Problem 94 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which of the following is a disaccharide? (a) Glucose (b) Ribulose (c) Lactose (d) Arabinose

Step-by-Step Solution

Answer

(c) Lactose is the disaccharide.

Step 1: Understanding Monosaccharides and Disaccharides

Begin by recalling that monosaccharides are the simplest form of carbohydrates and cannot be hydrolyzed into simpler sugars, while disaccharides consist of two monosaccharide molecules bonded together. Disaccharides can be hydrolyzed into two monosaccharides.

Step 2: Recognize the Provided Options

Identify each option: - Glucose is a monosaccharide. - Ribulose is a pentose sugar and also a monosaccharide. - Lactose is known to be composed of glucose and galactose. - Arabinose is a monosaccharide.

Step 3: Identify the Disaccharide

Among the given options, only lactose is composed of two monosaccharides (glucose and galactose). This makes lactose a disaccharide.

Key Concepts

Monosaccharides

Monosaccharides are the basic building blocks of carbohydrates. They are often referred to as simple sugars because they are the simplest form of carbohydrates. Monosaccharides cannot be broken down into simpler sugars by hydrolysis. Common examples of monosaccharides include glucose, fructose, and ribulose. From a structural viewpoint, they are usually composed of three to seven carbon atoms. Monosaccharides are crucial because they serve as energy sources for cells. Glucose, for example, is a primary energy source in cellular respiration. Apart from providing energy, monosaccharides also play a role in forming more complex carbohydrates. These complex carbohydrates, such as disaccharides and polysaccharides, are formed when monosaccharides bond together through glycosidic linkages.

Hydrolysis in carbohydrates

Hydrolysis is a fascinating process in carbohydrate chemistry. It involves the breaking down of complex carbohydrates into simpler units by the addition of water. In the context of disaccharides, hydrolysis specifically refers to the process of breaking down disaccharides into their individual monosaccharide components. When a disaccharide like lactose is hydrolyzed, the process uses water to break the bond between glucose and galactose, its two constituent monosaccharides. Enzymes often facilitate hydrolysis in biological systems, making it a vital process in digestion. Without hydrolysis, our bodies wouldn't be able to break down complex sugars into simple sugars readily absorbed by the bloodstream.

Carbohydrate classification

Carbohydrates are classified based on their complexity and the number of sugar units they contain. They fall into three primary groups:

• Monosaccharides: These are the simplest, single-unit sugars, like glucose, ribulose, and arabinose.
• Disaccharides: Comprising two monosaccharide units, such as lactose (glucose + galactose) or sucrose (glucose + fructose).
• Polysaccharides: These are complex carbohydrates made up of many monosaccharide units, like starch and cellulose.

This classification helps understand how carbohydrates are digested and used as energy sources in organisms. Each class has distinct characteristics and roles in living organisms, with disaccharides and polysaccharides serving as more efficient ways of energy storage and structural functions in cells.