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Complex Lipids Complex Lipids (Phospholipids & Sphingolipids) = Lipids containing parts other than or including fatty acids and glycerol. Found in cell membranes, brain/ nervous tissues, myelin sheaths of nerves and blood platelets Phosphoglycerides • A phospholipid is built from long chained fatty acids on a glycerol backbone attached to a phosphoric acid molecule containing an alcohol substituent. (usually an amino alcohol) Fatty acids and alcohols vary. Important Phosphoglycerides •Lecithins - Contain the amino alcohol choline. - Important cell membrane component. - Easily form micelles. •Cephalins - Contain ethanolamine or serine as the alcohol. - Found in most cell membranes, especially brain tissue and in blood platelets where they aid in clotting. Structural formula of a phosphatidylcholine molecule containing stearic acid (18:0) and oleic acid (18:1). Molecular model showing the “head and two tails” structure of a phosphatidylcholine molecule containing stearic acid (18:0) and oleic acid (18:1). 1 Sphingolipids •Sphingolipids are a class of lipids built from long chained fatty acids attached to a sphingosine backbone rather than glycerol. •There are two types of sphingolipids: - Sphingomylins - Glycolipids • Glycolipids contain carbohydrates unit. - They are often called cerebrosides because of their abundance in brain tissue. •Sphingomyelin contains a phosphate and choline group. - It is found in the myelin sheath surrounding nerve cells. Several human diseases are known to result from abnormal accumulation of Sphingomyelins and glycolipids due to the inherited absence of enzymes needed to break down these complex lipids. A typical cerebroside contains an α– D-glucopyranose or β–D-glactopyranose unit attached to the terminal sphingosine carbon via an ether bond. Notice how complex lipids are polar molecules. Each has a relatively polar head and a nonpolar tail(s) Complex lipids are found in cell membranes, liposomes and the myelin sheaths of nerve cells. Cell Membranes • Prokaryotic Cells – simple unicellular organisms with no internal membrane structures. • Eukaryotic Cells – cells containing membrane – enclosed organelles, particularly a nucleus. 2 Membrane/liposome Structure • Lipids are organized in a bi-layer with hydrophobic (long carbon chain) portions inside and hydrophilic (polar groups) exposed to the water environment. Cross section of a lipid bilayer. The circles represent the polar heads of the lipid components, and the wavy lines represent the nonpolar tails of the lipid components. The “heads” occupy surface positions, and the “tails” occupy internal positions. The kinks associated with cis double bonds in fatty acid chains prevent tight packing of the lipid molecules in a lipid bilayer. Fluid Mosaic Model of Membrane Structure Proteins are important structural components of cell membranes. •Lipids are free to move past one another across the membrane Integral vs. perpheral 3 • Glycolipids present in the lipid bi-layer decrease the rigidity of the cell membrane. • Cholesterol (a steroid) present in the lipid bi-layer increases the rigidity of the cell membrane 4