Editing Glycogen (section)

==Functions==

===Liver===
As a meal containing [[carbohydrate]]s or protein is eaten and [[digestion|digested]], [[blood glucose]] levels rise, and the [[pancreas]] secretes [[insulin]]. Blood glucose from the [[portal vein]] enters liver cells ([[hepatocyte]]s). Insulin acts on the hepatocytes to stimulate the action of several [[enzyme]]s, including [[glycogen synthase]]. Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this [[postprandial]] or "fed" state, the liver takes in more glucose from the blood than it releases.

After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. When it is needed for [[Food energy|energy]], glycogen is broken down and converted again to glucose. [[Glycogen phosphorylase]] is the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen is the primary source of blood glucose used by the rest of the body for fuel.

[[Glucagon]], another hormone produced by the pancreas, in many respects serves as a countersignal to insulin. In response to insulin levels being below normal (when blood levels of glucose begin to fall below the normal range), glucagon is secreted in increasing amounts and stimulates both [[glycogenolysis]] (the breakdown of glycogen) and [[gluconeogenesis]] (the production of glucose from other sources).

===Muscle===
[[File:Metabolism of common monosaccharides, and related reactions.png|thumb|Metabolism of common monosaccharides]]
[[Skeletal muscle|Muscle]] glycogen appears to function as a reserve of quickly available phosphorylated glucose, in the form of [[Glucose 1-phosphate|glucose-1-phosphate]], for muscle cells. Glycogen contained within skeletal muscle cells are primarily in the form of β particles.<ref>{{Cite journal |title=BrowZine |url=https://browzine.com/libraries/1453/journals/5912/issues/477500242?showArticleInContext=doi:10.1016/j.carbpol.2022.119710 |access-date=2023-05-12 |journal=Carbohydrate Polymers | date=2022 |doi=10.1016/j.carbpol.2022.119710| pmid=35989025 | last1=Liu | first1=Q. H. | last2=Wang | first2=Z. Y. | last3=Tang | first3=J. W. | last4=Mou | first4=J. Y. | last5=Ma | first5=Z. W. | last6=Deng | first6=B. | last7=Liu | first7=Z. | last8=Wang | first8=L. | volume=295 | s2cid=249489284 | url-access=subscription }}</ref> Other cells that contain small amounts use it locally as well. As muscle cells lack [[glucose-6-phosphatase]], which is required to pass glucose into the blood, the glycogen they store is available solely for internal use and is not shared with other cells. This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs.<ref>{{cite web |title=Glycogen Biosynthesis; Glycogen Breakdown |website=oregonstate.edu |url=https://oregonstate.edu/instruct/bb450/summer09/lecture/glycogennotes.html |access-date=2018-02-28 |archive-date=12 May 2021 |archive-url=https://web.archive.org/web/20210512113448/https://oregonstate.edu/instruct/bb450/summer09/lecture/glycogennotes.html |url-status=dead }}</ref>

Skeletal muscle needs [[Adenosine triphosphate|ATP]] (provides energy) for [[Muscle contraction#Skeletal muscle|muscle contraction]] and relaxation, in what is known as the [[sliding filament theory]]. Skeletal muscle relies predominantly on [[glycogenolysis]] for the first few minutes as it transitions from rest to activity, as well as throughout high-intensity aerobic activity and all anaerobic activity.<ref name="Lucia-2021">{{Cite journal |last1=Lucia |first1=Alejandro |last2=Martinuzzi |first2=Andrea |last3=Nogales-Gadea |first3=Gisela |last4=Quinlivan |first4=Ros |last5=Reason |first5=Stacey |last6=International Association for Muscle Glycogen Storage Disease study group |date=December 2021 |title=Clinical practice guidelines for glycogen storage disease V & VII (McArdle disease and Tarui disease) from an international study group |journal=Neuromuscular Disorders |volume=31 |issue=12 |pages=1296–1310 |doi=10.1016/j.nmd.2021.10.006 |issn=1873-2364 |pmid=34848128|s2cid=240123241 |doi-access=free }}</ref> During anaerobic activity, such as weightlifting and [[isometric exercise]], the phosphagen system (ATP-PCr) and muscle glycogen are the only substrates used as they do not require oxygen nor blood flow.<ref name="Lucia-2021" /> 

Different [[bioenergetic systems]] produce ATP at different speeds, with ATP produced from muscle glycogen being much faster than fatty acid oxidation.<ref>{{Cite web |title=Hormonal Regulation of Energy Metabolism - Berne and Levy Physiology, 6th ed (2008) |url=https://doctorlib.info/physiology/physiology/38.html |access-date=2023-10-21 |website=doctorlib.info |language=en}}</ref> The level of [[Exercise intensity#Fuel Used|exercise intensity]] determines how much of which substrate (fuel) is used for ATP synthesis also. Muscle glycogen can supply a much higher rate of substrate for ATP synthesis than blood glucose. During maximum intensity exercise, muscle glycogen can supply 40 mmol glucose/kg wet weight/minute,<ref>{{Cite journal |last1=Murray |first1=Bob |last2=Rosenbloom |first2=Christine |date=2018-04-01 |title=Fundamentals of glycogen metabolism for coaches and athletes |journal=Nutrition Reviews |volume=76 |issue=4 |pages=243–259 |doi=10.1093/nutrit/nuy001 |issn=1753-4887 |pmc=6019055 |pmid=29444266}}</ref> whereas blood glucose can supply 4 - 5 mmol.<ref name="Brooks-2020">{{Cite journal |last=Brooks |first=George A. |date=January 2020 |title=The Precious Few Grams of Glucose During Exercise |journal=International Journal of Molecular Sciences |language=en |volume=21 |issue=16 |pages=5733 |doi=10.3390/ijms21165733 |pmid=32785124 |pmc=7461129 |issn=1422-0067 |doi-access=free }}</ref><ref name="Glucose-Glycogen storage review" /> Due to its high supply rate and quick ATP synthesis, during high-intensity aerobic activity (such as brisk walking, jogging, or running), the higher the exercise intensity, the more the muscle cell produces ATP from muscle glycogen.<ref>{{Cite journal |last1=van Loon |first1=L. J. |last2=Greenhaff |first2=P. L. |last3=Constantin-Teodosiu |first3=D. |last4=Saris |first4=W. H. |last5=Wagenmakers |first5=A. J. |date=2001-10-01 |title=The effects of increasing exercise intensity on muscle fuel utilisation in humans |journal=The Journal of Physiology |volume=536 |issue=Pt 1 |pages=295–304 |doi=10.1111/j.1469-7793.2001.00295.x |issn=0022-3751 |pmc=2278845 |pmid=11579177}}</ref> This reliance on muscle glycogen is not only to provide the muscle with enough ATP during high-intensity exercise, but also to maintain blood glucose homeostasis (that is, to not become hypoglycaemic by the muscles needing to extract far more glucose from the blood than the liver can provide).<ref name="Brooks-2020" /> A deficit of muscle glycogen leads to [[muscle fatigue]] known as "hitting the wall" or "the bonk" ''(see below under glycogen depletion)''.