Animals do not store starch like plants. While plants utilize starch as their primary energy storage molecule, animals rely on a different, yet structurally related, polysaccharide: glycogen. This molecule is central to how animals manage their energy reserves.
Starch Versus Glycogen
Starch is the main energy storage polysaccharide in plants. Composed of linked glucose units, it exists in two primary forms: amylose and amylopectin. Amylose is a linear, unbranched polymer of glucose connected by alpha-1,4 glycosidic bonds. Amylopectin is a branched polymer, also with alpha-1,4 linkages, but featuring additional alpha-1,6 glycosidic bonds at its branch points, occurring less frequently than in glycogen.
Glycogen, often referred to as “animal starch,” is the analogous energy storage polysaccharide found in animals. However, glycogen is characterized by a much higher degree of branching compared to amylopectin, with branches typically occurring every 8-12 glucose units. Their distinct branching patterns reflect their differing roles and metabolic demands in plants versus animals.
Glycogen’s Vital Role in Animals
Glycogen is stored in two main locations within the animal body: the liver and the muscles. The liver can store approximately 100-120 grams of glycogen, while skeletal muscles, due to their greater mass, store about 400 grams. These glycogen stores serve distinct physiological functions.
Liver glycogen plays a role in maintaining stable blood glucose levels throughout the body. When blood glucose concentrations fall, the liver breaks down its glycogen reserves, releasing glucose into the bloodstream to provide fuel for other tissues, particularly the brain. In contrast, muscle glycogen serves as a localized energy source, providing immediate fuel for muscle contraction during physical activity. Muscle cells lack the enzyme necessary to release glucose into the bloodstream, meaning their glycogen stores are used exclusively for their own energy needs.
The highly branched structure of glycogen offers a significant advantage for animals. This extensive branching creates numerous non-reducing ends, which are the sites where enzymes can rapidly cleave off glucose molecules. This allows for quick mobilization of glucose when the body requires a sudden burst of energy, such as during intense exercise or in response to falling blood sugar. This rapid accessibility to stored glucose supports the dynamic metabolic demands of animal life.