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“Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999 Stipanuk M. Biochemical, physiological, and molecular aspects of human nutrition.
Two sugars having the same chemical form (aldoses, for example) and the same number of carbons, but that differ only in the stereochemical orientations of their carbons are referred to as diastereomers (Figure 2.149).
For example, glucose, galactose, and mannose all have the formula of C6H12O6, but are chemically distinct from each other in the orientation of hydroxyl groups around the carbons within them.
Fructose and other less well known sugars are ketones. Thus, there are glucose, galactose, sucrose, and many other ‘-oses’.
Other descriptive nomenclature involves use of a prefix that tells how many carbons the sugar contains.
For example, glucose, which contains six carbons, is described as a hexose.
The following list shows the prefixes for numbers of carbons in a sugar: •Tri- = 3 •Tetr- = 4 •Pent- = 5 •Hex- =6 •Hept- = 7 •Oct- = 8 Other prefixes identify whether the sugar contains an aldehyde group (aldo-) or a ketone (keto-) group. Glucose, which is a 6-carbon sugar with an aldehyde group, can be described as an aldohexose.However, studies suggest that some people get at least 55% to 75% of energy from carbohydrates.It may depend on the amount of physical work done by people: the harder the work, the more energy they need. Living in a cold climate means a person needs more energy.Figure 2.148 - Common sugar structures Monosaccharides The most common monosaccharides include glucose, fructose, galactose, ribose, and mannose.Of these sugars, all but one (fructose) exists as an aldehyde. By convention, the letters ‘ose’ at the end of a biochemical name flags a molecule as a sugar.Figure 2.150 - Epimers - D-Galactose and D-Glucose differ only in the configuration of carbon #4 Enantiomers and epimers Figure 2.151 - Enantiomers - D-Glucose (left) and L-Glucose (right) are mirror If two sugars are identical except for having one hydroxyl configured differently (such as images glucose and galactose - Figure 2.150), they are diastereomers known as epimers.If the configuration of all of the hydroxyls of one sugar is exactly the opposite of their configuration in another sugar, the two sugars are mirror images of each other (Figure 2.151). Please note that due to the way sugars are named, L-glucose is the mirror image of Dglucose.The list that follows gives the common sugars and their descriptors.•Ribose = aldo-pentose •Glucose = aldo-hexose •Galactose = aldo-hexose •Mannose = aldo-hexose •Fructose = keto-hexose Diastereomers Figure 2.149 - Diastereomers Sugars may have multiple asymmetric carbons and thus differ from each other in the configuration of hydroxyl groups on asymmetric carbons.On the basis of the number of forming units, three major classes of carbohydrates can be defined: monosaccharides, oligosaccharides and polysaccharides. omopolysaccharides if they contain only one type of monosaccharide as starch, glycogen and chitin; eteropolysaccharides, instead, contain two or more different kinds (e.g. Note: the term “saccharide” derives from the greek word “sakcharon”, which means sugar.