Problem 118 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

A sugar that is not a disaccharide among the following is : [KCET 2002] (a) galactose (b) lactose (c) maltose (d) sucrose

Step-by-Step Solution

Answer

Galactose is not a disaccharide.

Step 1: Understand the Difference Between Disaccharides and Monosaccharides

Disaccharides are carbohydrates formed by the combination of two monosaccharide molecules, whereas monosaccharides are simple sugars made of a single molecule. Examples of disaccharides include lactose, maltose, and sucrose.

Step 2: Analyze Each Option

Look at each option one by one: - **Galactose**: A monosaccharide, which means it has a single sugar unit. - **Lactose**: A disaccharide, made from glucose and galactose. - **Maltose**: Also a disaccharide, composed of two glucose units. - **Sucrose**: Another disaccharide, made up of glucose and fructose.

Step 3: Identify the Sugar That is Not a Disaccharide

From the analysis in Step 2, galactose is the only monosaccharide, while lactose, maltose, and sucrose are disaccharides. Therefore, the sugar that is not a disaccharide is galactose.

Key Concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. They consist of a single sugar molecule, making them fundamental building blocks for more complex carbohydrates. Because they are made of just one molecule, they are often referred to as simple sugars.

Common examples of monosaccharides include glucose, fructose, and galactose. These sugars have distinct roles in biology, primarily as a source of energy. Glucose, for instance, is essential for cellular respiration, a process that generates energy for cells to function.

Since monosaccharides do not need further digestion to be absorbed, they provide quick energy to the body. This makes them a fast-acting source of fuel, which is why they are often found in fast-acting energy products like sports drinks. When absorbed into the bloodstream, they can be swiftly utilized or stored for future energy needs.

Monosaccharides are characterized by their chemical formula, generally \(C_{n}H_{2n}O_{n}\). Each carbon atom is typically paired with a hydrogen atom and a hydroxyl group \((-OH)\), making their chemical structure both simple and predictable.

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide molecules linked together. This combination forms through a process called dehydration synthesis, where a water molecule is removed to join the sugars. Disaccharides function primarily as a more stable form of sugar that can be broken down into monosaccharides to release energy.

Common examples of disaccharides include:

• Lactose: Found in milk, composed of glucose and galactose.
• Maltose: Present in malted foods and beverages, consisting of two glucose molecules.
• Sucrose: Commonly known as table sugar, made from glucose and fructose.

Disaccharides must be broken down into their simpler monosaccharide units before absorption in the digestive system. For example, an enzyme called lactase breaks lactose into glucose and galactose in the small intestine.

This process accounts for some digestive sensitivities, such as lactose intolerance, where individuals lack enough enzymes to break down lactose effectively. Each disaccharide has distinct properties based on its structure and the types of monosaccharides involved.

Chemical Structure of Sugars

The chemical structure of sugars defines their properties and functions. Sugars are carbohydrates, typically having the general formula \(C_{n}H_{2n}O_{n}\), where "n" represents the number of carbon atoms.

In monosaccharides like glucose, fructose, and galactose, the structure includes a carbon backbone, with each carbon atom bound to hydrogen and hydroxyl groups. This arrangement not only ensures stability but also enables simple sugars to dissolve readily in water.

For disaccharides, the chemical structure involves two monosaccharide units. The linkage between these units, known as a glycosidic bond, plays a crucial role in determining the properties of the disaccharide. For instance, the enzyme that can break the bond in lactose will not work on the bond in sucrose. Different linkage types affect digestion and metabolism in the body.

• Simple monosaccharides often have a linear form, which can transform into ring structures in aqueous environments, a common feature seen in glucose.
• Disaccharides, through their glycosidic bonds, are suitable for storage and transport of energy in plants and animals.

Understanding the chemical structure of sugars helps us comprehend their biological roles and how they are utilized in various biochemical pathways.