CH_{2}OH CH_{2}OH CH_{2}OH CH_{2}OH | | | | H-C-OH H-C-OH H-C-OH HO-C-H | | | | H-C-OH H-C-OH HO-C-H H-C-OH | | | | HO-C-H H-C-OH HO-C-H H-C--OH | | | | H-C-OH HO-C-H H-C-OH HO-C-H | | | | CHO CHO CHO CHO Comparisons of the above formulas will show that the difference between the formulas for _d_- and _l_-glucose lies in the arrangement of the H atoms and the OH groups around the two asymmetric carbon atoms next the aldehyde end of the chain; while the _d_- and _l_-galactoses differ in that this arrangement is in the reverse order around all four of the asymmetric carbons. By similar variations in the grouping around the four asymmetric atoms, it is possible to produce the sixteen different space arrangements shown on page 37 for the groups of an aldohexose. Sugars corresponding to fourteen of these different forms have been discovered, three of which are of common occurrence in plants, either as single mono-saccharides or as constituent groups in the more complex carbohydrates; the remaining two forms have only theoretical interest. Similarly, for a ketohexose, which contains three asymmetric carbon atoms, there are eight possible arrangements. Three sugars of this type are known, only one (fructose) being common in plants; the others are of only theoretical interest. FOOTNOTES: [1] It is assumed that the reader, or student, is familiar with the theoretical and experimental evidence in support of the existence of the so-called "asymmetric" carbon atom and its relation to the effect of the compound which contains it, when in solution, in rotating the plane of polarized light. For purposes of review, or of study of this most interesting and important phenomenon, the reader is referred to any standard text-book on Organic Chemistry.