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 of the following pairs contain only reducing carbohydrates? (i) glucose, fructose (ii) maltose, lactose (iii) sucrose, cellulose (iv) starch, cellobiose (a) (i), (ii) and (iii) (b) (i), (ii) and (iv) (c) (i) and (ii) (d) (i), (ii), (iii) and (iv)

Short answer

Answer: (c) (i) and (ii)

Step-by-step solution

Let's analyze each pair individually

(i) Glucose is an aldose with a free aldehyde group, and fructose is a ketose with a free ketone group. Both can reduce other compounds. So, this pair contains reducing carbohydrates. (ii) Maltose and lactose both contain a free anomeric carbon, which is an aldehyde group. They can act as reducing sugars. So, this pair contains reducing carbohydrates as well. (iii) Sucrose is a non-reducing disaccharide, as it lacks a free anomeric carbon because of its glycosidic bond is between the anomeric carbon atoms of glucose and fructose. In contrast, cellulose is a long-chain polysaccharide containing only glucose subunits, and it does not have free anomeric carbon atoms as well. Therefore, this pair does not contain reducing carbohydrates. (iv) Starch is a polysaccharide containing only glucose subunits and no free anomeric carbon atoms. Thus, it is a non-reducing carbohydrate. However, cellobiose is a disaccharide and has a free anomeric carbon atom, allowing it to act as a reducing sugar. So, this pair contains only one reducing carbohydrate, cellobiose.

Choose the correct option

By analyzing each pair, we find that pairs (i) and (ii) contain only reducing carbohydrates. Thus, the correct option is: (c) (i) and (ii)

Key concepts

Carbohydrate Chemistry

Carbohydrates are essential biomolecules that serve various functions such as providing energy, storing energy, and forming structural components in cells. They are composed mainly of carbon (C), hydrogen (H), and oxygen (O) atoms and are commonly found in foods like grains, fruits, and vegetables.

Chemically, carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides are the simplest form of carbohydrates, consisting of single sugar units like glucose and fructose. When two monosaccharides join together, they form a disaccharide, such as sucrose (table sugar) or lactose (milk sugar). Oligosaccharides contain a few sugar units, and polysaccharides such as starch and cellulose contain many sugar units linked together.

In terms of chemical reactivity, carbohydrates can be either reducing or non-reducing sugars. Reducing sugars can donate electrons to other molecules during chemical reactions, a property used in tests to identify them. Non-reducing sugars lack this capability, often because of their specific bonding that prohibits the free aldehyde or ketone groups necessary for these reactions.

Aldose and Ketose

Monosaccharides, the building blocks of larger carbohydrates, can further be categorized into two groups: aldoses and ketoses. This classification is based on the presence of a specific functional group in their structure.

Aldoses contain an aldehyde group (CH₂O)) at one end of the carbon chain. A classic example of an aldose is glucose, which has a six-carbon structure where the aldehyde group is at the first carbon atom. Ketoses, on the other hand, contain a ketone group (C=O)) typically at the second carbon atom in the chain. Fructose, which is structurally similar to glucose but with a ketone group instead of an aldehyde group, is a well-known ketose.

The distinction between aldoses and ketoses is important because it influences their chemical properties, including their behavior as reducing sugars. Generally, aldoses are more likely to act as reducing sugars because of their free aldehyde group, but ketoses can also be reducing under certain conditions when they are are able to isomerize to an aldose in the presence of a base.

Anomeric Carbon

In carbohydrate chemistry, the concept of the anomeric carbon is pivotal. The anomeric carbon is a stereocenter created due to the cyclization of a monosaccharide, which is the carbon atom that was originally the carbonyl carbon (either the aldehyde or ketone carbon). This cyclization leads to the formation of ring structures that are typically more stable than their open-chain forms and are often depicted as Haworth projections.

In these ring structures, the anomeric carbon can adopt one of two configurations, referred to as alpha (α) or beta (β), depending on the orientation of the hydroxyl group attached to it. It is this carbon that is involved in the joining of monosaccharides through glycosidic bonds to form disaccharides and polysaccharides.

The significance of the anomeric carbon lies in its ability to revert back to an open-chain form, which in the case of aldoses and some ketoses, presents a free aldehyde or ketone group, making the sugar a reducing sugar. As seen in the exercise, sugar molecules with a free anomeric carbon, such as glucose, lactose, and maltose, are capable of reducing other compounds, hence classified as reducing sugars.