Both glycogen and starch are large carbohydrate molecules known as polysaccharides, built from long chains of simple glucose units. Their primary biological role is to serve as energy storage compounds within living organisms that can be broken down when fuel is needed. While they share the fundamental architecture of being glucose polymers, their differences in chemical structure and biological location reflect the distinct energy demands of the organisms that utilize them.
Where Starch and Glycogen are Stored
Starch is the primary energy reserve used by plants, and it is stored in specialized organelles called plastids, often concentrated in tissues that need long-term sustenance. This includes seeds, where it nourishes the germinating seedling, and tubers or roots, such as potatoes, where it provides energy for the plant’s survival over winter.
Glycogen, often called “animal starch,” is the main storage polysaccharide in animals. It is predominantly stored in the liver and skeletal muscle tissues. Liver glycogen is used to maintain stable blood glucose levels, releasing sugar into the bloodstream to fuel the brain and other organs during fasting. Muscle glycogen, however, is reserved almost exclusively for the muscle cells themselves, providing a readily available source of fuel for sudden bursts of movement or intense activity.
The Chemical Difference in Structure
The fundamental distinction between the two molecules lies in their branching pattern and composition. Starch is a mixture of two glucose polymers: amylose and amylopectin. Amylose makes up about 15% to 30% of starch and is a largely linear chain of glucose units linked by alpha-1,4 glycosidic bonds.
Amylopectin constitutes the remaining 70% to 85% of starch and is a branched structure. It uses alpha-1,4 linkages for its main chains but introduces branches through alpha-1,6 glycosidic bonds, which occur roughly every 20 glucose units.
Glycogen is a single polymer that is structurally similar to amylopectin but is significantly more branched. In glycogen, the alpha-1,6 branch points occur much more frequently, every 8 to 12 glucose units along the chain. This dense, bush-like structure makes the glycogen molecule compact and allows it to pack efficiently into storage granules within the cells.
How Structure Affects Energy Use
The degree of branching directly influences the speed at which stored energy can be accessed and released. Enzymes responsible for breaking down these polysaccharides, such as glycogen phosphorylase, work only at the non-reducing ends of the glucose chains. Each branch point introduces an additional end where an enzyme can attach and begin cleaving off glucose units.
Because glycogen is far more highly branched than amylopectin, it possesses a large number of free ends. This allows many enzymes to work simultaneously, enabling rapid mobilization of glucose. This capacity for quick energy release is suited to the high metabolic demands of mobile animals, providing immediate fuel during activity.
In contrast, the less-branched structure of starch, particularly the linear amylose component, offers far fewer terminal ends for enzymatic action. The consequence is a much slower rate of glucose release. This aligns with the plant’s need for sustained energy to support growth or maintain function over extended periods.