Lactose is the carbohydrate that is commonly known as milk sugar. It is a disaccharide composed of two sugar molecules, galactose and glucose, linked together by a β-glycosidic bond. Lactose is found in the milk of mammals, including humans, and is an important source of energy for infants.

Here is a detailed explanation of why lactose is called milk sugar:

Lactose in Milk:
- Lactose is primarily found in the milk of mammals. It is the main carbohydrate present in milk, accounting for about 4-5% of its composition.
- It provides energy for the growing infant and is essential for their development.
- As mammals produce milk to nourish their young, lactose is present in varying amounts in the milk of different species, including humans.

Structure of Lactose:
- Lactose is a disaccharide, which means it is composed of two sugar molecules linked together.
- It consists of one molecule of glucose and one molecule of galactose.
- The glucose and galactose molecules are joined by a β-glycosidic bond, which is a specific type of chemical bond between the two sugars.

Digestion of Lactose:
- Lactose is digested by the enzyme lactase, which is produced in the small intestine.
- Lactase breaks down lactose into its individual sugar components, glucose and galactose.
- These simple sugars can then be absorbed into the bloodstream and used as a source of energy by the body.

Intolerance to Lactose:
- Some individuals have a reduced ability to produce lactase, resulting in lactose intolerance.
- Lactose intolerance is characterized by symptoms such as bloating, gas, and diarrhea after consuming milk or dairy products.
- This occurs because undigested lactose reaches the large intestine, where it is fermented by bacteria, leading to the production of gas and other byproducts.

In conclusion, lactose is called milk sugar because it is the main carbohydrate found in milk. It consists of glucose and galactose linked together by a β-glycosidic bond. Lactose provides energy for infants and is digested by the enzyme lactase. Lactose intolerance can occur when there is a reduced ability to produce lactase, leading to digestive symptoms after consuming milk or dairy products.