Question

Identify the following disaccharides. Give a natural source for each of these disaccharides. (a) The disaccharide that contains two glucose units joined by an \(\alpha\) -glycosidic linkage. (b) The disaccharide that contains fructose and glucose. (c) The disaccharide that contains galactose and glucose.

Short answer

(a) Maltose (malted grains), (b) Sucrose (sugarcane), (c) Lactose (milk).

Step-by-step solution

Identify Disaccharide with Two Glucose Units

The disaccharide composed of two glucose molecules joined by an \( \alpha \) -glycosidic linkage is called maltose. It is also known as malt sugar and is commonly found in malted grains like barley.

Identify Disaccharide with Fructose and Glucose

The disaccharide that contains fructose and glucose is sucrose. Sucrose is widespread in nature and is primarily obtained from sugarcane and sugar beet.

Identify Disaccharide with Galactose and Glucose

The disaccharide that consists of galactose and glucose is lactose. It is naturally found in milk and dairy products.

Key concepts

Glucose

Glucose is a simple sugar crucial for life as it acts as a primary energy source for cells. It is a monosaccharide, which means it is the simplest form of sugar, consisting of just one sugar molecule. Glucose is essential in the formation of complex carbohydrates and disaccharides.
It plays a critical role in metabolism and is absorbed directly into the bloodstream during digestion. Given this importance, it's no wonder that glucose is often referred to as "blood sugar."

Glycosidic Linkage

A glycosidic linkage is a type of covalent bond that connects a carbohydrate (sugar) molecule to another molecule, which could be another carbohydrate or different type of molecule. This bond forms during a dehydration reaction, where a water molecule is removed, linking the oxygen of one sugar molecule to the carbon of another.
In the context of disaccharides, the glycosidic linkage is crucial as it determines the properties and functional behavior of the sugar. For instance, maltose is a disaccharide with an \(\alpha\)-glycosidic linkage between two glucose molecules.

Maltose

Maltose is a disaccharide consisting of two glucose molecules linked by an \(\alpha\)-1,4-glycosidic bond.
Also known as malt sugar, maltose is formed through the breakdown of starch and is naturally present in malted grains like barley.

• Maltose is less sweet compared to other sugars, like sucrose.
• It is often used in brewing beer and in malted milk beverages.

The presence of maltose can indicate the beginning stages of carbohydrate digestion in the body.

Sucrose

Sucrose is one of the most common disaccharides, made up of one glucose and one fructose molecule. Known widely as table sugar, sucrose is extracted primarily from sugarcane and sugar beet plants.

• Sucrose is sweet and used extensively as a natural sweetener in many foods and beverages.
• It is also important in photosynthesis, as it acts as a transport form of carbohydrate in plants.

When consumed, sucrose is broken down into its constituent sugars, glucose, and fructose, for absorption and use by the body.

Lactose

Lactose is a naturally occurring disaccharide found in milk and dairy products, composed of one galactose and one glucose molecule. It is sometimes referred to as milk sugar due to its prevalence in mammalian milk.

• Lactose is less sweet than sucrose and maltose.
• It is a key nutrient for infants and young mammals, providing essential energy and promoting healthy growth.

Some individuals have lactose intolerance, meaning they lack the enzyme lactase needed to efficiently digest lactose, leading to digestive challenges.