Lactose (a sugar)

From Proteopedia

Lactose is a sugar found in milk and other foods. It is formed by a condensation reaction (also called dehydration synthesis) of the building blocks glucose and galactose. A large part of the human adult population is lactose intolerant. Because they are unable to efficiently digest lactose into its building blocks, lactose builds up and causes symptoms such as stomach pain. Lactose-intolerant individuals either have to limit lactose intake or take supplements to help digest it.

This page assumes basic textbook knowledge of carbohydrate chemistry. For a refresher, see the relevant section in the first chapters of any college biology textbook, e.g. this one.

Structure

Lactose is a sugar; like many other sugars (glucose, fructose, cellulose, amylose), it has a name ending in -ose. Lactose (a disaccharide) is formed by a condensation reaction of glucose (a monosaccharide) and galactose (another monosaccharide). When lactose is hydrolyzed, it yields the building blocks glucose and galactose, see below. In the structure depicted on the right, the two sugar building blocks are distinguished by color. The sugar building blocks feature a chain of carbon atoms (in this case 6 carbon atoms each), with each carbon atom directly connected to at least one oxygen atoms. This is different from proteins (which contain nitrogen atoms) and lipids (which have carbons that are not directly connected to oxygen atoms). The two building blocks are connected by a bridging oxygen atom forming bonds to two carbon atoms.

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Function

Lactose is one of the main sources of energy (calories) in human breast milk and milk from other animals. As hunter-gatherers, adult humans had no significant source of lactose in their diet, and no need for the ability to digest it. This has changed in societies that include dairy as significant part of their diet. Some adults produce sufficient quantities of the enzyme lactase, enabling them to digest substantial amounts of lactose in their diets by hydrolyzing it, while others don't.

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