A condensation reaction occurs when two monosaccharide units are joined together to form a disaccharide. In the process, a water molecule is produced and lost. The OH group of each monosaccharide are the ones that get joined to each other, with the removal of H2O. A disaccharide containing an -O- bridge between the two monosaccharides is formed. Between glucose units, these bonds are usually between carbon 1 of one glucose molecule and carbon 4 or 6 on the other. Depending on the direction of the OH group at carbon 1, the link may be called an alpha or a beta linkage.