= Monosaccharide
{wiki}
A <monosaccharide> is a single-unit <carbohydrate> that cannot be split into smaller <sugars> by <hydrolysis>. <Ribose>, <deoxyribose>, and <glucose> are examples. The unit is a whole <molecule> containing many <chemical atoms>; a <disaccharide> instead contains two linked <monosaccharide> units.<Organic Chemistry by John McMurry/25.1 Classification of Carbohydrates>[McMurry, Organic Chemistry, §25.1: simple and complex carbohydrates]{ref}
Classification combines <carbon> count with open-chain connectivity. An <aldose> has an <aldehyde> group; a <ketose> has a <ketone> group. Thus <glucose> is an <aldohexose>, while <fructose> is a six-<carbon> <ketose>. These names describe the open-chain reference forms even when cyclic forms predominate in <water>.<Organic Chemistry by John McMurry/25.1 Classification of Carbohydrates>[McMurry, Organic Chemistry, §25.1: aldoses, ketoses and carbon-count classification]{ref}
\Image[/aldose-ketose-open-chain-connectivity.png]
{title=Open-chain D-<glucose> and D-<fructose>: the same <molecular formula>, different <carbonyl group> connectivity}
{description=Original numbered <Fischer projections>. At tetrahedral <stereocenters>, horizontal <chemical bonds> point toward the viewer and vertical <chemical bonds> away. The <carbonyl group> <carbon> is not a tetrahedral <stereocenter>. The C5 <hydroxy group> on the right specifies D here; D does not specify the sign of <optical rotation>. These reference structures do not show relative abundances in <water>.}
Check the distinction by counting the groups in the drawings. Open-chain <glucose> contains one $\mathrm{CHO}$, four $\mathrm{CHOH}$ and one $\mathrm{CH_2OH}$ group. Open-chain <fructose> contains two $\mathrm{CH_2OH}$, three $\mathrm{CHOH}$ and one $\mathrm{C{=}O}$ group. The respective <hydrogen> totals are $1+4(2)+3=12$ and $2(3)+3(2)=12$; each structure also has six <carbon> and six <oxygen> <chemical atoms>. Their shared <molecular formula> $\mathrm C_6\mathrm H_{12}\mathrm O_6$ therefore cannot identify the <carbonyl group> position or the <stereochemistry>.
The beauty is that a familiar <sugar> name can embrace an interconverting family of forms. <Monosaccharide ring-chain interconversion> connects those drawings to <mutarotation>, a change visible with <polarimetry>. Spatial arrangement also matters to biology: an <enzyme>'s <active site> encounters a particular arrangement of <hydroxy groups>, rather than merely an inventory of <chemical atoms>. The existing <enzyme> and <chirality (chemistry)> articles explain that recognition principle.
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