Glycogenolysis:  It is breakdown  of glycogen to glucose  by means  of the   reaction   series  shown  in figure 6.11 Like glycogenesis  it also  occurs in all  body  cells , But mainly in liver  and muscle cells ,to yield  glucose  required  for energy production , Muscle cells use this glucose themselves,  but liver  cells  release it into the blood to replenish the  amount  of glucose  that is being constantly  taken up by  body  cells   from the blood   .Thus  glycogen storage of liver serves  to maintain normal  blood sugar level  in between  meals. It is thus a reserve fuel, A fasting or starving person first consumes this liver glycogen. All of it is consumed in 12to 24 hours if not replenished by another meal.

Glycogensis  and glycogenolysis  are regulated by  double control  nervous as   well as hormonal. Hormonal control involves five  main  hormones  insulin   hormone  of pancreas, growth   hormone of pituitary  and cortisol of adrenal cortex stimulate  glycogenesis  whereas glucagon  of pancreas  and adrenaline  of adrenal medulla stimulate  glycogenolysis.