Study the hexose sugar (glucose) shown in Fig. 7-13 The top (number 1) carbon has quây three things attached to It: a hydrogen, an oxygen (by double bond), and the rest of the molecule. The bottom (number 6) carbon has only three kinds of things attached to it: two hydrogens, an -он, and the rest of the molecule. Each of the four car- bons in between has four different kinds of things attached to it, so each of those four carbons and attached atoms or groups could exist as two stereoisomers (two mirror. Im- ages). Note in Fig. 7-13 that the names of the sugars are prefaced by the letter D (written as a small capital). This designation indicates their stereoisomeric structure and refers to the position of the -он on the next-to-the-bottom carbon. If it were on the other side, we would use the letter L. (If only this -OH changes from one side to the other, a new sugar is produced with a new name, if the name stays the same, as o-glucose and L-glucose, all asymmetric car- bons in the two molecules are mirror images of each other.) The chain of carbons in a sugar molecule form a zigzag, but this three-dimensional pattern cannot be represented conveniently on a two-dimensional sheet of paper, so the carbon chain is usually shown as though it were straight.