LIPIDCOURSENOTES-shortcut2005 - LIPIDS – COURSE NOTES...

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Unformatted text preview: LIPIDS – COURSE NOTES (shortcut) © Dr. Ya ş ar Kemal Erdem 2004-2005/Fall, Hacettepe University Chapter 1. INTRODUCTION WHAT IS A LIPID; DEFINITIONS Although lipid analysts tend to have a firm understanding of what is meant by the term "lipid", there is no widely accepted definition. General textbooks usually describe lipids in woolly terms as a group of naturally occurring compounds, which have in common a ready solubility in such organic solvents as hydrocarbons, chloroform, benzene, ethers and alcohols. They include a diverse range of compounds, like fatty acids and their derivatives, carotenoids, terpenes, steroids and bile acids. It should be apparent that many of these compounds have little by way of structure or function to relate them. In fact, a definition of this kind is positively misleading, since many of the substances that are now widely regarded as lipids may be almost as soluble in water as in organic solvents. A more specific definition than one based simply on solubility is necessary, and most scientist active in this field would happily restrict the use of "lipid" to fatty acids and their naturally occurring derivatives (esters or amides). The definition could be stretched to include compounds related closely to fatty acid derivatives through biosynthetic pathways (e.g. prostanoids, aliphatic ethers or alcohols) or by their biochemical or functional properties (e.g. cholesterol). Our definition is; "Lipids are fatty acids and their derivatives, and substances related biosynthetically or functionally to these compounds." This treats cholesterol (and plant sterols) as a lipid, and could be interpreted to include bile acids, tocopherols and certain other compounds. It does not include such natural substances as steroidal hormones, petroleum products, all fat-soluble vitamins, carotenoids or terpenes, except in rare circumstances. If "lipids" are defined in this way, fatty acids must be defined also. They are compounds synthesised in nature via condensation of malonyl coenzyme A units by a fatty acid synthase complex. They usually contain even numbers of carbon atoms in straight chains (commonly C14 to C24), and may be saturated or unsaturated, and can contain a variety of substituent groups. The most common lipid classes in nature consist of fatty acids linked by an ester bond to the trihydric alcohol -glycerol, or to other alcohols such as cholesterol, or by amide bonds to sphingoid bases, or on occasion to other amines. In addition, they may contain alkyl moieties other than fatty acids, phosphoric acid, organic bases, carbohydrates and many more components, which can be released by various hydrolytic procedures. A further subdivision into two broad classes is convenient for chromatography purposes especially. Simple lipids are defined as those that on hydrolysis yield at most two types of primary product per mole; complex lipids yield three or more primary hydrolysis products per mole. Alternatively, the terms "neutral" and "polar" lipids respectively are used to per mole....
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LIPIDCOURSENOTES-shortcut2005 - LIPIDS – COURSE NOTES...

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