What is the major structural difference between starch and glycogen?

Starch and glycogen are both polysaccharides, which are long chains of repeating sugar units. They are primarily composed of glucose monomers linked together through glycosidic bonds.

The major structural difference between starch and glycogen lies in their branching patterns. Starch is a plant storage polysaccharide, whereas glycogen is an animal storage polysaccharide. Starch has a relatively linear structure, consisting of two main components: amylose and amylopectin.

1. Amylose: It is an unbranched chain of glucose molecules linked by α-1,4-glycosidic bonds. It forms a helical structure due to the spiral arrangement of glucose units. Amylose makes up about 20-30% of the starch structure.

2. Amylopectin: It is a highly branched chain of glucose molecules linked by both α-1,4-glycosidic and α-1,6-glycosidic bonds. The α-1,6-glycosidic bonds create branch points in the chain, causing the structure to resemble a tree-like structure. Amylopectin constitutes the majority (70-80%) of the starch structure.

On the other hand, glycogen is highly branched and extensively more complex than starch. Its structure is similar to amylopectin with many α-1,4-glycosidic bonds, but with much more frequent branch points formed by α-1,6-glycosidic bonds. Glycogen has a higher level of branching than starch, making it more compact and suitable for efficient storage of glucose in animals.

The branched structure of glycogen allows for rapid mobilization of glucose when the body needs energy. This is especially crucial during times of increased demand or low glucose availability, such as during exercise or fasting.

Overall, the major structural difference between starch and glycogen is the degree of branching. Starch has a more linear structure, while glycogen has a highly branched structure, enabling it to store and release glucose efficiently to meet the energy demands of animals.