Final answer:
The atom that joins monosaccharides to form a glycosidic bond is oxygen, which serves as a bridge between two monosaccharides, such as glucose and fructose, in the formation of disaccharides like sucrose.
Step-by-step explanation:
The atom that joins monosaccharides to form a glycosidic bond is oxygen. Monosaccharides like glucose and fructose can link together by a dehydration reaction, which involves the removal of a water molecule. When this occurs, a glycosidic bond is formed between the hydroxyl group of one monosaccharide and the hydrogen of the other, with an oxygen atom bridging the two. Specifically, in the formation of disaccharide sucrose, a glycosidic bond forms between c