GLYCOGEN, THE STORAGE FORM OF GLUCOSE IN ANIMAL CELLS, IS COMPOSED OF GLUCOSE RESIDUES. THE PRIMARY FUNCTION OF GLYCOGEN VARIES IN DIFFERENT TISSUES. IN SKELETAL MUSCLE, STORED GLYCOGEN IS A SOURCE OF FUEL THAT IS USED FOR
SHORT-TERM, HIGH-ENERGY CONSUMPTION DURING MUSCLE ACTIVITY: IN THE BRAIN, THE SMALL AMOUNT OF STORED GLYCOGEN IS USED DURING BRIEF PERIODS OF HYPOGLYCEMIA OR HYPOXIA AS AN EMERGENCY SUPPLY OF ENERGY. IN CONTRAST, THE LIVER TAKES UP GLUCOSE FROM THE BLOODSTREAM AFTER A MEAL AND STORES IT AS GLYCOGEN. WHEN BLOOD GLUCOSE LEVELS START TO FALL, THE LIVER CONVERTS GLYCOGEN BACK INTO GLUCOSE AND RELEASES IT INTO THE BLOOD FOR USE BY TISSUES SUCH AS BRAIN AND ERYTHROCYTES THAT CANNOT STORE SIGNIFICANT AMOUNTS OF GLYCOGEN.