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: Jan 13, 2026
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Solution and Explanation

Sugars with a free aldehyde or ketone group are classified as reducing sugars because they can reduce Tollens' reagent or Fehling’s solution by donating electrons. Common monosaccharides such as glucose, fructose, and galactose are reducing sugars. Disaccharides like lactose can also be reducing if they possess a free reducing end. Sucrose, conversely, is a non-reducing sugar as its anomeric carbons are both engaged in a glycosidic bond, rendering it incapable of 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: Jan 13, 2026
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Solution and Explanation

- Monosaccharides: Basic sugars, not further reducible. Examples include Fructose and Galactose, each a single sugar unit.
- Disaccharides: Composed of two monosaccharides joined by a glycosidic bond. Sucrose (glucose + fructose) and Lactose (glucose + galactose) are examples, formed from two monosaccharide units.
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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: Jan 13, 2026
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Solution and Explanation

Glycogen, also termed ‘animal starch,’ is a branched polymer of glucose. It serves as the principal glucose storage molecule in animals, predominantly within the liver and muscles. Resembling plant-based amylopectin, glycogen exhibits more frequent branching. The designation ‘animal starch’ highlights its analogous function to plant starch in energy storage for later utilization.
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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: Jan 13, 2026
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Solution and Explanation

$\alpha$-D-Glucose and $\beta$-D-Glucose are isomers of glucose that differ solely in the configuration of the –OH group at C-1 in their cyclic forms. These molecules are classified as anomers, a specific category of stereoisomers distinguished by the orientation of a substituent group (here, the –OH group) at the anomeric carbon (C-1). In $\alpha$-D-glucose, the –OH group at C-1 is oriented on the opposite side of the ring from the CH2OH group. Conversely, in $\beta$-D-glucose, the –OH group at C-1 is positioned 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: Jan 13, 2026
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Solution and Explanation

The reaction of glucose with Br\(_2\) water serves as a test for the aldehyde group (CHO). This is due to the aldehyde group's ability to reduce bromine water, undergoing oxidation in the process. Therefore, interaction with Br\(_2\) water is a method for confirming the presence of the aldehyde functional group in reducing sugars like glucose.
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