As I said previously, steroid hormones can readily cross the membrane because they're lipid-soluble. In fact, cholesterol is a major important component of the cell membrane. And because steroid hormones will readily move through the membrane, they actually tend to have intracellular receptors or just receptors that are found inside the cell. And they tend to act in ways that modify gene expression. They'll do this either by binding to a receptor and having that receptor hormone complex act as some type of transcription factor, or they will trigger some signal in the cell that will activate other transcription factors. Now, the elements that a hormone receptor complex will bind to help initiate transcription is known as a hormone response element. You can see that whole process playing out in this image behind me, where we have a steroid hormone that would have had to be transported through the blood, assisted by proteins because it's not water-soluble but is going to easily diffuse through the cell membrane, bind to this intracellular receptor, and that's going to actually move into the nucleus where this is going to find its hormone response element and it will modify transcription. You can see in this case it's going to lead to the translation of some new protein.

Now, it's worth noting that although thyroid hormones are amine hormones, they behave like steroid hormones because they are non-polar. This is due to the iodine atoms found in thyroid hormones. We'll talk more about thyroid hormones in just a bit. I do just want to point out this exception because thyroid hormones are going to behave similarly to what we see happening in this image as opposed to how water-soluble hormones will behave.

Now, water-soluble hormones are going to bind to cell surface receptors because they can't cross the membrane. And what they'll end up doing is activating some type of signal transduction pathway that's going to communicate that hormone signal within the cell. Often they're going to be using G-protein coupled receptors as well as second messengers to transmit these signals. You can see a little model of that happening here where this hormone is going to go ahead and bind to this G-protein coupled receptor. Here you can see the GTP being exchanged for the GDP. This is going to activate this protein here, which is adenylyl cyclase. Adenylyl cyclase is going to take ATP and turn it into cyclic AMP. Cyclic AMP is a very common second messenger used, and will frequently be part of these signal transduction pathways. Now, a second messenger, to be clear, is just any non-protein intracellular signaling molecule. Often these signal transduction cascades will involve a series of activations or inactivations, or both, of various molecules, frequently involving protein kinases and phosphorylases and dephosphorylases, phosphorylating and dephosphorylating each other in a series of steps that will transmit the signal. What's cool about that is along the way, the signal can get amplified. Let's say one signaling molecule can actually activate two other downstream signaling molecules. If those guys can both activate two, then you can see how over time, or rather, over the course of the transmission of the signal, it gets amplified. More and more molecules are communicating that signal. These are just common features of these water-soluble hormones.

With that, let's flip the page.