Action of Hormones
We said earlier that hormones are released into the blood stream or extracellular fluid and therefore, reach most of the cells of the body. However, they are specific and influence only their destined target cells. Other cells do not react. For instance, insulin passes throughout the body but only the liver and muscle cells respond to it by taking up glucose. Similarly the target cells respond to the hormones differently are different times. Injections of thyroxine will not make a very young tadpole develop into an adult since the cells cannot respond to the hormones as they will in later life. This suggests that only specific receptor molecules at the target cells recognise and bind to particular hormones, and other cell do not possess these receptor molecules. When the hormone reaches a target cell many changes may occur in the cell.