Starch and glycogen are the main molecules living organisms use to store energy. Both are polysaccharides built entirely from repeating glucose units. The differences between them lie in which organisms use them, where they are stored, and their specific molecular structure. These variations suit the distinct biological needs of the organisms that rely on them.
Storage Location and Biological Role
Starch is the main energy storage compound for plants. It is stored in the form of dense granules within specialized cellular compartments called amyloplasts, found in organs such as roots, seeds, and tubers. This reserve provides a long-term, static energy supply, nourishing the plant when photosynthesis is not possible, such as at night or during winter dormancy.
Glycogen serves the same energy storage purpose in animals and fungi. In the human body, it is predominantly stored in the liver and the skeletal muscles. Liver glycogen maintains stable blood glucose levels by releasing glucose into the bloodstream when needed. Muscle glycogen is used exclusively by the muscle cells themselves, providing a readily available fuel source for physical activity.
Molecular Architecture and Branching
The structures of starch and glycogen differ significantly in their degree of branching. Starch is a mixture of two polysaccharides: amylose and amylopectin. Amylose is the linear component, forming a coiled helix with glucose units linked exclusively by \(\alpha-1,4\) glycosidic bonds.
Amylopectin is the branched component of starch, making up the majority of the molecule (70–90%). Its main chains also use \(\alpha-1,4\) bonds, but branch points occur through \(\alpha-1,6\) glycosidic bonds, typically spaced every 20 to 30 glucose units. This moderate branching creates a somewhat compact structure.
Glycogen is structurally similar to amylopectin but is far more densely branched. Branching occurs much more frequently, with a new \(\alpha-1,6\) bond appearing approximately every 8 to 12 glucose units. This extensive branching makes the glycogen molecule highly compact, resembling a dense shrub. This dense, globular structure also increases its solubility in water, which facilitates its rapid use within the cytoplasm of animal cells.
Speed of Energy Release and Breakdown
The difference in branching directly determines the speed at which the stored energy can be accessed. Glucose is released from these polysaccharides by enzymes, such as glycogen phosphorylase, which only act on the free ends of the molecule. These enzymes break the bonds to liberate individual glucose units from the non-reducing ends of the chains.
Because glycogen is highly branched, it possesses a large number of non-reducing ends available for simultaneous enzyme attack. This structural feature enables an extremely rapid breakdown, allowing an animal to mobilize large amounts of glucose almost instantly. This capacity for quick energy release is necessary for sudden, high-demand activities, such as the fight-or-flight response or intense muscle contraction.
In contrast, starch’s structure, particularly the linear amylose component, provides far fewer ends for enzyme action. Even the amylopectin portion is less branched than glycogen, leading to a significantly slower rate of glucose liberation. This slower, more sustained release of energy is perfectly suitable for the needs of plants, which require a steady, long-term fuel supply rather than sudden bursts of activity.