Lipids are a heterogeneous family of substances classified according to their solubility qualities.

• They are insoluble in water but soluble in diethyl ether, acetone, and dichloromethane.

• The most prevalent lipids are triglycerides (triacylglycerols), which are triesters of glycerol and fatty acids.

• Fatty acids are long-chain carboxylic acids generated from the breakdown of animal fats, vegetable oils, and biological membrane phospholipids.

• The number of carbons in the chain and the amount of carbon-carbon double bonds present are used to identify fatty acids (e.g., 18:2)

• Almost all fatty acids have an odd number of carbon atoms, often between 12 and 20.

In naturally occurring fatty acids, carbon-carbon double bonds are virtually always present in the cis orientation.

A triglyceride's melting point rises as (1) the length of its hydrocarbon chains lengthens and (2) the degree of saturation rises (cis double bonds add kinks to the chains, reducing packing and decreasing melting points).

• Triglycerides high in saturated fatty acids are solids or semisolids at room temperature and are referred to as fats.

• Triglycerides high in unsaturated fatty acids are liquids at room temperature and are referred to as oils.

• Oils are partially hydrogenated (H2 is limited in the presence of a metal catalyst) to minimize certain unsaturated fatty acids.

Soaps are fatty acid sodium or potassium salts.

• Soaps create micelles in water, which "dissolve" nonpolar organic grease and oil.

• Because of the strong interaction between carboxylates and these ions, natural soaps precipitate as water-insoluble salts with Mg(II), Ca(II), and Fe(III) ions in hard water.

• Because metal ion salts of sulfonates are substantially more soluble in water, synthetic detergents employ sulfonates instead of carboxylates.

• Linear alkylbenzenesulfonates are the most prevalent and commonly used synthetic detergents.

Prostaglandins are a class of exceptionally physiologically active chemicals with a 20-carbon structure similar to prostanoic acid.

• Prostaglandins are produced in response to physiological cues from phospholipid-bound arachidonic acid and other 20-carbon fatty acids.

• This conversion is inhibited by aspirin and other nonsteroidal anti-inflammatory medications (NSAIDs).

Eicosanoids are a large family of natural lipids with 20 carbons that are produced from fatty acids and include prostaglandins, leukotrienes, thromboxanes, and prostacyclins, all of which are common and significant physiologically active chemicals.

Steroids are a class of plant and animal lipids with a distinctive tetracyclic structure made of three six-membered rings and one five-membered ring.

• Because of the common trans-anti-trans-anti-trans orientation of ring fusions, the steroid skeleton is rigid and relatively flat.

• Cholesterol is an integral part of animal membranes, and it is the compound from which human sex hormones, adrenocorticoid hormones, bile acids, and vitamin D are biosynthesized.

Low-density lipoproteins (LDLs) carry cholesterol from its manufacturing site in the liver to tissues and cells where it is needed.

High-density lipoproteins (HDLs) carry cholesterol from cells to the liver, where it is broken down into bile acids and excreted in the stool.

Oral contraceptives contain a synthetic progestin (for example, norethindrone), which prevents pregnancy.

Phospholipids, the second most frequent class of naturally occurring lipids, are formed from phosphatidic acids, which are glycerol esters esterified with two molecules of fatty acid and one molecule of phosphoric acid. A phospholipid is formed by esterifying the phosphoric acid portion with low-molecular-weight alcohol, most often ethanolamine, choline, serine, or inositol.

Phospholipids are the primary constituents of biological membranes. When phospholipids are introduced in water solution, they spontaneously form lipid bilayers.

According to the fluid mosaic model, membrane phospholipids form lipid bilayers with membrane proteins as both peripheral and integral proteins connected with the bilayer.

Vitamin A's most well-known job is in the visual cycle, where it is utilized to produce the photoactive component of rhodopsin, the light-sensitive pigment in our eyes.

Vitamin D is the term given to a group of structurally similar compounds that play an important role in calcium and phosphorus metabolism control. The action of UV light on 7-dehydrocholesterol results in the synthesis of vitamin D3 in the skin of animals. Vitamin E is a series of similar-structured chemicals, the most active of which is a-tocopherol. Vitamin E acts as an antioxidant in the body by trapping peroxy radicals.