The other groups that might be attached are denoted by R .
Sugars have an aldehyde group and lots of alcoholic -OH groups that can react.
They are along the same chain and thus form a ring. It cannot, however, react again with groups in its own ring. The hemiacetal reaction is reversible. Depending on which sugar, there may be a high percentage in either an open chain or a ring. Reactions in which the aldehyde group of a sugar can participate will take advantage of this reversibility to attack the open chain form as more is made by opening the ring.