Some monosaccharides and most disaccharides have a reactive carbonyl group or carbon that can be oxidized.
Examples: glucose, maltose, cellobiose, lactose
Detected by the ability to reduce Cu2+ --> Cu+ with Benedict’s reagent (blue --> red-orange).
Non-reducing sugars have both carbons of the functional aldehyde or ketone group, locked in a glycosidic bond (e.g. sucrose).
The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group. If the hydroxyl group is on the right, the molecule is a D sugar, otherwise it is an L sugar. Because D sugars are biologically far more common, the D is often omitted
D-glucose is an aldohexose with the formula (C·H2O)6. The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde;
Ring-straight chain isomerism
The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.
Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis. When monosaccharides are not immediately needed by many cells they are often converted to more space efficient forms, often polysaccharides. In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same purpose.
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