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Monosaccharide with its carbonyl group found on end rather than middle carbon.
Aldoses are sugars as too are ketoses and indeed all monosaccharides as well as disaccharides. All monosacharides, of the molecular formula (CH2O)n, possess one carbonyl group and n - 1 hydroxyl groups. In the case of aldoses, that carbonyl group is an aldehyde, that is, found on an end carbon. Biologically important aldoses include the hexoses, glucose and galactose, and the pentose, ribose (of RNA fame), and the triose, glyceraldehyde.
Because sugars have a molecular formula of (CH2O)n, in their non-ring form all carbons but one will have a hydroxyl group attached. The odd carbon out, by contrast, will have a carbonyl group. For example, with a triose, one carbon, the middle carbon, will consist of a HC-OH unit (plus two C-C bonds), one end will instead consist of HO-CH2 (plus one C-C bond), and the other end will consist of HC=O (plus one C-C bond). Adding up all of the atoms you can see that there are 2 + 3 + 1 = 6 hydrogens, 1 + 1 + 1 = 3 oxygens, and of course 3 carbons = (CH2O)3. Since a carbonyl group found on an end carbon is an aldehyde, this triose therefore is an aldehyde. Indeed, it is the aldose, glyceraldehyde.
Note also that the middle carbon on glyceraldehyde is chiral since HO-CH2, -OH, H, and HC=O are not identical. That is, glyceraldehyde, like all sugars that are trioses or larger, in principle can possess enantiomers (i.e., optical isomers).
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