Lipids are a variety of organic molecules grouped together on the basis of their solubility in nonpolar solvents. Their varied biological functions include energy source, energy storage, cell membrane structural components, hormones, vitamins, vitamin adsorption, protection, and insulation.

The spectrum of lipid functions can be condensed into the three broad areas of:
1. Storage of energy
2. Structure of cell membranes
3. Signal of chemical biological activities

The four main groups of lipids include:
1. Fatty acids (saturated and unsaturated)
2. Glycerides (glycerol-containing lipids)
3. Nonglyceride lipids (sphingolipids, steroids, waxes)
4. Complex lipids (lipoproteins, glycolipids)

Lipids can also be broadly subdivided as either saponifiable or nonsaponifiable.
Saponifiable lipids Nonsaponifiable lipids
Waxes (some)

Fatty Acids
The carboxylic acid products found in the saponifiable lipids are referred to as fatty acids. The fatty acids are long, unbranched monocarboxylic acids containing 10 to 22 carbon atoms. They typically have an even number of carbon atoms due to their biosynthetic pathway. The Fatty acids can be classified into families based on chain length and on the number of C=C double bonds present. Saturated fatty acids contain no C=C double bonds. (Saturated = bonded to the maximum number of hydrogens; will not accept any more hydrogen) Unsaturated fatty acids contain C=C double bonds. (These will accept more hydrogen.) The double bonds are typically cis.

Free fatty acids are rare in the cell; free fatty acids are somewhat toxic to cells. Fatty acids are typically found as components of larger lipid molecules. Free fatty acids are transported through the blood bound to serum albumin.

Common Saturated and Unsaturated Fatty Acids
Capric Decanoic  32 C9H19COOH CH3(CH2)8COOH
Lauric Dodecanoic 44 C11H23COOH CH3(CH2)10COOH
Myristic Tetradecanoic 54 C13H27COOH CH3(CH2)12COOH
Palmitic Hexadecanoic 63 C15H31COOH  CH3(CH2)14COOH
Stearic Octadecanoic 70 C17H35COOH CH3(CH2)16COOH 
Arachidic Eicosanoic 77 C19H39COOH CH3(CH2)18COOH

Palmitic acid and stearic acid, which have 16 and 18 carbons respectively, are the two most abundant saturated fatty acids.

Common Unsaturated Fatty Acids
# of
Double Bond Position
Palmitoleic cis-9-Hexadecenoic 0 C15H29COOH 1 9
Oleic  cis-9-Octadecnoic 16 C17H33COOH  1 9
Linoleic  cis,cis-9,12-Octadecadienoic 5 C17H33COOH  2 9, 12
Linolenic All cis-9,12,15-Octadecatrienoic -11 C17H31COOH  3 9, 12, 15
Arachidonic All cis-5,8,11,14-Octadecatrienoic -50 C19H31COOH  4 5, 8, 11, 14

Condensed Formulas
Palmitoleic CH3(CH2)5CH=CH(CH2)7COOH
Oleic CH3(CH2)7CH=CH(CH2)7COOH

Physical Properties of Fatty Acids:
1. Fatty acids are carboxylic acids and are therefore weak acids. For fatty acids, the value of pKa is around 4.5. Therefore, generally speaking, fatty acids are neutral below pH 4.5 and charged above pH 4.5.

2. Fatty acids do not dissolve in water; instead they are dispersed by the formation of micelles in water.

3. As the fatty acid carbon chain length increases, the melting point and boiling point increases.

4. Unsaturated fatty acids have lower melting points and boiling points than saturated fatty acids. This is because the cis C=C cause "kinks" in the hydrocarbon chains, which reduce the extent of association of the molecules. Less energy is required to separate the molecules.

Reactions of Fatty Acids
Fatty acids undergo the reactions typical of any carboxylic acid.
Fatty acids react with alcohols to form esters and water.

Acid Hydrolysis (reverse of esterification)

Saponification (Base hydrolysis)
The sodium or potassium salt resulting from the base hydrolysis of an ester of a fatty acid is referred to as a soap.

Reaction at the Double Bond (unsaturated fatty acids)

Hydrogenation is used in the food industry to convert polyunsaturated vegetable oils into saturated fats.

An eicosanoid is any of a group of structurally related hormonelike molecules derived from arachidonic acid. The principle function of the eicosinoids is to signal specific biological acitivities. For example, the prostaglandins are made in most tissues, and exert their biological effects on the cells that produce them and on other cells in the immediate vicinity. A hormone on the other hand, is made in one organ and transported to another, which it stimulates to function by its chemical activity.

The term "eicosanoid" is derived from the Greek word "eikos", meaning twenty, because they are all derivatives of 20 carbon fatty acids. The eicosanoids include prostaglandins, leukotrienes, and thromboxanes.

Arachidonic acid (5,8,11,14-eicosatetraenoic acid) is an unsaturated fatty acid, C20H32O2, found in animal fats, essential in human nutrition and is a precursor in the biosynthesis of prostaglandins.

Mammals can synthesize most of all the fatty acids they need. The two major exceptions are linolenic & linoleic acids. Mammals lack the enzymes required to introduce double bonds at carbon atoms beyond C-9 in the fatty acid chain. Fatty acids that cannot be made and must be supplied through diet are referred to as essential fatty acids. Both linolenic and linoleic are obtained from plants. Linoleic acid is required for the biosynthesis of arachidonic acid, the precursor of the eicosanoids.

Prostaglandins, leukotrienes and thromboxanes regulate, blood clotting, the inflammatory response, the reproductive system, the gastrointestinal tract, the kidneys, and the respiratory tract.

Prostaglandins are unsaturated carboxylic acids consisting of a twenty-carbon skeleton that contains a five-carbon ring. As stated earlier, prostaglandins exert their biological effects in the cells that produce them or on other cells in the immediate vicinity.

Leukotrienes are involved in allergic and inflammation responses.

Thromboxane A2 is a potent inducer of platelet aggregation.

Glycerides are lipid esters of the glycerol molecule and fatty acids. The primary function of the glycerides is energy storage. Glycerides can be subdivided into two categories. The first group, the neutral glycerides are nonionic and nonpolar. The second group, the phosphoglycerides contain a polar region, the phosphoryl group.

Both of these two types of glycerides can be seen as possessing a three carbon "backbone" of the glycerol molecule. Esterification of glycerol with a fatty acid produces a neutral glyceride.

Esterification may occur at one, two or all three positions producing monoglycerides (monoacylglycerols), diglycerides (diacylglycerols), or triglycerides (triacylglycerols). The most prevalent and most important are the triglycerides.

Triglycerides are named by combining the "backbone" name, glyceryl (from glycerol), with the fatty acid name. The prefix tri- is used because three stearic acid units are attached to the glycerol backbone. The fatty acid groups can be the same or different.

Normally, a fat is solid at room temperature, while an oil is liquid. The state depends on extent of unsaturation of the fatty acid components. The term, rancid, describes the oxidation of double bonds to yield volatile organic acids and aldehydes. Agents to protect fats from oxidation:BHA (butylated hydroxyanisole) BHT (butylated hydroxytoluene). Partial hydrogenation of double bonds can extend the shelf life of some fat containing products.

As stated earlier, triglycerides are efficient long-term energy storage molecules. When we need the chemical energy of the triglyceride stored in our fat tissue, the enzyme lipase catalyzes their complete hydrolysis. The fatty acid molecules, bound to proteins (albumins) in the blood, are then sent to the liver. Fat has 9 kcal/gram while sugar and amino acid molecules have 4 kcal/gram. Also, fats are anhydrous while sugars are hydrated. Anhydrous molecules represent more concentrated storage forms. Fat is also suitable for insulation and protection of internal organs.

Phosphoglycerides (phospholipids) are polar lipids, whereas triglycerides are neutral lipids. Another way of stating this is that phospholipids are amphipathic molecules due to the presence of both polar and nonpolar areas within the molecule. Amphipathic comes for Latin for "feeling both sides". (Soaps are amphipathic.) They are amphipathic due to the presence of a polar "head" and a nonpolar "tail".

Phosphoglycerides are also among the primary building blocks of all cellular membranes. Vital organs such as the liver and reproductive tract, and muscles contain high concentration of phosphoglycerides. Membrane functions include cellular transport of nutrients and wastes, internal cellular pressure regulation, and ion exchange.

Phosphoglycerides have the glycerol backbone structure as the neutral glycerides but differ in that one of the ester linkages results from a phosphoryl ester. In other words, they are composed of glycerol, 2 fatty acids and a phosphoryl ester group bonded to the third alcohol carbon of the glycerol backbone. The simplest phosphoglyceride is phosphatidate. It is rare in nature.

Phosphatidyl choline (lecithin) is a phosphoglyceride made from the glycerol backbone, two fatty acids, and a phosphoryl ester. The R group of the phosphoryl ester is choline.

Like lecithin, phosphatidyl ethanolamine (cephalin) is a phosphoglyceride made from the glycerol backbone, two fatty acids, and a phosphoryl ester. The R group of the phosphoryl ester in this case is ethanolamine, NH2CH2CH2-OH.

Nonglyceride Lipids
Sphingolipids though not derived from glycerol, can still be visualized as a three-carbon backbone molecule just as the triglycerides or the phospholipids. Instead of the three-carbon backbone of glycerol, the three-carbon backbone is sphingosine. Sphingosine is a nitrogen-containing alcohol (amino alcohol). They also are amphipathic, having a polar head group and two nonpolar fatty acid tails. They also are structural components of cellular membranes.

Sphingolipids contain the sphingosine backbone, the amide of a fatty acid, and a polar molecule. As suggested, the fatty acid portion of a sphingolipid is linked to the sphingosine molecule by an amide linkage.

The sphingolipids are divided into the two subcategories of sphingomyelins and the glycosphingolipids. The sphingomyelins are the only class of sphingolipids that are also phospholipids. The general structure of a sphingomyelin is shown below.

Glycosphingolipids, or glycolipids, are built on the backbone structure of ceramide, which is an amide derivative of sphingosine.

Subgroups of the glycolipids include the cerebrosides, the sulfatides, and the gangliosides. The cerebrosides are characterized by a single monosaccharide head group. Sulfatides are a sulfate group containing derivative of the cerebroside galactocerebroside. Gangliosides are glycolipids possessing oligosaccharide groups, including one or more molecules of N-acetylneuroaminic acid (sialic acid).

Stereroids are lipids with the principle function of signaling chemical biological activities. Steroids are members of a large, diverse collection of lipids called the isoprenoids. All of these compounds are built from one or more five-carbon units called isoprene.

Terpene is the general term for lipids that are synthesized from isoprene units. Thus the steroid and bile salts, the lipid-soluble vitamins, chlorophyll, and certain plant hormones are all terpenes.

Steroids have a "steroid nucleus" which is a fused four-ring system.

Cholesterol is the most abundant steroid, and it is the precursor for all the other important steroids of mammalian metabolism. It is amphipathic.

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Cholesterol is a lipid found in the body tissues and blood plasma of vertebrates. A steroid, cholesterol can be found in large concentrations in the brain, spinal cord, and liver. The liver is the most important site of cholesterol biosynthesis, although other sites include the adrenal glands and reproductive organs. By means of several enzymatic reactions, cholesterol is synthesized from acetic acid; it then serves as the major precursor for the synthesis of vitamin D3, of the various steroid hormones, including cortisol, cortisone, and aldosterone in the adrenal glands, and of the sex hormones progesterone, estrogen, and testosterone. Cholesterol is excreted from the liver in the form of a secretion known as bile; it sometimes crystallizes in the gall bladder to form gallstones.

The insolubility of cholesterol in water is also a factor in the development of atherosclerosis, the pathological deposition of plaques of cholesterol and other lipids on the insides of major blood vessels, a condition associated with coronary artery disease. This buildup of cholesterol in the blood vessels may constrict the passages considerably and inhibit the flow of blood to and from the heart. Recent research has shown that the relative abundance of certain protein complexes, called lipoproteins, to which cholesterol becomes attached may be the real cause of cholesterol buildup in the blood vessels. High-density lipoprotein (HDL) carries cholesterol out of the bloodstream for excretion, while low-density lipoprotein (LDL) carries it back into the system for use by various body cells. Researchers believe that HDL and LDL levels in the bloodstream may be at least as important as cholesterol levels, and now measure both to determine risk for heart disease. Reducing consumption of foods containing cholesterol and saturated fat has been found to lower blood cholesterol levels. Cholesterol levels can also be reduced with drugs, most especially with HMG-CoA reductase inhibitors (commonly called "statins"), such as lovastatin (Mevacor) and atorvastatin (Lipitor), and by regular exercise.

Bile Salts
Bile salts are amphipathic, excretion products of cholesterol. Mammals lack the enzymes to break down the steroid nucleus. The liver makes cholesterol more water soluble as a bile acid and excretes it. Strategy: nonpolar molecules are retained longer in the body, while polar molecules pass through quicker.

Steroid hormones
Four families of stereoid hormones include estrogens, androgens, progestins, and corticosteroids.

Estrogens prepare the female reproductive system prior to ovulation. The androgen testosterone is an anabolic steroid. Progestin maintains the female reproductive system after ovulation. An example of an adrenocorticoid is cortisol which is an anti-inflammatory agent used to treat skin inflammations, arthritis, and asthma.

Waxes are mixtures comprising chiefly esters of long chain monohydroxy alcohols and long chain fatty acids. [Paraffin wax is a mixture of solid hydrocarbons (normally straight-chain).] Waxes differ from fats in that fats contain chiefly esters of glycerol. Waxes are generally harder and less greasy than fats, but like fats they are less dense than water and are soluble in alcohol and ether but not in water. Due to their high molecular weights, waxes are generally solids at room temperature. Waxes are found naturally as coating on fruits, leaves, insect exoskeleton (water retaining). Birds have glands producing wax for feathers (water repelling).

One component of beeswax is myricin (myricyl palmitate, CH3(CH2)14COO(CH2)12CH3). Myricyl palmitate is a saturated 16 carbon fatty acid esterified to a 30 carbon alcohol.

Another naturally occurring wax is one of the many components of whale oil, spermacetti (cetyl palmitate CH3(CH2)14CO2(CH2)15CH3). It is a saturated 16 carbon fatty acid esterified to a 16 carbon alcohol.

Complex Lipids
Complex lipids are bonded to other types of molecules. Because lipids are mostly insoluble in water, the movement of lipids from organ to organ through the bloodstream is facilitated by plasma lipoproteins.

There are 4 major classes of plasma lipoproteins.
1. Chylomicrons [density = < 0.95 g/mL]
        Function: carry dietary triglycerides from intestines to other tissues
2. VLDL (very low density lipoproteins) [density = 0.95-1.019 g/mL]
        Function: carry triglycerides from liver
3. LDL (low density lipoproteins) [density = 1.019-1.063 g/mL]
        Function: carry cholesterol to peripheral tissues
4. HDL (high density lipoproteins) [density = 1.063-1.210 g/mL]
        Function: carry cholesterol peripheral tissues to live