A very important method for detection of carbohydrates is the use of periodic acid-Schiff (PAS) reagent. Periodate oxidation of the hydroxyl groups cleaves sugars and generates aldehydes. These aldehydes are then detected with Schiff's; reagent, which transforms the aldehydes into colored imines.

The imine product of such a reaction is the source of the term 'Schiff base'. 'Schiff base' has been adopted as a general term for a stable imine, or in other words, one in which the nitrogen atom is not connected to a hydrogen but to an aryl or alkyl group. Remember this because a recurring motif in metabolism is the formation of a Schiff' base (imine) between an aldehyde or ketone substrate and a lysine residue of an enzyme. You might not think it is fair, but from an MCAT writer's perspective, Schiff base is the kind of thing you put in a passage as a comprehension hurdle. It's technically organic chemistry, but you only talk about it for five minutes in general organic chemistry. You spend quite a bit of time on it in biochemistry.

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