Which of the following polysaccharides contains a chemically modified monosaccharide unit in its structure?

Introduction / Context:
This biochemistry related chemistry question asks about polysaccharides, which are long chains of monosaccharide units. Some polysaccharides are made only from simple glucose units, while others contain monosaccharides that have been chemically modified, for example by adding nitrogen containing groups. Understanding which polysaccharide contains such a modified sugar helps students distinguish between different structural carbohydrates found in plants, animals, and fungi.

Given Data / Assumptions:

• Polysaccharides listed are starch, chitin, glycogen, and cellulose.
• Starch and glycogen are storage polysaccharides.
• Cellulose and chitin are structural polysaccharides.
• A modified monosaccharide means a sugar unit that has an additional functional group such as an acetyl amide group compared to simple glucose.

Concept / Approach:
Starch, glycogen, and cellulose are primarily polymers of glucose molecules. The glucose residues may be linked in different ways, but the basic monosaccharide is still glucose. Chitin, in contrast, is made from N acetylglucosamine units. N acetylglucosamine is a derivative of glucose where one hydroxyl group is replaced by an acetylated amino group. Because of this extra nitrogen containing modification, chitin is said to contain a modified monosaccharide. Therefore, we look for the polysaccharide whose building blocks are not plain glucose but a chemically altered form.

Step-by-Step Solution:
Step 1: Recall the composition of starch. Starch is a polymer of alpha glucose units and does not usually contain modified sugars. Step 2: Recall the composition of glycogen. Glycogen is a highly branched polymer of alpha glucose, similar to starch but found in animals. Step 3: Recall the composition of cellulose. Cellulose is a linear polymer of beta glucose units forming plant cell walls. Step 4: Recall the composition of chitin. Chitin consists of repeating N acetylglucosamine residues, which are glucose units modified by an acetylated amino group. Step 5: Conclude that chitin is the only option containing a modified monosaccharide.

Verification / Alternative check:
Biology and chemistry textbooks describe chitin as the main component of the exoskeletons of arthropods and the cell walls of fungi. They explicitly state that the monomer is N acetylglucosamine rather than simple glucose. In contrast, starch and glycogen are energy storage polymers of glucose, while cellulose is a structural polymer of unmodified beta glucose. This cross check confirms that chitin is unique among the listed polysaccharides in containing a chemically modified sugar unit.

Why Other Options Are Wrong:
Option A: Starch is made from alpha glucose units and does not require a modified sugar to form its structure. Option C: Glycogen is also built from alpha glucose units, similar to starch, and functions as animal energy storage. Option D: Cellulose is composed of beta glucose units and is not based on a nitrogen containing sugar derivative.

Common Pitfalls:
Students sometimes think that any structural polysaccharide must contain a modified sugar, which leads them to consider cellulose alongside chitin. Another confusion is between the names glucosamine and glucose; although they sound similar, glucosamine and N acetylglucosamine have extra functional groups. Remember that chitin is special for containing nitrogen and is often described as a nitrogenous polysaccharide, which helps to separate it from purely glucose based polymers such as starch and cellulose.

Final Answer:
The polysaccharide that contains a modified monosaccharide unit is chitin.