In animals, which substance serves as the primary storage form of glucose that can be broken down later to release energy?

Introduction / Context:
Animals need a way to store excess glucose so that it can be released quickly when energy demand rises, such as during exercise or fasting. This storage form must be compact, efficient, and easily mobilised. Understanding which molecule acts as the main storage form of glucose in animals is a basic concept in biology and human physiology, and it often appears in exam questions about carbohydrates and metabolism.

Given Data / Assumptions:

• The question asks specifically about animals, not plants or microorganisms.
• The focus is on the primary storage form of glucose.
• The options mention starch, free glucose, glycogen, and none of the above.
• We assume standard textbook descriptions of carbohydrate storage in animals and plants.

Concept / Approach:
Glucose can exist as a free simple sugar in the blood, but this form is not suitable for long term storage because high levels would disturb the body's osmotic balance. Instead, animals store glucose in the form of a polysaccharide called glycogen. Glycogen is a highly branched polymer of glucose molecules, mainly stored in liver and skeletal muscles. Starch is a storage polysaccharide in plants, not in animals. Therefore, when the question refers to the storage form of glucose in animals, the correct answer is glycogen.

Step-by-Step Solution:
Step 1: Recall that carbohydrates can be monosaccharides (like glucose), disaccharides, or polysaccharides (like glycogen and starch). Step 2: Understand that free glucose in the bloodstream is tightly regulated and cannot be stored in large amounts because it would draw water into cells and blood by osmosis. Step 3: Recognise that animals convert excess glucose into glycogen through glycogenesis and store it mainly in liver and muscle cells. Step 4: When energy is needed, glycogen can be rapidly broken down to release glucose through glycogenolysis. Step 5: Compare this with starch, which is the storage polysaccharide in plants such as potatoes and grains, not in animal tissues. Step 6: Conclude that glycogen is the specific storage form of glucose in animals.

Verification / Alternative check:
Standard biology and physiology textbooks clearly state that animals store carbohydrates as glycogen, while plants store them as starch. Diagrams of the liver and muscles often highlight glycogen granules as energy reserves. In discussions of metabolic diseases such as glycogen storage disorders, the role of glycogen as the main storage form of glucose in humans is emphasised. These consistent references confirm that glycogen, not starch or free glucose, is the correct answer for animals.

Why Other Options Are Wrong:
Starch, the storage polysaccharide mainly found in plants, is incorrect because animals do not synthesise starch; they digest it into glucose. Glucose, the free sugar circulating in the bloodstream, is not a storage form; it is an immediate energy source kept within a narrow concentration range. None of the above substances store glucose in animal tissues is wrong because glycogen clearly serves this function in liver and muscles.

Common Pitfalls:
A common mistake is to confuse plant and animal storage polysaccharides and think that starch is used by both. Another pitfall is to assume that because glucose is the key energy sugar, it must also be the storage form. To avoid these errors, remember the simple pairing: plants store glucose as starch, animals store glucose as glycogen. This association will help you quickly choose glycogen in multiple choice questions.

Final Answer:
The correct choice is Glycogen, a branched polysaccharide stored in liver and muscles, because glycogen is the primary storage form of glucose in animals.