These are the simplest carbohydrates with single polyhydroxy aldehyde (aldose) or ketone (ketose) units each.
They cannot be further hydrolyzed into simpler monosaccharides. Glucose is a single polyhydroxy aldehyde or aldose while fructose is a single polyhydroxy ketone or ketose. The minimum number of carbon in a monosaccharide is three and it can go up to seven.
They have the empirical formula of Cn (H20) n. The monosaccharides are named as trioses, tetroses, pentoses, hexoses and heptoses depending on the presence of three, four, five, six or seven carbon atoms respectively.
Depending on the number of carbon atoms and types of functional group (aldehyde/ketone) monosaccharides can be named as aldohexose (glucose) or keto hexose (fructose), aldotriose or ketotriose, aldopentose or ketopentose etc.
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The hexose sugars are generally sweet in taste and crystalline in structure. Fructose, a ketohexose is the sweetest sugar and is also known as fruit sugar as it is commonly found in fruits except a few like grapes.
Grapes contain glucose; hence glucose is known as grape sugar (also as blood sugar as it occurs in blood). Fructose is also found in nectar and honey. Similarly glucose is found in corn and is known as dextrose. Fructose is known as laevulose.
The pentoses and hexoses can exist either in open chain structure or in ring structure. Two types of ring structure are common with pentoses and hexose: These are pyranose from and furanose form. Pyranose has a six member ring with five carbon atom and one oxygen atom.
The furanose form has five member rings with four carbon atoms and one oxygen atoms. The rest of the carbon atoms remain outside the ring. In solution the pyranose forms of sugars are more stable than the fruranose forms.