and the orthophosphate group (-OPO32-). Compounds containing these groups will have polar properties, whereas those containing just carbon and hydrogen will have much less polarity. The presence of an electronegative atom does not always give polarity to a molecule - if it is part of a chain and balanced by a similar atom this property may be lost. For instance, the ester link in a tria-cylglycerol molecule (discussed below) contains two oxygen atoms but has no polar properties.

Examples of relatively polar (and thus water-soluble) compounds which will be frequent in this book are sugars (with many -OH groups), organic acids such as lactic acid (with a COO- group) and most other small metabolites. Most amino acids also fall into this category (with their amino and carboxyl groups), although some fall into the amphipathic ('mixed') category discussed below.

Another important point about polarity in organic molecules is that within one molecule there may be both polar and non-polar regions. They are called amphipathic compounds. This category includes phospholipids and long-chain fatty acids (Fig. 1.4). Cell membranes are made up of a double layer of phospholipids, interspersed with specific proteins such as transporter molecules, ion channels and hormone receptors, and molecules of the sterol, cholesterol (Fig. 1.5). The phospholipid bilayer presents its polar faces - the polar 'heads' of the phospholipid molecules - to the aqueous external environment and to the aqueous internal environment; within the thickness of the membrane is a nonpolar, hydrophobic region. The physico-chemical nature of such a membrane means that, in general, molecules cannot diffuse freely across it: non-polar molecules would not cross the outer, polar face and polar molecules would not cross the inner, hydrophobic region. Means by which molecules move through membranes are discussed in Chapter 2 (Box 2.1).

The long-chain fatty acids fall into the amphipathic category - they have a long, non-polar hydrocarbon tail but a more polar carboxylic group head (-COO-). Another compound with mixed properties is cholesterol (Fig. 1.6); its ring system is very non-polar, but its hydroxyl group gives it some polar properties. However, the long-chain fatty acids and cholesterol may lose their polar aspects completely when they join in ester links. An ester is a compound formed by the condensation (elimination of a molecule of water) of an alcohol (-OH) and an acid (e.g. a carboxylic acid, -COO-). Cholesterol (through

Palmitic acid (C16:0), a saturated fatty acid

HO -CHj CH, ^CHj CH, ^CH, ,CH2. ^CHj /CHj XC XC«; CH) XCH; XCH, XCH; XCH, 0

Fatty acids:

Oleic acid (C18:1), an unsaturated fatty acid

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