The suffix -ose always denotes a carbohydrate.
Figure 2.10 Monosaccharides may be aldoses or ketoses. The three carbon sugars (a) glyc-eraldehyde and (b) dihydroxyacetone share the same molecular formula, but have different functional groups. The two molecules are isomers (see Box 2.2)
have the general formula (CH2O)„. They are classed as either aldoses or ketoses, according to whether they contain an aldehyde group or a ketone group (Figure 2.10). Monosaccharides can further be classified on the basis of the number of carbon atoms they contain. The simplest are trioses (three carbons) and the most important biologically are hexoses (six carbons) (see Boxes 2.2 and 2.3).
Monosaccharides are generally crystalline solids which are soluble in water and have a sweet taste. They all reducing sugars, so called because they are able to reduce alkaline solutions of cupric ions (Cu2+) to cuprous ions (Cu+).
A disaccharide is formed when two monosaccharides (which may be of the same type or different), join together with a concomitant loss of a water molecule (Figure 2.11). Further monosaccharides can be added, giving chains of three, four, five
Glucose + Glucose = Maltose
Galactose + Glucose = Lactose
Figure 2.11 Monosaccharides such as two glucose molecules can be joined by a glycosidic linkage to form a disaccharide. The reaction is a condensation reaction, in which a molecule of water is lost. a- (a) and ^-linkages (b) result in different orientations in space
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