Comprehension

Carbohydrates are essential for life in both plants and animals. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Chemically, they are polyhydroxy aldehydes or ketones. On the basis of their behavior on hydrolysis, carbohydrates are classified as monosaccharides, oligosaccharides, and polysaccharides.
All monosaccharides are reducing sugars, i.e., they are oxidized by Tollens' reagent and Fehling’s solution. A monosaccharide like glucose is an aldose, and its molecular formula was found to be C6H12O6. After reacting with different reagents like HI, H2N–OH, Bromine water, (CH3)2O, etc., its structure was found to contain one aldehyde group, one primary alcoholic group (–CH2OH), and four secondary alcoholic groups (>CHOH).
Despite having the aldehyde group, glucose does not give some of the reactions of aldehyde groups like Schiff’s test or NaHSO3 addition. This explains the existence of glucose in two cyclic hemiacetal forms, which differ only in the configuration of the hydroxyl group at C–1.

Question: 1

What are reducing sugars?

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- Reducing sugars are sugars that reduce chemical reagents like Tollens' or Fehling’s solution. - The presence of free aldehyde or ketone groups in the sugar molecules allows them to act as reducing agents.
Updated On: Feb 25, 2025
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Solution and Explanation

Reducing sugars are sugars that can reduce Tollens' reagent or Fehling’s solution due to the presence of a free aldehyde or ketone group. These groups are capable of donating electrons to reduce the reagents. Most monosaccharides like glucose, fructose, and galactose are reducing sugars. Disaccharides like lactose can also be reducing sugars if they have a free reducing end. Sucrose, however, is a non-reducing sugar since both of its anomeric carbons are involved in a glycosidic bond, preventing it from reducing reagents.
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Question: 2

Classify the following into monosaccharides and disaccharides: Fructose, Sucrose, Lactose, Galactose

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- Monosaccharides like glucose, fructose, and galactose cannot be further hydrolyzed. - Disaccharides like sucrose and lactose can be broken down into two monosaccharides through hydrolysis.
Updated On: Feb 25, 2025
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Solution and Explanation

- Monosaccharides: These are the simplest sugars and cannot be hydrolyzed into simpler sugars. Fructose and Galactose are both monosaccharides, as they consist of a single sugar unit.
- Disaccharides: These consist of two monosaccharides linked by a glycosidic bond. Sucrose and Lactose are disaccharides, formed by the combination of two monosaccharide units. Sucrose consists of glucose and fructose, while lactose consists of glucose and galactose.
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Question: 3

Name the polysaccharide which is known as ‘animal starch’. Why is it called ‘animal starch’?

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- Glycogen is more highly branched than amylopectin, making it more readily accessible for quick energy release. - The structure of glycogen enables rapid mobilization of glucose when required by the body.
Updated On: Feb 25, 2025
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Solution and Explanation

The polysaccharide known as ‘animal starch’ is Glycogen. Glycogen is a branched polymer of glucose molecules, and it is the primary storage form of glucose in animals, particularly in the liver and muscles. It is similar to amylopectin, which is found in plants, but glycogen has more frequent branching. The name ‘animal starch’ reflects its role as the animal equivalent of plant starch, storing energy for later use.
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Question: 4

Name the isomers of glucose which in the cyclic form differ only in the configuration of the –OH group at C – 1.

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- Anomers are cyclic sugars that differ in the configuration of the –OH group at the anomeric carbon. - $\alpha$-D-Glucose and $\beta$-D-Glucose are examples of anomers formed by glucose in its cyclic form.
Updated On: Feb 25, 2025
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Solution and Explanation

The isomers of glucose that differ only in the configuration of the –OH group at C-1 in the cyclic form are $\alpha$-D-Glucose and $\beta$-D-Glucose. These are anomers, which are a type of stereoisomer that differ in the orientation of the substituent group (in this case, the –OH group) at the anomeric carbon (C-1). In $\alpha$-D-glucose, the –OH group at C-1 is on the opposite side of the ring relative to the CH2OH group, whereas in $\beta$-D-glucose, the –OH group at C-1 is on the same side as the CH2OH group.
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Question: 5

Presence of which functional group was detected when glucose reacted with Br\(_2\) water?

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- The aldehyde group in glucose reacts with bromine water, indicating its reducing nature. - This reaction is a common test for the presence of aldehydes, which undergo oxidation to carboxylic acids.
Updated On: Feb 25, 2025
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Solution and Explanation

When glucose reacts with Br\(_2\) water, the presence of the aldehyde group (CHO) is detected. This is because the aldehyde group in glucose is capable of reducing bromine water, which leads to the oxidation of the aldehyde group. The reaction with Br\(_2\) water helps identify the presence of the aldehyde functional group in reducing sugars like glucose.
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