Geoff P. answered • 02/20/16
Learn biology from a university instructor who understands students!
This question requires a step-by-step approach to answer effectively. Remember that hormones are just a form of chemical signal. It's a way for one part of the body to communicate with another. The signal may be sent throughout the body, but only the cells that have any use for the hormone signal will actually make use of it. For example, if adrenocorticotropic hormone (ACTH) is released by the anterior pituitary gland into the bloodstream, every cell in the body will ignore the hormone signal except for those cells that are in the adrenal cortex, the targets for ACTH. How does the adrenal cortex know to respond to that specific hormone signal, ACTH, and not thyroid stimulating hormone (TSH) or some other hormone? The target cells have specific receptors that respond only to their corresponding hormone. The hormone fits very nicely into the receptor, which then activates a signal within the cell that some new action must be taken (or not taken). It's a lot like a lock and key - only the right key (the hormone) will open a lock (the receptor); no other key will work. This is because the shape of the key is specifically formed by its chemical structure and properties. If the receptor is sticking out of the cell membrane and it initiates a change right when the hormone locks into it, then it's an extracellular receptor. If a hormone is bound by a receptor and the receptor then sends a signal to a different molecule to do the work, instead, then it's using a second messenger. An intracellular receptor is just like an extracellular receptor, except that it lies inside the cytosol or even the cell nucleus, which means the hormone must be lipid-soluble in order to pass through the cell membrane and get to the receptor in the first place. If you have any other questions or need further clarification, let me know.
