Lipids are naturally occurring organic molecules that are nonpolar and therefore dissolve in nonpolar organic solvents but not in water.

Since lipids are defined by solubility in nonpolar solvents (a physical property) rather than by chemical structure, there are many different kinds and that they serve a variety of functions in the body.

The lipid molecules contain large hydrocarbon portions and not many polar groups, which accounts for their solubility behaviour. Many lipids have hydrocarbon or modified hydrocarbon structures, properties, and behaviour. This similarity to hydrocarbons and their derivatives unifies a set of highly diverse molecules into one class.

• Waxes are esters of unbranched fatty acids and alcohols.

• Sterols contain four interconnecting rings.

• Fats and oils are triacylglycerols—triesters of glycerol and fatty acids. In fats, the fatty acid chains are mostly saturated; in oils, the proportions of unsaturated fatty acid chains vary.

• Fatty acids are long-chain carboxylic acids.

Fatty acids may or may not contain carbon–carbon double bonds.

Those containing only carbon–carbon single bonds are known as saturated fatty acids; those containing one or more carbon–carbon double bonds are known as unsaturated fatty acids.

If double bonds are present in naturally occurring fats and oils, the double bonds are usually cis rather than trans.

The polyunsaturated fatty acids have more than one carbon–carbon double bond. The number of double bonds present in a fatty acid is referred to as the degree of unsaturation.

A wax is a mixture of fatty acids and long-chain alcohol esters. The acids usually have an even number of carbon atoms, generally from 16 to 36 carbons, whereas the alcohols have an even number of carbon atoms ranging from 24 to 36 carbons.

All fats and oils are composed of triesters of glycerol (propane 1,2,3-triol, also known as glycerin) with three fatty acids. They are named chemically as triacylglycerols but are often called triglycerides.

Properties of the Triacylglycerols in Natural Fats and Oils

• Nonpolar and hydrophobic

• No ionic charges

• Solid triacylglycerols (fats)—high proportion of saturated fatty acid chains

• Liquid triacylglycerols (oils)—high proportion of unsaturated fatty acid chains

Soap is the mixture of salts of fatty acids formed by saponification of animal fat. When soap is dispersed in water, the big, organic anions cluster together so that their long, hydrophobic hydrocarbon tails are in contact, creating a nonpolar microenvironment.

• At the same time, their hydrophilic ionic heads on the surface of the cluster stick out into the water. The resulting spherical clusters are called micelles.

• Grease and dirt become coated by the nonpolar tails of the soap molecules and trapped in the center of the micelles as they form. Once suspended within micelles, the grease and dirt can be rinsed away.

Phospholipids contain a phosphate ester link between phosphoric acid and an alcohol. They are built up from either glycerol (to give glycerophospholipids) or from the alcohol sphingosine (to give sphingomyelins).

• Because phospholipids have ionized phosphate groups at one end, they are similar to soap molecules in having ionic, hydrophilic heads and hydrophobic tails. They differ, however, in having two tails instead of one.

Glycerophospholipids (also known as phosphoglycerides) are triesters of glycerol 3-phosphate and are the most abundant membrane lipids. Two of the ester bonds are with fatty acids, which provide the two hydrophobic tails.

• The fatty acids may be any of the fatty acids normally present in fats or oils. The fatty acid acyl group (R¬C“O) bonded to C1 of glycerol is usually saturated, whereas the fatty acyl group at C2 is usually unsaturated.

• At the third position in glycerophospholipids, there is a phosphate ester group. This phosphate has a second ester link to one of several different OH-containing compounds, often ethanolamine, choline, or serine.

Because of their combination of hydrophobic tails and hydrophilic head groups, the glycerophospholipids are emulsifying agents—substances that surround droplets of nonpolar liquids and hold them in suspension in water.

In sphingolipids, the amino alcohol sphingosine provides one of the two hydrophobic hydrocarbon tails. The second hydrocarbon tail is from a fatty acid acyl group connected by an amide link to the ¬NH2 group in sphingosin.

Glycolipid is a lipid with a fatty acid bonded to the C2¬NH2 group and a sugar bonded to the C1¬OH group of sphingosine.

Sterol is a lipid whose structure is based on a fused tetracyclic (four-ring) carbon skeleton.

• Sterols have many roles throughout both the plant and animal kingdoms. In human biochemistry, the main sterol is cholesterol, which is an important component of cell membranes.

• The major functions of sterols other than cholesterol are as the bile acids that are essential for the digestion of fats and oils in the diet and as hormones.

Lipid bilayer is the basic structural unit of cell membranes; composed of two parallel sheets of membrane lipid molecules arranged tail to tail.

Liposome is a spherical structure in which a lipid bilayer surrounds a water droplet.

The overall structure of cell membranes is represented by the fluid-mosaic model. The membrane is described as fluid because it is not rigid and molecules can move around within it and as a mosaic because it contains many kinds of molecules.

Passive transport is the movement of a substance across a cell membrane without the use of energy, from a region of higher concentration to a region of lower concentration.

Active transport is the movement of substances across a cell membrane with the assistance of energy (e.g., from ATP).

Simple diffusion is passive transport by the random motion of diffusion through the cell membrane.

Facilitated diffusion is passive transport across a cell membrane with the assistance of a protein that changes shape.

Properties of Cell Membranes:

• Cell membranes are composed of a fluid-like phospholipid bilayer.

• The bilayer incorporates cholesterol, proteins (including glycoproteins), and glycolipids.

• Small nonpolar molecules cross by simple diffusion through the lipid bilayer.

• Small ions and polar molecules diffuse across the membrane via protein pores (simple diffusion).

• Glucose and certain other substances (including amino acids) cross with the aid of proteins and without energy input (facilitated diffusion).

• Na +, K+, and other substances that maintain concentration gradients across the cell membrane cross with expenditure of energy and the aid of proteins (active transport).