All such monosaccharides, being alcohol-aldehydes, can easily be reduced to the corresponding polyatomic alcohols, containing the same number of carbon atoms as the original monosaccharides, each with one OH group attached to it. All aldose monosaccharides are converted, by gentle oxidation, into the corresponding monobasic acid, having a COOH group in the place of the original CHO group. Further oxidation either changes the alcoholic groups into COOH groups, producing polybasic acids, or breaks up the chain. When ketose monosaccharides are submitted to similar oxidation processes, they are broken down into shorter chain compounds.
The various monosaccharides which have thus far been found as constituents of plant tissues, or as parts of other more complex compounds which occur in plants, are shown in the following table:
| Trioses (C3H6O3) | Tetroses (C4H8O4) | |||
| Aldose— | Glyceric aldehyde, or glycerose | Aldoses— | d- and l-Erythrose, | |
| l-Threose | ||||
| Ketose— | Dioxyacetone | |||
| Pentoses (C5H10O5) | Methyl Pentoses (C6H12O5) | |||
| Aldoses— | d- and l-Arabinose | Aldoses— | Rhamnose | |
| d- and l-Xylose | Fucose | |||
| l-Ribose | Rhodeose | |||
| l-Lyxose | Chinovose | |||
| Hexoses (C6H12O6) | ||||
| Mannitol series | Dulcitol series | |||
| Aldoses— | d- and l-Glucose | d- and l-Galactose | ||
| d- and l-Mannose | d- and l-Talose | |||
| d- and l-Gulose | ||||
| d- and l-Idose | ||||
| d-Altrose | ||||
| d-Allose | ||||
| Ketoses— | d-Fructose | d-Tagatose | ||
| d-Sorbose | ||||
| Heptoses (C7H14O7) | Octoses (C8H16O8) | Nonoses (C9H18O9) |
| Glucoheptose | Gluco-octose | Glucononose |
| Mannoheptose | Manno-octose | Mannononose |
| Galactoheptose | Galacto-octose | |
| Persuelose | ||
| Sedoheptose |
The hexoses are by far the most important group of monosaccharides. They are undoubtedly the first products of photosynthesis, and all the other carbohydrates may be considered to be derived from them by condensation. Because of their biochemical significance and their immense importance as the fundamental substances for all plant and animal energy-producing materials, the following detailed studies of their chemical composition and molecular configuration are fully warranted.
That all the hexoses contain five alcoholic groups is proved by the experimental evidence that each one forms a penta-ester, by uniting with five acid radicals, when treated with mineral or organic acids under proper conditions. Thus, glucose penta-acetate, penta-nitrate, penta-benzoate, etc., have all been prepared. The presence of the aldehyde group is proved by the fact that all aldohexoses have been converted, by gentle oxidation, into pentaoxy-monobasic acids, and the ketohexoses broken down into shorter chain compounds by similar gentle oxidations; these reactions being characteristic of compounds containing an aldehyde and a ketone group respectively. This experimental evidence establishes the nature of the characteristic groups in the molecule, in each case.
The molecular configurations illustrated in the following table are those suggested by Emil Fischer, as a result of his exhaustive studies of the chemical constitution of the various carbohydrates. There is, of course, no thought that the printed formulas here presented accurately represent the actual relationships in space of the different groups; but there is fairly conclusive evidence that the variations in special groupings in the different sugars are properly referable to the particular asymmetric carbon atoms as indicated in the several formulas as presented.
