Editing Glycogen (section)

{{short description|Glucose polymer used as energy store in animals}}
{{use dmy dates |date=October 2020}}
[[File:Glycogen structure.svg|thumb|upright=1.25|Schematic two-dimensional cross-sectional view of glycogen: A core protein of [[glycogenin]] is surrounded by branches of [[glucose]] units. The entire globular granule may contain around 30,000&nbsp;glucose units.<ref>{{cite book |first1=William D. |last1=McArdle |first2=Frank I. |last2=Katch |first3=Victor L. |last3=Katch |year=2006 |title=Exercise physiology: Energy, nutrition, and human performance |edition=6th |page=12 |publisher=Lippincott Williams & Wilkins |isbn=978-0-7817-4990-9 |url=https://books.google.com/books?id=SRptlOx7yj4C}}</ref>]] [[File:Glycogen spacefilling model.jpg|thumb|A view of the [[atom]]ic structure of a single branched strand of [[glucose]] units in a glycogen [[molecule]].]]

[[File:Parasite130059-fig7 Spermiogenesis in Pleurogenidae (Digenea).tif|thumb|right|Glycogen (black granules) in [[spermatozoa]] of a [[flatworm]]; [[transmission electron microscopy]], scale: 0.3&nbsp;μm]]
'''Glycogen''' is a multibranched [[polysaccharide]] of [[glucose]] that serves as a form of energy storage in [[animal]]s,<ref>{{cite book |first1=David E. |last1=Sadava |first2=William K. |last2=Purves |first3=David M. |last3=Hillis |first4=Gordon H. |last4=Orians |first5=H. Craig |last5=Heller |year=2011 |title=Life |edition=9th |publisher=W. H. Freeman |isbn=9781429254311}}</ref> [[fungi]], and bacteria.<ref name="oclc913469736">{{cite book |vauthors=Berg JM, Tymoczko JL, Gatto GJ, Stryer L |date=2015-04-08 |title=Biochemistry |edition=8th |publisher=W. H. Freeman |location=New York |isbn=9781464126109 |oclc=913469736}}</ref> It is the main storage form of glucose in the human body. 

Glycogen functions as one of three regularly used forms of energy reserves, [[creatine phosphate]] being for very short-term, glycogen being for short-term and the [[triglyceride]] stores in [[adipose tissue]] (i.e., body fat) being for long-term storage. Protein, broken down into amino acids, is seldom used as a main energy source except during starvation and glycolytic crisis ''(see [[bioenergetic systems]])''. 

In [[human]]s, glycogen is made and stored primarily in the cells of the [[liver]] and [[skeletal muscle]].<ref name="Glucose-Glycogen storage review">{{cite journal |vauthors=Wasserman DH|date=January 2009 |title=Four grams of glucose |journal=American Journal of Physiology. Endocrinology and Metabolism |volume=296 |issue=1 |pages=E11–21 |doi=10.1152/ajpendo.90563.2008 |pmc=2636990|pmid=18840763 |quote=Four grams of glucose circulates in the blood of a person weighing 70&nbsp;kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4&nbsp;g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. ... The brain consumes ~60% of the blood glucose used in the sedentary, fasted person. ... The amount of glucose in the blood is preserved at the expense of glycogen reservoirs (Fig.&nbsp;2). In postabsorptive humans, there are ~100&nbsp;g of glycogen in the liver and ~400&nbsp;g of glycogen in muscle. Carbohydrate oxidation by the working muscle can go up by ~10&nbsp;fold with exercise, and yet after 1&nbsp;{{sup|h}}, blood glucose is maintained at ~4&nbsp;g.}}</ref><ref name="pmid1615908" /> In the liver, glycogen can make up 5–6% of the organ's fresh weight: the liver of an adult, weighing 1.5&nbsp;kg, can store roughly 100–120&nbsp;grams of glycogen.<ref name="Glucose-Glycogen storage review" /><ref>{{cite book |first1=Arthur C. |last1=Guyton |first2=John Edward |last2=Hall |year=2011 |title=Guyton and Hall Textbook of Medical Physiology |publisher=Saunders/Elsevier |location=New York, New York |isbn=978-5-98657-013-6 |url=https://books.google.com/books?id=X491kgEACAAJ}}</ref> In skeletal muscle, glycogen is found in a low [[concentration]] (1–2% of the muscle mass): the skeletal muscle of an adult weighing 70&nbsp;kg stores roughly 400&nbsp;grams of glycogen.<ref name="Glucose-Glycogen storage review" /> Small amounts of glycogen are also found in other tissues and cells, including the [[kidney]]s, [[red blood cells]],<ref>{{cite journal |vauthors=Moses SW, Bashan N, Gutman A |date=December 1972 |title=Glycogen metabolism in the normal red blood cell |journal=Blood |volume=40 |issue=6 |pages=836–843 |pmid=5083874 |doi=10.1182/blood.V40.6.836.836 |doi-access=free }}</ref><ref>{{cite journal |journal=J Exp Biol |vauthors=Ingermann RL, Virgin GL |title=Glycogen content and release of glucose from red blood cells of the sipunculan worm themiste dyscrita |volume=129 |pages=141–149 |year=1987 |doi=10.1242/jeb.129.1.141 |url=http://jeb.biologists.org/cgi/reprint/129/1/141.pdf }}</ref><ref>{{cite journal |vauthors=Miwa I, Suzuki S |date=November 2002 |title=An improved quantitative assay of glycogen in erythrocytes |journal=Annals of Clinical Biochemistry |volume=39 |issue=Pt 6 |pages=612–13 |pmid=12564847 |doi=10.1258/000456302760413432}}</ref> [[white blood cells]],<ref>{{cite journal |last1=Murray |first1=Bob |title=Fundamentals of glycogen metabolism for coaches and athletes |journal=Nutrition Reviews |date=April 2018 |volume=76 |issue=4 |pages=243–259 |doi=10.1093/nutrit/nuy001 |pmid=29444266 |pmc=6019055 }}</ref> and [[glial]] cells in the [[brain]].<ref>{{cite journal |vauthors=Oe Y, Baba O, Ashida H, Nakamura KC, Hirase H |date=June 2016 |title=Glycogen distribution in the microwave-fixed mouse brain reveals heterogeneous astrocytic patterns |journal=Glia |volume=64 |issue=9 |pages=1532–1545 |pmid=27353480 |pmc=5094520 |doi=10.1002/glia.23020}}</ref> The uterus also stores glycogen during pregnancy to nourish the embryo.<ref>{{cite book |last1=Campbell |first1=Neil A. |first2=Brad |last2=Williamson |first3=Robin J. |last3=Heyden |year=2006 |title=Biology: Exploring Life |publisher=Pearson Prentice Hall |location=Boston, MA |isbn=978-0-13-250882-7 |url=http://www.phschool.com/el_marketing.html}}</ref>

The amount of glycogen stored in the body mostly depends on [[Skeletal muscle#Fiber types|oxidative type 1 fibres]],<ref name="Jensen 2020">{{Cite journal |last1=Jensen |first1=Rasmus |last2=Ørtenblad |first2=Niels |last3=Stausholm |first3=Marie-Louise Holleufer |last4=Skjærbæk |first4=Mette Carina |last5=Larsen |first5=Daniel Nykvist |last6=Hansen |first6=Mette |last7=Holmberg |first7=Hans-Christer |last8=Plomgaard |first8=Peter |last9=Nielsen |first9=Joachim |date=1 October 2020 |title=Heterogeneity in subcellular muscle glycogen utilisation during exercise impacts endurance capacity in men |journal=The Journal of Physiology |language=en |volume=598 |issue=19 |pages=4271–4292 |doi=10.1113/JP280247 |pmid=32686845 |s2cid=220653138 |issn=1469-7793 |doi-access=free}}</ref><ref name="Jensen 2021">{{Cite journal |last1=Jensen |first1=Rasmus |last2=Ørtenblad |first2=Niels |last3=Stausholm |first3=Marie-Louise H. |last4=Skjærbæk |first4=Mette C. |last5=Larsen |first5=Daniel N. |last6=Hansen |first6=Mette |last7=Holmberg |first7=Hans-Christer |last8=Plomgaard |first8=Peter |last9=Nielsen |first9=Joachim |date=1 May 2021 |title=Glycogen supercompensation is due to increased number, not size, of glycogen particles in human skeletal muscle |journal=Experimental Physiology |language=en |volume=106 |issue=5 |pages=1272–1284 |doi=10.1113/EP089317 |pmid=33675088 |s2cid=232131416 |issn=0958-0670 |doi-access=free}}</ref> physical training, [[basal metabolic rate]], and eating habits.<ref>{{Cite journal |last1=Bergström |first1=Jonas |last2=Hermansen |first2=Lars |last3=Hultman |first3=Eric |last4=Saltin |first4=Bengt |date=October 1967 |title=Diet, Muscle Glycogen and Physical Performance |journal=Acta Physiologica Scandinavica |language=en |volume=71 |issue=2–3 |pages=140–150 |doi=10.1111/j.1748-1716.1967.tb03720.x |pmid=5584523 |issn=1365-201X}}</ref> Different levels of resting muscle glycogen are reached by changing the number of glycogen particles, rather than increasing the size of existing particles<ref name="Jensen 2021"/> though most glycogen particles at rest are smaller than their theoretical maximum.<ref>{{Cite journal |last1=Marchand |first1=I. |last2=Chorneyko |first2=K. |last3=Tarnopolsky |first3=M. |last4=Hamilton |first4=S. |last5=Shearer |first5=J. |last6=Potvin |first6=J. |last7=Graham |first7=T. E. |date=2002-11-01 |title=Quantification of subcellular glycogen in resting human muscle: granule size, number, and location |journal=Journal of Applied Physiology |language=en |volume=93 |issue=5 |pages=1598–1607 |doi=10.1152/japplphysiol.00585.2001 |pmid=12381743 |issn=8750-7587 |doi-access=free}}</ref>
 
Approximately 4&nbsp;grams of glucose are present in the [[blood]] of humans at all times;<ref name="Glucose-Glycogen storage review"/> in fasting individuals, [[blood glucose]] is maintained constant at this level at the expense of glycogen stores, primarily from the liver (glycogen in skeletal muscle is mainly used as an immediate source of energy for that muscle rather than being used to maintain physiological blood glucose levels).<ref name="Glucose-Glycogen storage review"/> Glycogen stores in skeletal muscle serve as a form of energy storage for the muscle itself;<ref name="Glucose-Glycogen storage review"/> however, the breakdown of muscle glycogen impedes muscle glucose uptake from the blood, thereby increasing the amount of blood glucose available for use in other tissues.<ref name="Glucose-Glycogen storage review"/> Liver glycogen stores serve as a store of glucose for use throughout the body, particularly the [[central nervous system]].<ref name="Glucose-Glycogen storage review"/> The [[human brain]] consumes approximately 60% of blood glucose in fasted, sedentary individuals.<ref name="Glucose-Glycogen storage review"/>

Glycogen is an analogue of [[starch]], a glucose [[polymer]] that functions as energy storage in [[plant]]s. It has a structure similar to [[amylopectin]] (a component of starch), but is more extensively branched and compact than starch. Both are white [[powder (substance)|powders]] in their dry state. Glycogen is found in the form of granules in the [[cytosol]]/cytoplasm in many [[cell (biology)|cell]] types, and plays an important role in the [[glucose cycle]]. Glycogen forms an [[energy]] reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of [[triglycerides]] ([[Fat|lipids]]). As such it is also found as storage reserve in many parasitic protozoa.<ref>{{cite journal |last1=Ryley |first1=J.F. |date=March 1955 |title=Studies on the metabolism of the protozoa. 5:&nbsp;Metabolism of the parasitic flagellate Trichomonas foetus |journal=The Biochemical Journal |volume=59 |issue=3 |pages=361–369 |pmid=14363101 |pmc=1216250 |doi=10.1042/bj0590361}}</ref><ref>{{cite journal |last1=Benchimol |first1=Marlene |last2=Elias |first2=Cezar Antonio |last3=de&nbsp;Souza |first3=Wanderley |date=December 1982 |title=''Tritrichomonas foetus'': Ultrastructural localization of calcium in the plasma membrane and in the hydrogenosome |journal=Experimental Parasitology |volume=54 |issue=3 |pages=277–284 |doi=10.1016/0014-4894(82)90036-4 |pmid=7151939 |issn=0014-4894}}</ref><ref>{{cite journal |last1=Mielewczik |first1=Michael |last2=Mehlhorn |first2=Heinz |last3=al&nbsp;Quraishy |first3=Saleh |last4=Grabensteiner |first4=E. |last5=Hess |first5=M. |title=Transmission electron microscopic studies of stages of ''histomonas meleagridis'' from clonal cultures |journal=Parasitology Research |date=1 September 2008 |volume=103 |issue=4 |pages=745–750 |doi=10.1007/s00436-008-1009-1 |pmid=18626664 |s2cid=2331300 |language=en |issn=0932-0113}}</ref>