Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Table sugar is (a) D-glucose (b) a disaccharide consisting of D-glucose and D-fructose (c) a monosaccharide (d) a disaccharide of D-glucose

Step-by-Step Solution

Answer

The correct answer is (b): a disaccharide consisting of D-glucose and D-fructose.

Step 1: Understanding Table Sugar

When we refer to 'table sugar', we are talking about a common type of sugar used in households and cooking. The chemical name for table sugar is sucrose.

Step 2: Identifying Sucrose Composition

Sucrose is a disaccharide, which means it is made up of two monosaccharides. The two monosaccharides that compose sucrose are D-glucose and D-fructose. Thus, sucrose is made by joining one molecule of D-glucose with one molecule of D-fructose.

Step 3: Analyzing the Options

Now, let's analyze the options given: (a) D-glucose: Table sugar is not just D-glucose. (b) a disaccharide consisting of D-glucose and D-fructose: This correctly describes sucrose. (c) a monosaccharide: Table sugar is not a single sugar unit. (d) a disaccharide of D-glucose: This would imply it consists only of glucose units, which is incorrect for sucrose.

Step 4: Conclusion

Based on the composition of sucrose being one molecule of D-glucose and one molecule of D-fructose, the correct answer is option (b).

Key Concepts

Disaccharide

A disaccharide is a type of carbohydrate formed by the combination of two monosaccharides. These simple sugars are the building blocks, which when linked together, form more complex sugars. Sucrose, commonly known as table sugar, is a well-known disaccharide. It is composed of one molecule of D-glucose and one molecule of D-fructose. This connection occurs through a process called dehydration synthesis, where a water molecule is removed to form the bond. Disaccharides are important in our diet as they provide energy and can be broken down into their monosaccharide components through digestion.

Key points about disaccharides include:

• Formed by two monosaccharides joined together.
• Include common sugars like sucrose, lactose, and maltose.
• Provide a quick source of energy when digested.

D-glucose

D-glucose is one of the most essential and widespread monosaccharides found in nature. It is a simple sugar, also referred to as dextrose, and is a primary source of energy for cells. In the context of sucrose, D-glucose pairs with D-fructose to form the disaccharide. D-glucose is often derived from carbohydrates we consume and plays a critical role in cellular respiration, where it is broken down to release energy.

Here are some fundamental facts about D-glucose:

• Exists naturally in fruits, vegetables, and honey.
• Critical for energy production in the body.
• Isomer forms such as L-glucose exist, but D-glucose is predominant in living organisms.

D-fructose

D-fructose, often called fruit sugar, is another important monosaccharide that combines with D-glucose to form sucrose. It is found naturally in many plants, where it is often bonded with glucose to create various disaccharides. Fruits, honey, and root vegetables contain high levels of D-fructose.

D-fructose is noteworthy for a few reasons:

• It is sweeter than D-glucose and is often used as a sweetener in foods and beverages.
• Despite its sweetness, it is absorbed differently in the body compared to glucose.
• Consumption of pure fructose has different metabolic pathways, affecting blood sugar levels less directly than glucose.

Understanding D-fructose's role in sucrose helps illustrate how sugars function in both diet and metabolism.