Monosaccharides such as glucose can be linked together in condensation reactions. For example, sucrose (table sugar) is formed from one molecule of glucose and one of fructose, as shown below. Molecules composed of two monosaccharides are called disaccharides.
Click on the step numbers to see the steps in the formation of sucrose. Click on the mouse icon at left to clear the steps to see them again.
First, two monosaccharides are brought together such that two hydroxyl groups are close to each other.
Note that the glucose half of sucrose has the α configuration at C1. Glycosidic bonds are labeled α or β depending on the anomeric configuration of the C1 involved in the glycosidic bond. Maltose, which links two glucose molecules, has an α glycosidic bond like sucrose. Lactose, the primary sugar in milk, links glucose and galactose in a β glycosidic bond instead.
Can glycosidic bonds only be formed between C1 and C4, as in sucrose, maltose, and lactose?
Glycosidic bonds can also be formed between other carbons of monosaccharides. For example, several polymers of glucose involve glycosidic bonds between C1 and C6 in addition to bonds between C1 and C4. This fact makes polymers of monosaccharides potentially much more complex than polymers of amino acids (proteins) or nucleotides (DNA), as you will see shortly.