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A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule. Glycogen (black granules) in spermatozoa of a flatworm; transmission electron microscopy, scale: 0.3 μm. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, [2] fungi, and bacteria. [3]
Glycogen is analogous to starch, a glucose polymer in plants, and is sometimes referred to as animal starch, [16] having a similar structure to amylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked with α(1→6)-linked branches.
Amylopectin is a key component in the crystallization of starch's final configuration, [4] [5] [6] accounting for 70-80% of the final mass. [7] Composed of α-glucose, it is formed in plants as a primary measure of energy storage in tandem with this structural metric.
Starch has been classified as rapidly digestible starch, slowly digestible starch and resistant starch, depending upon its digestion profile. [45] Raw starch granules resist digestion by human enzymes and do not break down into glucose in the small intestine - they reach the large intestine instead and function as prebiotic dietary fiber. [46]
Glycogen is a highly branched structure, consisting of the core protein Glycogenin, surrounded by branches of glucose units, linked together. [2] [12] The branching of glycogen increases its solubility, and allows for a higher number of glucose molecules to be accessible for breakdown at the same time. [2]
B-type chains, making half of the branches, have two branch points, and all chains have the same length. E. Meléndez-Hevia, R. Meléndez and E. I. Canela (2000) "Glycogen Structure: an Evolutionary View", pp. 319–326 in Technological and Medical Implications of Metabolic Control Analysis (ed. A. Cornish-Bowden and M. L. Cárdenas), Kluwer Academic Publishers, Dordrecht
Glucans serve a diverse set of functions. Within the cell, certain glucans store energy, fortify cellular structure, behave in recognition, and enhance virulence in pathogenic organisms. [13] Glycogen and starch are notable glucans responsible for storing energy for the cell.
Two common examples are cellulose, a main component of the cell wall in plants, and starch, a name derived from the Anglo-Saxon stercan, meaning to stiffen. [ 2 ] To name a polysaccharide composed of a single type of monosaccharide, that is a homopolysaccharide, the ending “-ose” of the monosaccharide is replaced with “-an”. [ 3 ]