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CHY 204 (10)
Chapter 10

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Ryerson University
CHY 204
Mario Estable

CHAPTER 10: LIPIDS -Lipids are chemically diverse compounds & their common & defining feature is their insolubility in water. Eg. Fats/oils (principal forms of storage), phospholipids/sterols (major structural elements of membranes), enzyme co-factors, electron carriers, light- absorbing pigments, hormones, chaperones (hydrophobic anchors for pro to help pro fold), emulsifying agents in GI tract & intracellular messengers. 10.1 Storage lipids -fats & oils universally used as stored forms of energy are derivatives of FA which are hydrocarbon derivatives, at ~the same low oxi state, ie. Highly reduced, as hydrocarbons in fossil fuel. The cellular oxi of FACO2 + H2O is highly exergonic (-ΔG). -FA are –COOH acids w/ hydrocarbon chains rangin from 4-35 C. Simplified nomenclature of unbranched FA specify chain length : # of double bonds, eg. 16:0palmitic acid, 18:1 (Δ^9)oleic caid. For polyunsat FA, an alt convention numbers C in the opp direction, assigning 1 to CH3 C at the other end of the chain which is designated as w (omega); the posn of = bonds are indicated relative to w C. -Types: Saturated FA, monounsat FA (usually Δ^9), polyunsat FA (usually Δ^9, Δ^12 & Δ^15). Most common naturally-occuring FA have even #of C’s & are unbranched chains of 12-24 C’s. Synthesis of FA occur via condensation of 2C acetate. Nearly all naturally occurring unsat FA have = bonds in cis configuration; = bonds of PUFA are almost never conjugated, they have alternating single/double bonds but are separated by methelene group. -Dietary w-6 PUFA:dietary w-3 PUFA should be 1:1 to 4:1 in the diet but NA diets is 10:1 to 30:1 ratio which may be why there is a high prevalence of heart disease & stroke. Fish oils are rich in: EPA (Eicosapentanoic acid; EPA; 20:5 (Δ^5,8,11,14,17)) & DHA (docosahexaenoic acid; DHA; 22:6 (Δ^4,7,10,13,16,19). EPA & DHA are made from ALA, alpha-linolenic acid; w-3; 18:3 (Δ^9,12,15) which is req in diet b/c humans can’t synthesisze it; can get it from oils, nuts, soybeans, etc. -The physical properties of FA & compounds that contain them are largely determined by degree of length & degree of unsaturation of the hydrocarbon chain; the nonpolar hydrocarbon chain accts for poor solubility in H2O; the longer the fatty acyl chain & the fewer = bonds, the lower the solubility in H2O. Mp are also strongly influenced by length & degree of unsaturation in hydrocarbon chain; at room temp (25°C), sat FA from 12:0 to 24:0 have a waxy consistency vs. unsat FA of these lengths are oily. The COOH is polar which accts for some solubility thus shorter FA have inc solubility in H2O; COOH ionizes at pH 7. This diff in mp are due to diff degrees of packing in FA acid molec. In fully sat compounds, free rotation around each C-C bond gives hydrocarbon chain great flexibility & the most stable form is the fully extended form in which steric hindrance of neighbouring atoms is minimized; these molec can pack together tightly in nearly crystalline arrays w/ atoms along their lengths in van der Waals contact w/ atoms of other molecules. In unsat FA, a cis = bond forces a kink in the hydrocarbon chain, FA w/ more than 1 kinks can’t pack together as tightly as fully sat FA & interactions w/ each other are therefore weaker. B/c less thermal energy is needed to disorder these poorly ordered arrays of unsat FA, they have much lower mp than sat FA of the same lengths. -TAGs (triacylglycerols): free FA have low solubility which can be carried by albumin; FA are mostly present as carboxylic acid derivatives & derivatives w/ glycerol are TAGs. TAGs are the simplest lipids constructed from FA, they are carboxylic acid derivatives. It composes of a glycerol backbone + 3 FA that are ester-linked; simple TAGs have same 3 FA & complex/mixed TAGs have diff FA. B/c the polar hydroxyls of glycerol & polar cho of the FA are bound in ester linkages, TAGs are nonpolar, hydrophobic molecules essentially insoluble in H2O; lipids have lower specific gravity in water which explains why mix of oil & water have 2 phases, oil floats. When glycerol has 3 diff FA at C1 & C3, C2 is the chiral center. -TAGs serve as storage fuel. In vertebrates, specialized cells called adipocytes store large amts of TAGs as fat droplets that nearly fill the cell. Adipocytes & germinating seeds contain lipases which are enzymes that hydrolyze TAGs to FA for export to sites where they are req as fuel. Adv to using TAGs as stored fuels is: C atoms of FA are more reduced than those of sugars & oxidation of TAG yield 2x the energy; TAGs are hydrophobic thus unhydrated so the organism that carries fat as fuel doesn’t have to carry the extra weight of H2O of hydration assoc’ w/ polysachhs. Cotyledon cests have pro bodies surrounded by stored oils; the part of the seed that becomes 1 leaves. -Sperm whales: fatheads of deep. ~90% of the weight of head is made up of spermaceti organ which is a bluberry mass that contains up to 3,600kg off spermaceti oil, a mix of TAGs & waxes containing an abundance of unsat FA. Mixture is liquid at normal body temp (37°C) but crystallizes & becomes solid when temp drops 31°C. These mammals feed almost exclusively on squid in very deep water, undergoes changes in buoyancy to match the density of its surrounding to be able to remain at a given depth w/o constant swimming effort. When the temp of oil drops during a deep dive, oil crystallizes & becomes denser; as whale returns to the surface, the oil warms & melts dec density to match surface of water. -Humans have fat tissue, composed primarily of adipocytes, under the skin, in abdominal cavity & m
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