Lactose (milk sugar) is a disaccharide, obtained by the condensation of one molecule of  D-galactopyranose and one molecule of  D-glucopyranose.

Answer:

Lactose is a disaccharide sugar that is composed of two monosaccharides: glucose and galactose. It is commonly found in milk and dairy products and serves as a source of energy for infants and young mammals.

Explanation:

1. Lactose:
Lactose is a sugar that is found in milk and dairy products. It is a disaccharide, which means it is composed of two monosaccharides joined together by a glycosidic bond. In the case of lactose, the two monosaccharides are glucose and galactose.

2. Glucose:
Glucose is a monosaccharide and is the most abundant sugar molecule in nature. It is a simple sugar that serves as a primary source of energy for cells. Glucose is commonly found in foods such as fruits, vegetables, and grains.

3. Galactose:
Galactose is also a monosaccharide and is less abundant than glucose. It is commonly found in dairy products and can also be produced in the body from glucose. Galactose is an important component of lactose and is converted into glucose in the liver for energy.

4. Composition of Lactose:
Lactose is formed by the condensation reaction between glucose and galactose. The hydroxyl group (-OH) of the glucose molecule reacts with the hydroxyl group of the galactose molecule, resulting in the formation of a glycosidic bond. This glycosidic bond links the two monosaccharides together to form lactose.

5. Importance of Lactose:
Lactose is an important nutrient for infants and young mammals, as it provides a source of energy and aids in the development of the digestive system. Lactose is broken down by the enzyme lactase in the small intestine, which hydrolyzes the glycosidic bond and releases glucose and galactose for absorption into the bloodstream.

Summary:
Lactose is a disaccharide sugar composed of glucose and galactose. It is commonly found in milk and dairy products and serves as a source of energy for infants and young mammals. The composition of lactose is formed by the condensation reaction between glucose and galactose, resulting in the formation of a glycosidic bond. Lactose is an important nutrient that is broken down by the enzyme lactase in the small intestine for absorption into the bloodstream.