What is used to store sugar in liver and muscle cells of animals? Glycogen Explained

December 4, 2025 •

3 min read

An estimated 400 grams of glycogen is stored in the skeletal muscle of an average adult, accounting for the largest share of the body's total stores. This molecule, known as glycogen, is what is used to store sugar in liver and muscle cells of animals, acting as a crucial short-term energy reserve.

Quick Summary

Glycogen is the polysaccharide that stores glucose in animal liver and muscle cells, providing a readily available energy source. It is critical for regulating blood glucose levels and fueling intense muscle activity.

Key Points

Glycogen is the key molecule: Glycogen, a branched polysaccharide, is the storage form of sugar in animal liver and muscle cells.
Liver glycogen regulates blood sugar: The liver stores glycogen to maintain stable blood glucose levels for the entire body, especially the brain.
Muscle glycogen fuels local activity: Muscles use their own glycogen stores as an immediate energy source for contraction during exercise.
Storage requires the enzyme glycogenin: Each glycogen granule is built around a protein core called glycogenin.
Hormones control synthesis and breakdown: Insulin promotes glycogen synthesis (glycogenesis), while glucagon stimulates its breakdown (glycogenolysis).
Glycogen disorders exist: Genetic defects affecting the enzymes involved in glycogen metabolism can cause serious inherited conditions known as Glycogen Storage Diseases (GSDs).
Muscle glycogen cannot be shared: Unlike the liver, muscle cells cannot release glucose into the bloodstream because they lack the necessary enzyme, glucose-6-phosphatase.

The storage of sugar is a vital process for all animals, ensuring a quick energy supply is available when needed. While sugar circulates in the bloodstream as glucose, it is converted into a more compact form for storage within specific cells. This complex carbohydrate polymer is known as glycogen. This article explores the structure, function, and importance of glycogen, explaining how it is stored and utilized differently in the liver and muscle cells.

The Function of Glycogen: Liver vs. Muscle

Glycogen's purpose varies depending on where it's stored.

The Liver's Role in Blood Sugar Regulation

The liver maintains overall blood glucose levels for the body. It absorbs excess glucose after meals, storing it as glycogen through glycogenesis. When blood sugar drops, the liver breaks down glycogen into glucose via glycogenolysis and releases it into the bloodstream for other tissues. An adult liver stores about 100–120 grams of glycogen, which can be quickly used during fasting.

Muscle Glycogen: Fuel for Contraction

Muscle cells store glycogen mainly for their own energy during physical activity. Lacking the enzyme glucose-6-phosphatase, muscles cannot release stored glucose into the bloodstream. Muscle glycogen provides immediate fuel for contraction, especially during intense exercise. Skeletal muscles store a larger total amount of glycogen than the liver due to greater muscle mass.

How Glycogen is Formed and Broken Down

Glycogen metabolism is regulated by hormones.

The Synthesis of Glycogen (Glycogenesis)

After eating, insulin promotes glucose uptake and glycogenesis in liver and muscle cells. The process involves converting glucose through several enzymatic steps, including the action of glycogen synthase to elongate the chain and a branching enzyme to create branches.

The Breakdown of Glycogen (Glycogenolysis)

During fasting or exercise, glucagon stimulates glycogen breakdown. Glycogen phosphorylase cleaves glucose units from glycogen branches. In the liver, glucose-6-phosphatase allows glucose release into the bloodstream. This enzyme is absent in muscle cells, keeping their glycogen for internal use.

The Structure of a Glycogen Molecule

Glycogen is a branched polymer of glucose. A protein called glycogenin forms the core. Glucose units are linked by $\alpha$(1→4) bonds, with $\alpha$(1→6) bonds creating branches. This structure increases solubility and provides many ends for quick synthesis and breakdown.

Glycogen and Exercise Performance

Muscle glycogen is crucial for high-intensity exercise. Increased exercise intensity relies more on muscle glycogen. Glycogen depletion causes fatigue, known as "hitting the wall". Carbohydrate loading before endurance events maximizes glycogen stores to delay fatigue. Muscles are highly receptive to glycogen replenishment after exercise with sufficient carbohydrate intake.

Comparison of Liver and Muscle Glycogen


Feature	Liver Glycogen	Muscle Glycogen
Primary Function	Maintains overall blood glucose homeostasis.	Provides immediate fuel for muscle contraction.
Total Storage Amount	Lower total amount (approx. 100-120g).	Higher total amount (approx. 400g in adults).
Concentration	Higher concentration by organ weight (5-6%).	Lower concentration by tissue weight (1-2%).
Glucose Release	Can release glucose into the bloodstream due to the presence of glucose-6-phosphatase.	Cannot release glucose into the bloodstream due to the absence of glucose-6-phosphatase.
Influencing Factor	Varies based on fasting and meal consumption.	Primarily influenced by physical activity intensity and duration.

Understanding Glycogen Storage Disorders (GSDs)

Glycogen storage diseases are genetic disorders causing abnormal glycogen accumulation due to enzyme defects in glycogen metabolism. Different types of GSDs affect specific tissues, leading to varied health problems like hypoglycemia or muscle weakness. The MSD Manuals offer a comprehensive overview of GSDs.

Conclusion

Glycogen is the primary molecule used to store sugar in animal liver and muscle cells. Liver glycogen regulates blood glucose, while muscle glycogen fuels muscle activity. Hormones like insulin and glucagon control glycogen metabolism. Defects in this process cause glycogen storage diseases.

Optional Outbound Link: For further reading on glycogen metabolism and related disorders, refer to the National Center for Biotechnology Information (NCBI) for a technical review: Biochemistry, Glycogen.

Frequently Asked Questions

The primary substance used to store glucose in the body is glycogen, a complex, branched chain of glucose molecules.

In humans and other animals, the majority of glycogen is stored in the skeletal muscles, with a smaller but more concentrated amount found in the liver.

Liver glycogen maintains overall blood glucose levels for the entire body, while muscle glycogen provides a localized and immediate energy source for the muscle cells themselves.

Muscle cells cannot release glucose into the bloodstream because they lack the enzyme glucose-6-phosphatase, which is required to convert stored glycogen back into a form that can be transported out of the cell.

The process of forming glycogen from glucose is called glycogenesis.

The breakdown of glycogen, known as glycogenolysis, is primarily triggered by the hormone glucagon in response to low blood sugar.

GSDs are a group of rare, inherited metabolic disorders caused by defects in the enzymes involved in glycogen metabolism. These defects lead to an abnormal accumulation or deficiency of glycogen in tissues.

The branched structure of glycogen is important because it increases the molecule's solubility and provides a large number of terminal ends, allowing for rapid breakdown and access to glucose.

Every glycogen granule has a protein at its core called glycogenin, which serves as a primer for glycogen synthesis.