In biochemistry, which substance is a multibranched polysaccharide of glucose that serves as a major form of energy storage in animals and fungi?

Introduction / Context:
This biochemistry question tests your understanding of important biological macromolecules, specifically polysaccharides that store or provide structural support. Living organisms use different glucose polymers for different purposes. Plants mainly store energy as starch and use cellulose for structure, whereas animals and fungi store energy as glycogen. Recognizing which polymer serves as the primary energy storage molecule in animals and fungi is essential for mastering carbohydrate metabolism and for interpreting textbook descriptions of liver and muscle energy reserves.

Given Data / Assumptions:
- The question refers to a multibranched polysaccharide made of glucose units.
- The polysaccharide serves as a form of energy storage specifically in animals and fungi.
- The options are glycogen, pectin, chitin, and cellulose, which are all polysaccharides found in nature.
- Only one of these correctly matches all the clues given in the stem.

Concept / Approach:
The key concept centers on the classification of polysaccharides into storage and structural types. Glycogen is a highly branched glucose polymer stored mainly in the liver and muscles of animals and also present in fungi, where it acts as an energy reserve. Cellulose is a linear structural polysaccharide that reinforces plant cell walls. Pectin is a component of plant cell walls and is important in the middle lamella, contributing to fruit firmness and gelling. Chitin is a structural polysaccharide found in fungal cell walls and the exoskeletons of arthropods. Only glycogen matches the description of a multibranched glucose polysaccharide used widely for energy storage in animals and fungi.

Step-by-Step Solution:
Step 1: Focus on the words multibranched polysaccharide of glucose, which point toward a storage form rather than a purely structural one. Step 2: Recall that animals store excess glucose mainly in the form of glycogen in their liver and muscle tissues. Step 3: Remember that fungi also use glycogen as an energy storage molecule, paralleling the role of starch in plants. Step 4: Compare the other options. Pectin is a plant cell wall polysaccharide, chitin is structural in fungi and arthropods, and cellulose forms plant cell walls but is not a multibranched storage polysaccharide. Step 5: Conclude that glycogen is the only option that fits all parts of the definition, making Glycogen the correct answer.

Verification / Alternative check:
Verification can be done by consulting a biochemistry textbook or a standard biology reference that discusses carbohydrates. These resources consistently describe glycogen as a branched polymer of alpha glucose, analogous to the branched components of starch but with even more frequent branching. They also explicitly state that animals and fungi use glycogen as a primary storage form of glucose. Pectin, chitin, and cellulose are described as structural or gel forming components rather than the main rapid release energy stores in animals. This agreement across multiple sources confirms that glycogen is the correct choice.

Why Other Options Are Wrong:
Pectin is a complex polysaccharide present in plant cell walls and is important in fruit ripening and jam making, but it is not the main energy storage compound in animals or fungi. Chitin is a structural polysaccharide found in fungal cell walls and arthropod exoskeletons and is not primarily used for energy storage. Cellulose is a linear glucose polymer that strengthens plant cell walls and is not multibranched or used as a major energy reserve in animals or fungi. Therefore, these three polysaccharides do not fit the description in the question.

Common Pitfalls:
Students sometimes confuse glycogen with starch because both are glucose storage polymers. The clue about animals and fungi is important here, since starch is mainly a plant storage polysaccharide. Others may be tempted by cellulose because it is a very common polysaccharide, but they forget that it is linear and structural. Confusion between structural and storage functions of polysaccharides is a frequent source of mistakes. To avoid such errors, clearly associate glycogen with animal and fungal storage, starch with plant storage, and cellulose and chitin with structural roles.

Final Answer:
The multibranched glucose polysaccharide that stores energy in animals and fungi is Glycogen.