Carbohydrates

Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or larger molecules that yield such compounds on hydrolysis. Many of the simplest ones fit a general formula $\text{C}_x(\text{H}_2\text{O})_y$, which is where the name "carbohydrate" (hydrate of carbon) originally came from, though the name is really a historical accident rather than a strict structural rule -- several genuine carbohydrates, such as deoxyribose, do not fit this formula exactly, and a few compounds that do fit it, such as acetic acid, are not carbohydrates at all.

Carbohydrates are grouped into three broad classes based on how many monomer units they contain. Monosaccharides are the simplest carbohydrates: single polyhydroxy aldehyde or ketone units that cannot be hydrolysed into any smaller carbohydrate. Glucose and fructose, the two monosaccharides this chapter focuses on, are both six-carbon monosaccharides (hexoses). Oligosaccharides are formed when a small number of monosaccharide units (conventionally two to ten) are joined together by glycosidic bonds; sucrose, made of one glucose and one fructose unit, is the disaccharide (a two-unit oligosaccharide) this chapter covers. Polysaccharides are much larger molecules built from very many monosaccharide units joined together in long chains; starch and cellulose, this chapter's two polysaccharides, are both built entirely from glucose units, joined by glycosidic linkages, but arranged with a crucial structural difference that gives them completely different physical and biological behaviour.

Carbohydrates are also described as being "reducing" or "non-reducing", according to whether they can reduce mild oxidising agents such as Fehling's solution or Tollens' reagent. This ability depends on whether the sugar has a free (or freely-generated, via ring-opening) aldehyde group or an alpha-hydroxy ketone group available to be oxidised. All monosaccharides, including both glucose and fructose, are reducing sugars. Among disaccharides the answer depends on exactly how the two monosaccharide units are joined -- as later sections of this chapter show for sucrose specifically, a sugar can be non-reducing if the glycosidic bond ties up the carbon that would otherwise generate the free carbonyl group.

The sections that follow build up this chapter's WBCHSE-prescribed scope in order: first the classification and detailed open-chain and cyclic structures of glucose and fructose, then the D/L configurational convention used to name them, then sucrose as a worked example of a non-reducing disaccharide, and finally starch and cellulose as two polysaccharides built from the very same monomer. The chapter then moves from carbohydrates to proteins (amino acids, the peptide bond, primary structure and denaturation), to enzymes, and finally to a basic introduction to the nucleic acids DNA and RNA.