<P> 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). </P> <P> Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells . 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 . </P> <P> Glycogen was discovered by Claude Bernard . His experiments showed that the liver contained a substance that could give rise to reducing sugar by the action of a "ferment" in the liver . By 1857, he described the isolation of a substance he called "la matière glycogène", or "sugar - forming substance". Soon after the discovery of glycogen in the liver, A. Sanson found that muscular tissue also contains glycogen . The empirical formula for glycogen of (C) was established by Kekulé in 1858 . </P> <P> Glycogen synthesis is, unlike its breakdown, endergonic--it requires the input of energy . Energy for glycogen synthesis comes from uridine triphosphate (UTP), which reacts with glucose - 1 - phosphate, forming UDP - glucose, in a reaction catalysed by UTP--glucose - 1 - phosphate uridylyltransferase . Glycogen is synthesized from monomers of UDP - glucose initially by the protein glycogenin, which has two tyrosine anchors for the reducing end of glycogen, since glycogenin is a homodimer . After about eight glucose molecules have been added to a tyrosine residue, the enzyme glycogen synthase progressively lengthens the glycogen chain using UDP - glucose, adding α (1 → 4) - bonded glucose to the reducing end of the glycogen chain . </P>

Gycogen fiber and starch are all built from which material