i like to envision the hormone insulin as a key. after a meal, when carbohydrate causes blood sugar stream and attach themselves to keyhole-like receptor sites on muscle and fat cells. the insulin key must match the cell's keyhole, insulin can then unlock the muscle or fat cells and allow blood sugar to enter these opened cells become storage sites for future fuel needs. note that insulin and insulin receptors must match to enable the fit that allows the key to open the door to the cell