When consumed, glucose is absorbed by the intestines into the blood. In response to the rise in blood-glucose levels, the pancreas releases insulin, the central metabolic hormone. Cells that need glucose have specific insulin receptors on their surface, permitting glucose entry into the cells. When present in the blood, insulin directs some tissue cells to take up glucose, other cells to store glucose (in the form of glycogen) in liver and muscle cells, and still other cells to hold lipids (fats) in adipose tissue (body fat). Insulin’s absence signals cells to turn off the uptake of glucose, break down glycogen, release lipids from adipose tissue, and put glucose into the bloodstream. When oxidized in the body, in the process called cellular respiration, glucose provides energy for cells; carbon dioxide, and water are the waste products of that process.