The glucose molecule has an open as opposed to cyclic
and unbranched backbone of six carbon atoms, C-1 through C-6; where C-1 is part of an aldehyde group
H(C=O)-, and each of the other five carbons bears one hydroxyl group -OH. The remaining bonds
of the backbone carbons are satisfied by hydrogen
atoms -H.Each of the four carbons C-2 through C-5 is chiral
, meaning that its four bonds connect to four different substituents. In D-glucose, these four parts must be in a specific three-dimensional arrangement. Namely, when the molecule is drawn in the Fischer projection
, the hydroxyls on C-2, C-4, and C-5 must be on the right side, while that on C-3 must be on the left side.