Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. Which among the following is a non-reducing sugar? (a) Maltose (b) Glucose (c) Cellulose (d) Cellobiose

Short answer

Answer: Cellobiose is a non-reducing sugar.

Step-by-step solution

Understanding Reducing and Non-Reducing Sugars

Reducing sugars are those that can reduce other compounds due to the presence of a free carbonyl group (aldehyde or ketone) in their structure. Non-reducing sugars do not have a free carbonyl group. Therefore, to identify a non-reducing sugar among the given options, we need to look for one which does not have a free carbonyl group in its structure.

Analyzing the Structure of Maltose

Maltose is a disaccharide composed of two glucose molecules joined by an α(1→4) glycosidic linkage. One of the glucose molecules has a free aldehyde group, making maltose a reducing sugar. So, option (a) is not correct.

Analyzing the Structure of Glucose

Glucose is a monosaccharide sugar with an aldehyde group in its open-chain form. This means that glucose is a reducing sugar. So, option (b) is not correct.

Analyzing the Structure of Cellulose

Cellulose is a polysaccharide composed of glucose subunits, linked by β(1→4) glycosidic linkages. All the glucose molecules, except the end molecules, do not have free carbonyl groups as they are involved in glycosidic linkages. However, the end molecules have free carbonyl groups making cellulose a reducing sugar. So, option (c) is not correct.

Analyzing the Structure of Cellobiose

Cellobiose is a disaccharide consisting of two glucose molecules linked by a β(1→4) glycosidic linkage. Both of the glucose molecules are involved in glycosidic linkage, and there are no free carbonyl groups in cellobiose. Therefore, cellobiose is a non-reducing sugar. So, option (d) is the correct answer. In conclusion, cellobiose is the only non-reducing sugar among the given options.

Key concepts

Reducing Sugars

Reducing sugars play a crucial role in biochemistry. They are characterized by their ability to act as reducing agents due to the presence of a free carbonyl group. When in solution, these sugars can easily switch from a ring form to a linear form where the carbonyl group is exposed and able to reduce other compounds, such as copper in Benedict’s solution, resulting in a color change.

Common examples of reducing sugars include glucose, fructose, and lactose. The presence of this free aldehyde or ketone group makes them highly reactive, which has significant biological implications, such as in browning reactions in fruits and the Maillard reaction during cooking.

Carbonyl Group

The carbonyl group is a functional group in organic chemistry with the formula \( C=O \) and consists of a carbon atom double bonded to an oxygen atom. It's pivotal in determining the chemical properties of a compound and is categorized as either an aldehyde if the carbon of the carbonyl group is at the end of a carbon chain, or a ketone if it's within the chain.

When found in sugars, this carbonyl group can interact with other molecules. In the context of carbohydrate chemistry, the reactivity of the carbonyl group is crucial since it can form glycosidic linkages with other sugar molecules or participate in redox reactions, as seen in reducing sugars.

Glycosidic Linkage

Glycosidic linkage is the bond formed between two monosaccharide units through a dehydration reaction, which involves the removal of water. Depending on the carbon atoms involved in the bond, glycosidic linkages can be named α (alpha) or β (beta), and the numeric position of each carbon involved is specified.

This linkage defines the structure and properties of disaccharides, oligosaccharides, and polysaccharides. The nature of the glycosidic bond, i.e., α or β, affects the sugar's digestibility and whether it can be classified as a reducing or non-reducing sugar. For example, a bond between two sugar molecules can render a disaccharide non-reducing if it involves the carbon atom belonging to the carbonyl group.

Monosaccharides and Disaccharides

Monosaccharides are the simplest form of carbohydrates and consist of a single sugar unit with the general chemical formula \( C_nH_{2n}O_n \). Examples include glucose, fructose, and galactose. Because they cannot be hydrolyzed into simpler sugars, they serve as the building blocks for larger carbohydrate molecules.

In contrast, disaccharides are made up of two monosaccharide units joined together by a glycosidic linkage. Examples include sucrose (table sugar), lactose (milk sugar), and maltose (malt sugar). Disaccharides exhibit diverse properties depending on their glycosidic linkage and whether they retain a free carbonyl group, influencing whether they are reducing or non-reducing sugars.