A Level Biology Practice Exam

A disaccharide is characterized as a molecule that consists of two monosaccharide units chemically bonded together. This characteristic is fundamental to its definition, as disaccharides are formed when two monosaccharides undergo a condensation reaction, resulting in the release of a water molecule and the formation of a glycosidic bond. Common examples of disaccharides include sucrose, which is formed from glucose and fructose, and lactose, which is composed of glucose and galactose.

The other choices do not capture the essence of what a disaccharide is. For instance, a molecule made up of three monosaccharides describes a trisaccharide, while a type of starch molecule refers to polysaccharides, which consist of multiple sugar units. A simple sugar molecule typically refers to monosaccharides themselves, not disaccharides. Therefore, identifying a disaccharide as a molecule made of two monosaccharides is accurate and essential for understanding carbohydrate structures in biology.