Lipid structure is composed of more fundamental molecules--if you know the simple structures, it is relatively easy to put them together into the complex structure.

Lipids are composed of hydrophobic moieties which are not soluble in water. Some, like fats and oils are primarily storage forms of energy. Some have additional hydrophilic moieties which make the molecule amphipathic and useful for structure and signaling.

Fats and oils:

Two or three fatty acids esterified to glycerol.

Phospholipid:

Fatty acids    +      Glycerol      ±       Charged moiety
(controls bulk        (backbone)         (controls chemical                            
phase properties)                          specificity)

Cholesterol:

Rigid, planar steroid with polar hydroxyl

Sphingolipids:

Ceramide (fatty acid amide-bonded to amino fatty alcohol) esterified to phosphorylcholine (sphingomyelin) or in beta-glycoside with sugars (glycosphingolipid). Sphingomyelins are found in brain and nerves. Notice the lack of a glycerol in the structure.

The fatty acids normally contain an even number of carbon atoms (e.g. 14, 16, 18 or 20). Those without C=C double bonds are saturated, those with C=C double bonds are unsaturated. Double bonds in unsaturated fatty acids are normally separated by three carbon atoms (-C=C-C-C=C-) and are not conjugated (-C=C-C=C-). One notable exception is conjugated linoleic acid which is being intensively studied at present for possible health benefits.

Double bonds in fatty acids are normally in the cis configuration and keep the fatty acyl chain from assuming a linear configuration. Unsaturated fatty acids with trans bonds are linear. Unsaturation increases the fluidity of lipids (decreases the temperature at which they solidify).

Fatty acids with one double bond are monounsaturated. Fatty acids with multiple double bonds are polyunsaturated. Many food products are made more solid (less fluid) by decreasing the number of double bonds in constituent fatty acids by hydrogenation.

The concentration of free fatty acids in cells is very low. Free fatty acids are essentially soaps and will dissolve membranes and cause major problems.

Many common fatty acids are designated by common names.

C18:0 is stearate, a saturated fatty acid

C18:1 (9) is oleate, with a double bond at C-9

C18:2 (9,12) is linoleate, a polyunsaturated fatty acid with double bonds at C-9 and C-12

C18:3 (9,12,15) is linolenate, a polyunsaturated fatty acid with double bonds at C-9 , C-12 and C-15

The first carbon adjacent to the carboxyl is termed the alpha carbon. The last carbon is termed the 'omega' carbon. Since the last double bond in linoleate (18 carbons) is at C-12, this is also termed an omega-6 fatty acid. Similarly, linolenate, with a double bond at C-15, is an omega-3 fatty acid. Humans and other animals need to obtain polyunsaturated fatty acids from the diet, since they can't make them. These are essential fatty acids. These essential fatty acids are used to produce leukotrienes, prostaglandins and thromboxanes as well as long-chain fatty acids (24 to 34 carbons) who's specific biochemical role remains obscure. Linolenic acid is found in green leaves, whereas linoleic acid is in highest concentration in seeds (see linoleic acid content of corn, safflower, soybean and sunflower seeds on p. 241). The USDA RDA for these essential fatty acids is 1 to 2% of the total caloric intake. Lack of these essential fatty acids from the diet can cause a male's testicles to atrophy. They are also essential for brain development and vision.

Fish oils contain high levels of omega-3 fatty acids. Diets rich in omega-3 fatty acids appear to reduce plasma triglyceride levels and diminish blood clots in the vascular system, but don't seem to lower levels of LDL cholesterol.

NEUTRAL LIPIDS	AMPHIPATHIC LIPIDS
storage form of energy	chemically diverse
source of high energy content	many structures = many functions
thermal insulation	phosphatadic acid (-)
major form present in mammals; tends to be relatively saturated	phosphatidyl glycerol (-)
diacylglycerols	phosphatidyl choline (-/+) (lecithin)
triacylglycerols	diphosphatidylglycerol (-/-) (cardiolipin)
	phosphatidyl ethanolamine (-/+) (cephalin)
	phosphatidyl serine (-/-/+)
	sphingomyelin (-/+)

Lipase--an enzyme that hydrolyzes diacylglycerols and triacylglycerols, producing free fatty acids. When accomplished by chemical means (heating in acid or base) this hydrolysis is called saponification.

Phospholipase--an enzyme that hydrolyzes phospholipids. Phospholipases are common components in venoms since they produce amphipathic products that hydrolyze membranes and cause hemolysis.

Waxes are esters of a fatty acid and a fatty alcohol.

Terpenes or isoprenoids--an extremely interesting and diverse class of molecules derived from isopentyl pyrophosphate, a five-carbon molecule. This class of molecule includes flavors (menthol), fragrances (pinene), pigments (carotenoids), vitamins (K and E), bile salts and steroids. The pathway for forming and utilizing isopentyl pyrophosphate is shown on p. 837. Isoprenoids are often classified based on the degree of polymerization of the basic 5-carbon isoprene unit. Monoterpenes have 10 carbons (e.g. menthol and geraniol), sesquiterpenes have 15 carbons, diterpenes have 20 carbons (phytol and vitamin A), triterpenes have 30 carbons (e.g. squalene) and tetraterpenes have 40 carbons (e.g. carotenoids).