The sugars are classified according to following system. Monosaccharides are part of a homologous series with the general formula Cx(H2O)y. The most common sugars are when x and y are 5 or 6. These are called pentoses (C5H10O5) and hexoses(C6H12O6). The names of all sugars end with ‘ose’.
The aldehyde group is present in some sugars, called ‘reducing sugars’ because the aldehyde group is a good reducing agent. They all have a CHO group in them. Their general name is aldoses, of which glucose and ribose are the most common members . These compounds are aldehyde-oses, abbreviated to aldoses.
Reduction and oxidation are explained in Chapter 10. Generally a reducing material will take oxygen away from another molecule and use it to add to its own structure. The CHO group takes on oxygen to form a COOH group or even to break up into CO2 and H2O.
The sugar molecules contain many asymmetric carbon atoms. They are therefore optically active and have both D and L isomers. There are 16 optically active aldehyde hexoses alone. Some of them are of value to us in our metabolism and others not. Glucose is the most useful hexose. Our bodies are very selective in what chemicals they use and reject unsuitable ones.
Note that the D and L forms are mirror images of each other (better seen if a threedimensional model is made of the top three groups and set opposite each other as mirror images). The second carbon atom from the top of the chain is also an important asymmetric carbon atom in glucose. Our bodies are very selective in which isomer they like to use: they use D-glucose and not L-glucose.
Glucose, and other sugars, can also rearrange their structure from a linear shape and form a ring
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