What defines a disaccharide?

A disaccharide is defined by its composition, which consists of two sugar units bonded together. This structure is formed through a glycosidic bond that occurs when two monosaccharides undergo a dehydration reaction, releasing a molecule of water in the process. Common examples of disaccharides include sucrose, which is formed from glucose and fructose, and lactose, which is made up of glucose and galactose.

The concept of sugar units is fundamental in distinguishing different types of carbohydrates—monosaccharides have one sugar unit, oligosaccharides consist of a few sugar units (typically three to ten), and polysaccharides are made up of many sugar units. Disaccharides play an important role in nutrition and energy supply, providing a quick source of glucose upon digestion.

The other options, while related to carbohydrates in general, do not accurately capture the specific definition of a disaccharide. For instance, stating that disaccharides consist of one sugar unit or many sugar units would misidentify their structure. Additionally, solubility in water is a characteristic of many sugars, including disaccharides, but it does not define what a disaccharide is.