Structure of saccharides

Carbohydrates exist as aldoses, which are polyhydroxyaldehydes, and ketoses, being polyhydroxyketones. Their open–chain forms can be suitably represented by Fischer projections. Aldoses and ketoses can be of different chain length and are all derived from glyceraldehyde. A formal insertion of a stereogenic (HCOH)–group between the carbonyl carbon atom and the α–C–atom of glyceraldehyde leads to aldotetroses and further successive (HCOH)–group insertions between the carbonyl carbon atom and the adjacent stereogenic center to aldopentoses and aldohexoses. By an analogous sequence the group of ketoses branches out from 1,3–dihydroxyacetone, leading to tetruloses, pentuloses, and hexuloses.