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 FACO2 + 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.
16:0palmitic 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
-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