We've previously defined hormones as chemical messengers that travel through the blood, but that definition encompasses a whole bunch of molecules that can be chemically and functionally quite different from each other sometimes. So here, we want to start to put hormones into groups so that we can talk about them a little bit more specifically, and we're going to start by breaking them up into 2 major groups based on their chemical structure. Before we do that, let's just remind ourselves that hormones are chemical messengers used by the endocrine system for whole-body signaling. And when I say that, remember hormones go in the blood. Where does the blood go?
Virtually everywhere in your body. We're going to start by saying that hormones can be grouped by their chemical structure. And when we do that, we get 2 major groups to start. And we're going to go through these groups here, and they're illustrated here in these two boxes. We'll start with the box on the left, this blue box, and this represents the amino acid-based hormones.
And to illustrate an amino acid-based hormone here, we have epinephrine drawn out, and epinephrine, also known as adrenaline, is a hormone used in the fight or flight response. Now, amino acid-based hormones, this is still a really big diverse group of molecules. You can see here epinephrine, this is a pretty small molecule. It's a single modified amino acid. We have other amino acid-based hormones that are strings of a few amino acids.
We call those peptide hormones. And then we have some that we call protein hormones because there are a few 100 amino acids linked together. Now I just say that now, just know that as we go through this, there are going to be exceptions. Not every amino acid-based hormone follows every one of these rules that we're going over here, but generally this is true. Amino acid-based hormones tend to be water-soluble.
Now, because they're water-soluble, well, your blood is mostly made of water, so these are transported through the blood just dissolved in the blood. These hormones just go into the blood. They're dissolved. Now, right away, they can go everywhere that the blood goes. But this does create a problem when they get to the target cell because things that are dissolved in the blood, well, they're hydrophilic.
But the inside of the cell membrane, remember, the cell membrane is a lipid bilayer. The inside of the cell membrane is hydrophobic, so these generally cannot cross the cell membrane. They need to send their signal from outside the cell. So knowing that, you can probably figure out where the receptor for these hormones are. That means the receptor must be on the outside of the cell on the cell membrane.
And really, most hormones that we talk about are going to be amino acid-based hormones unless we're talking about sex hormones. Again, there are some other exceptions, but generally, most non-sex hormones are going to be amino acid-based hormones. Now just to illustrate this all out, we can see here the adrenal glands here sitting on top of the kidneys. They have released the hormone epinephrine. We can see epinephrine is just in water there.
It's dissolved in the water of your blood. And when it gets to the cell, you can see that this arrow stops on the outside of the cell because it can't get into it cannot cross the cell membrane. So when we think of the receptors, we could even draw them in if we wanted. The receptors would be here on the outside of the cell. Alright.
Now that's all going to be quite different from our steroid hormones. Our steroid hormones are going to be synthesized from cholesterol. And to illustrate a steroid hormone here, we have estrogen, sort of the female sex hormone. Now cholesterol is a lipid, so steroid hormones are generally lipid-soluble. Well, if they're lipid-soluble, that means that they do not dissolve in water.
So to get around, they are transported bound to transport proteins. To go through the blood, they need to be but do they need to be bound to another protein so that they can be dissolved and then travel and go everywhere the blood goes. But this means when they get to the cell, they can cross the cell membrane. They can cross that lipid bilayer because these hormones are hydrophobic. Well, if they can cross the cell membrane, that means that generally the receptor location can be inside the cell.
Now to illustrate this all out, again, we see here that generally these are going to be your sex hormones. Examples here, we have estrogen and testosterone. In fact, when you think of a steroid hormone, you might think that somebody is taking steroids maybe to build muscle. That's because they're taking testosterone or a hormone similar to testosterone because one of the effects of testosterone is to increase muscle mass. But here we're talking about estrogen, so we have an illustration of the female reproductive system.
And here we see the ovaries. They will release this hormone estrogen, and the estrogen here we have illustrated traveling in a boat there because the hormone needs a transport protein. It can't be directly in the water of the blood. It needs to be bound to something else. But when it gets to the cell, we can see that this arrow actually goes all the way into the cell.
This hormone can cross the cell membrane. So if we think where are those receptors, well, we could draw a few in. They might actually be in the cytoplasm, or sometimes they're actually in the nucleus. Alright. So those are our 2 major categories.
I just want to remind you here one more time, exceptions exist. One notable exception is going to be thyroid hormone. Thyroid hormone is an amino acid-based hormone that is hydrophobic, and so it needs a transport protein and its receptors are going to be inside the cell. So again, general categories, but for the most part, this is how you want to remember them. We're going to dive into more specifically what this means in terms of the functional response of how these hormones actually work and interact with cells.
We'll do that coming up. But first, we have examples and practice problems. You should give them a try.