Aldoses
An aldose, like a ketose, is a monosaccharide (a simple sugar), which have a carbon backbone chain with many alcohol (hydroxy groups) and an aldehyde. They are polar molecules and very soluble in water due to the many -OH groups they possess. Monosaccharides are a type of carbohydrate, organic structures which can be thought of as having the ingredients for many water (H2O) molecules attached to the carbons in the chain, hence the 'hydrate" of carbohydrate. For most organisms, carbohydrates are an important source of energy that is taken in as food, and broken down in a series of processes called metabolism. Aldoses feature prominently in two metabolic regimes, glycolysis which is the break down of sugars, and gluconeogenesis which is the opposite.
An aldose contains only one aldehyde (−CH=O) group per molecule, whereas a ketose contains a ketone group. The chemical formula takes the form Cn(H2O)n. The simplest possible aldose is the diose glycolaldehyde, which only contains two carbon atoms.
Because they have at least one asymmetric carbon center, aldoses with three or more carbon atoms exhibit stereoisomerism. Aldoses containing stereogenic centers can exist in either a D- form or L- form. The determination is made based on the chirality of the penultimate carbon (the second-furthest from the aldehyde), where alcohol groups on the right of the Fischer projection result in D-aldoses, and epimers with alcohols on the left result in L-aldoses. Biological systems tend to recognize D-aldoses more than L-aldoses.
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