma membrane defines cell and maintains differences btwn cytosol and extracellular environment.
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