Chapter 2 

 CARBOHYDRATES 



The universal distribution and physiological importance of carbohydrates have en- 

 titled them to rather full treatment in general texts of organic and biochemistry. There- 

 fore, many elementary aspects of their chemistry can be passed over lightly in a work 

 such as the present one. As early products of photosynthesis carbohydrates are key com- 

 pounds in the biochemistry of green plants. Ultimately, all other constituents can be 

 derived from them. Aside from this role as precursors, the different varieties of carbo- 

 hydrates themselves serve several quite different functions. Starch and the simple sugars 

 are generally involved with the storage and utilization of the energy required for the proc- 

 esses of growth, ion transport, water uptake, etc. As cellulose and the hemicelluloses, 

 carbohydrates contribute to structural strength and binding cells together. Other less 

 common derivatives -- glycosides, esters, esters, gums -- have less clear roles and 

 are frequently assigned a protective function in wound healing or as being toxic to parasites. 

 Linkage with a carbohydrate moiety may improve solubility characteristics. 



As the different classes of carbohydrates are discussed, it must be noted that no- 

 menclature described with reference to one class is frequently directly transferable to 

 another class (e.g. prefixes such as arabo, threo defined for the monosaccharides may 

 be applied just as well to the sugar acids or alcohols. ) Comprehensive rules of carbohy- 

 drate nomenclature are given in reference (1). 



MONOSACCHARIDES 



The monosaccharides or simple sugars are fundamentally polyhydroxy aldehydes or 

 ketones, although glycolaldehyde with only one hydroxyl group can be included. They are 

 colorless, optically active, water-soluble compounds. The vast majority have straight 

 carbon chains. 



When the structures are written vertically with the carbonyl group nearest the top 

 of the chain, the configuration around the lowest asymmetric carbon atom determines 

 whether the sugar belongs to the D or L series. Sugars of both series occur naturally 

 although L-arabinose is the only common L-sugar (as a component of many polysaccharides 

 and free in the heartwood of conifers). Structures of the D-family of sugars are given in 

 Figure 2-2. The open-chain aldehyde formulas are shown for convenience although it is 

 well-known that sugars normally exist as cyclic hemiacetals where such a structure is 

 possible. The hexoses are more frequently found with a six-membered pyranose ring, 

 but may in some cases have the five-membered furanose ring. Pentoses are also found 

 with either a furanose or pyranose ring. This type of structural representation is given 

 for some common sugars in Figure 2-3. It will be noted that free hydroxyl groups written 

 to the right in the straight chain formula are written below the plane of the ring. The des- 

 ignations a and /3 describe the position of the new free hydroxyl group generated by ring 

 closure, the a form having this hydroxyl group below the plane of the ring for D-sugars. 

 The pair of isomers differing only in the configuration around this carbon are called 

 "anomers." If asymmetric carbons are present in the "tail" portion extending from the 

 ring their configurations are represented by the straight chain convention(cf. glucofuranose). 



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