Question

How does glycogen differ from starch in structure and function?

Short answer

Glycogen is highly branched and found in animals; it provides quick glucose. Starch, found in plants, is less branched and serves as an energy reserve.

Step-by-step solution

Understanding Glycogen and Starch

Start by understanding what glycogen and starch are. Both are polysaccharides, meaning they are long chains of glucose molecules. Glycogen is primarily found in animals, while starch is found in plants.

Structure of Glycogen

Glycogen has a highly branched structure. The glucose units are linked by α-1,4-glycosidic bonds with branches formed by α-1,6-glycosidic bonds approximately every 8-12 glucose units.

Structure of Starch

Starch is composed of two types of molecules: amylose and amylopectin. Amylose is mostly a linear chain of glucose units connected by α-1,4-glycosidic bonds, while amylopectin is branched but less so than glycogen. Branching in amylopectin occurs every 24-30 glucose units.

Function of Glycogen

Glycogen functions as a storage form of glucose in animals. It is stored in liver and muscle cells and can be rapidly mobilized to meet sudden needs for glucose.

Function of Starch

Starch serves as the primary energy storage polysaccharide in plants. It is stored in chloroplasts and other plastids, and provides a carbohydrate reserve that plants can use during times where photosynthesis is not possible, such as at night or during winter.

Summary of Differences

Glycogen is more extensively branched than starch and is stored in animals to quickly supply glucose when needed. Starch, which includes both amylose and amylopectin, is the main carbohydrate storage form in plants, providing energy during periods without photosynthesis.

Key concepts

Polysaccharides

Polysaccharides are large, complex carbohydrates composed of long chains of monosaccharides linked together by glycosidic bonds. These molecules play key roles in both plants and animals.
Common examples of polysaccharides include glycogen, starch, and cellulose. Glycogen and starch serve as energy storage, whereas cellulose provides structural support.
Their structure and function depend on the arrangement of the glucose units and the types of glycosidic bonds connecting them.

α-1,4-glycosidic Bonds

α-1,4-glycosidic bonds are a type of covalent bond that links glucose molecules in polysaccharides. These bonds are formed between the first carbon (C1) of one glucose molecule and the fourth carbon (C4) of another glucose molecule.
This type of bond is common in the linear portions of both glycogen and starch. In the case of amylose, which is part of starch, all glucose units are connected exclusively by α-1,4-glycosidic bonds.

• Linear chains
• Found in amylose
• Essential for structural stability

α-1,6-glycosidic Bonds

Unlike α-1,4-glycosidic bonds, α-1,6-glycosidic bonds create branch points within polysaccharide molecules. These bonds occur between the first carbon (C1) of one glucose and the sixth carbon (C6) of another glucose.
Glycogen has numerous α-1,6-glycosidic bonds, leading to a highly branched structure, which enhances its solubility and allows quick mobilization of glucose.
Amylopectin, the branched component of starch, also has α-1,6-glycosidic bonds, but they occur less frequently than in glycogen. Glycogen branches every 8-12 glucose units, while amylopectin branches every 24-30 units.

Amylose

Amylose is one of the two components of starch. It is a mostly linear polymer of glucose units connected solely by α-1,4-glycosidic bonds.
This linearity means amylose tends to form helical structures, which make it less soluble in water yet contribute to the compact storage of glucose in plants.
While amylose represents a smaller fraction of starch, it is significant because it influences the texture and digestibility of starchy foods.

Amylopectin

Amylopectin is the other component of starch and has a branched structure due to the presence of both α-1,4 and α-1,6-glycosidic bonds.
While amylopectin has fewer branches than glycogen, its branching pattern occurs every 24-30 glucose units. These branches help in making amylopectin water soluble and easily accessible for enzymatic breakdown.
In plants, amylopectin acts as an efficient storage molecule, providing a readily available energy source during periods without photosynthesis.