 Hey everybody, Dr. O'Hare. In this video, we're going to talk about the two main ways that hormones are going to impact their target cells. We're going to start with the easy ones, the ones that are lipid soluble. So you see here, steroid hormones and then thyroid hormone are both lipid soluble, meaning they can walk right through. They don't need a receptor on the surface. They can walk right into a cell and even right into the nucleus if they need to. So steroid hormones can do so because they're lipids and likes dissolve likes with lipid soluble. Thyroid hormones, it has to do with their structure, it has to do with their benzene ring and iodine, but it doesn't matter. But steroid hormone, so the steroid hormones, which would be your sex hormones and then the adrenal hormones like cortisol and aldosterone, they're going to walk right in and they can bind to receptors right in the cytoplasm, the group of the cell, or right in the nucleus on the DNA. So they can impact protein synthesis, they can impact gene expression directly. Your thyroid hormones are going to do the same thing. They can dock and bind right on your DNA, but they can also bind right on the mitochondria because they're going to be responsible for your metabolism. So they're going to impact higher mitochondria function. So let me just read to these quickly. So the lipid soluble hormones are going to fuse right through the plasma membrane. So they don't need to be asked in, they don't need a receptor. The hormones are going to bind with the receptor and form a receptor hormone complex, which is going to go in and impact the DNA directly. It's going to trigger transcription, which is the conversion of DNA into RNA and then translation of RNA into proteins. So these hormones, your steroid hormones and thyroid hormone, they can walk right into the cell. Now let's look at the water soluble hormones, the ones that can't do this. They're going to be called first messengers because they actually don't enter the cell. They have to pass their message on to what's called a second messenger. And the most important second messenger to remember is cyclic AMP, or it looks like can't. Little C, capital A, MP. Excuse me, sorry about that. So they have to pass the message on. So most of your hormones are never actually going to enter cells. They're going to dock on a receptor as you see here. And then the second messenger is going to take over and we'll talk about why this can be beneficial. So number one, water soluble hormones, they're incible, in the membrane incible, they can't walk through the lipid bilayer, so they have to bind to a receptor. That's going to be their target receptor. The binding is going to activate this cascade. I don't care about G proteins or all the things that happen, but it's important to know that the hormones don't actually enter. They're going to trigger the production of cyclic AMP, and that's the most important second messenger. You should also note that some cells use calcium as a second messenger. It's a relatively minor deal. Cyclic AMP is more important, but then once this happens, the same type of thing is going to occur inside the cell. Once cyclic AMP has been elevated, then you're going to start to see proteins being produced and altered cell activity. So that's why the second messenger is so important. Let's talk about this cascade, though, and why it's important that why these hormones actually work the way they do. So you would think, well, wouldn't it make sense for the hormone to actually do the work itself? Well, the cascade effect that happens here, where a little tiny bit of hormone can lead to a massive production of second messengers, like cyclic AMP, actually makes it much more effective, I would say. So you're going to see a huge increase. There's this tiny little snowball rolling down the hill, and it's just going to blow up. You're going to see a huge response from a low amount of hormones. That's going to be the main benefit of what we call the cascade effect of these second messengers. So it won't take a lot of hormones. The duration, though, is going to be much shorter, because as soon as cyclic AMP is being produced, it's going to start to be broken down. So the impact is going to be bigger, but it's going to be shorter. That would be the only negative, I guess you could say. Last thing I'll say about this, this idea of a second messenger system utilizing usually cyclic AMP would be there actually are hormones that are going to decrease the amount of cyclic AMP. These would be your inhibiting hormones. So most hormones are going to up-regulate what a cell is doing, but there are hormones that actually down-regulate and tell cells to do less. They do so by decreasing. So their G protein would decrease the level of cyclic AMP instead of increasing it. All right, so that is how your lipid soluble and water soluble hormones actually reach their targets and impact their cells. I hope this helps. Have a wonderful day. Be blessed.