Lipids are the third major type of biochemical molecule found in humans. Although one of their major functions relates to the formation of biological membranes (phospholipids and cholesterol), lipid molecules are also essential for energy storage and transport (triacylglycerols), cellular binding and recognition and other biological processes (glycolipids), signaling (steroid hormones), digestion (bile salts), and metabolism (fatty acids, ketone bodies, and vitamin D). Lipid molecules are mainly hydrophobic and are, therefore, found in areas away from water molecules or are involved in mechanisms such as lipoprotein complexes that allow their movement in and through water environments. The smaller hydrophilic parts of lipids are, themselves, important in formation of biological membranes and in the several specific functions of lipids and lipid-derived molecules.
A major role of lipid molecules is to provide the building blocks for biological membranes, including phospholipids, glycolipids, and cholesterol. However, other lipids known as triacylglycerols (also referred to as triglycerides or fats) function in the storage of biological energy. Bile salts, derived in the liver from cholesterol, participate in the digestion of dietary fat. Finally, several lipid-derived molecules serve as important hormones and intracellular messengers.
It is important to note that the major part of every lipid molecule is hydrophobic in nature and, like the hydrophobic parts of proteins discussed earlier (Chapter 1), prefers to be away from and protected against interaction with water molecules. This hydrophobic character is fundamental in membrane formation, lipid transport, and in many of the functions that the various types of lipid molecules perform.
BASIC MEMBRANE LIPID STRUCTURE
A membrane lipid is composed of three basic components that are as follows:
Fatty acids are composed of long chains of carbon molecules with a carboxylic acid (COOH) at carbon 1 and a CH3 (methyl) group at the end of the chain (Figure 3-1A). The carboxylic acid group is involved in bonding of the fatty acid to the other components of a lipid molecule. In humans, fatty acids are usually 12–24 carbons long and most often the number of carbons in the fatty acid backbone is even. Fatty acids can contain single (C—C), double (C=C), or triple (C≡C) carbon–carbon bonds.
Saturated fatty acids contain only single carbon–carbon bonds, and all of the carbon molecules are bonded to the maximum number of hydrogen molecules.
Unsaturated fatty acids have at least one double carbon–carbon bond with the potential for additional hydrogen atom bonding still existing for some of the carbon atoms in the backbone chain. If more than one double bond is present, the term polyunsaturated is used. These double bonds can exist in either a “kinked” cis double bond or a more linear trans double bond (Figure 3-1B–C).
Glycerol is a simple three-carbon molecule with hydroxyl groups at each carbon (Figure 3-2A). ...