24-1 Names and Structures of Carbohydrates ugars are c sified as aldoses and k CHAPTER 24 &omanagsctere8aimela9asanehoeorketo Carbohydrates: Polyfunctional Compounds in the Nature (Ca).pn(C).tr 25。 伦8 ebythe ndbilng block ofthe 粉 sare chiral and optically active enter has the same absolute contig s that of p 2 Toe (-iydoypn nmesa 11 CHAPTER 24 Carbohydrates: Polyfunctional Compounds in Nature 24-1 Names and Structures of Carbohydrates Sugars are classified as aldoses and ketoses. Carbohydrate is the general name for the various forms of sugars (monosaccharides, disaccharides, trisaccharides, polysaccharides). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxy groups. Aldoses are aldehydic sugars. Ketoses are ketotic sugars. Complex sugars are those formed by the linkage of simple sugars through ether bridges. Based on chain length, sugars are called trioses (C3), tetroses (C4), pentoses (C5) and hexoses (C6). Glucose, also known as dextrose, blood sugar or grape sugar is an aldohexose. Glucose is present in many fruits and plants and is present in blood at concentrations of 0.08-0.1%. Fructose is an isomeric ketohexose of glucose. Fructose is the sweetest natural sugar and is present in many fruits and in honey. Ribose is an aldopentose and is a building block of the ribonucleic acids. A disaccharide is derived from two monosaccharides by the formation of an ether (usually acetal) bridge. Hydrolysis regenerates the monosaccharides. Trisaccharides, tetrasaccharides and eventually polysaccharides are formed through additional ether bridges. Starch and cellulose are two important biological polysaccharides. Most sugars are chiral and optically active. The simplest chiral sugar is 2,3-dihydroxypropanal (glyceraldehyde). With the exception of the ketose, 1,3-dihydroxyacetone, most biological sugars contain at least one stereocenter. The older D-L convention for naming sugars is still in general use. In this convention, monosaccharides whose highest numbered stereocenter has the same absolute configuration as that of D- (+)-2,3-dihydroxypropanal (D-glyceraldehyde) are labeled D. Those having the opposite absolute configuration are labeled L. Two diastereomers that differ only at one stereocenter are called epimers