Okay, so when I introduced the endocrine system, I mentioned that hormones can be classified by where they function. We talked about autocrine function and paracrine function and endocrine function. But maybe even more importantly, hormones can also be classified by structure. And I say more importantly because the structure of a hormone really determines how it works.
And so that's what I want to talk about today, the three major types of hormones. And the first major type of hormones are proteins and polypeptides. And just as a refresher, proteins and polypeptides are made up of amino acids, and these amino acids are linked together with peptide bonds, and so many peptide bonds come together to form a polypeptide, or a protein. And these proteins and polypeptides form most of our body's hormones. And these hormones can range from small to large.
And to give you an example of what I mean by that, imagine three or so amino acids linked together, forming a hormone. That would be a small polypeptide. In three amino acids, we're talking about a handful of atoms. Maybe a few of them. maybe 20 or so atoms, and just as a frame of reference, a cell in your body, one cell has on the order of a trillion atoms just inside that one cell, and there are 100 trillion cells in your body, and so we're talking about very, very, very small things, and they can range from these small collections of amino acids all the way up to hundreds and hundreds of amino acids, so they can get quite large, and the break point becomes right around 100, and that's where we shift from calling them polypeptides to proteins.
And just like all proteins in your body that are gonna be excreted, proteins and polypeptide hormones are made in the rough endoplasmic reticulum of the cell, and I'll shorten that to RER, and they go from the rough endoplasmic reticulum to the Golgi apparatus. And from the Golgi apparatus, they're kind of repackaged into vesicles that can eventually be excreted from the cell. And because proteins and polypeptides are made of amino acids, they're typically charged, which makes them water soluble, but it also gives them a really hard time crossing cell membranes.
And so typically, their receptors are located in or on a cell surface. And because the receptors are located in or on a cell surface and these protein and polypeptide hormones can't actually travel into the cell, what they do is they initiate a cascade effect of secondary messengers inside the cell. And I'm going to do an entire video about how that signaling cascade using secondary messengers actually works. But the main idea is that when these protein and polypeptide hormones bind to the cell surface, they initiate a response inside the cell.
And we refer to that as a secondary messenger system. And so to save some time, I went ahead and drew in an example of a polypeptide hormone. And I'm going to kind of fade it in for us. And I want to show you on this drawing where the peptide bonds are because these peptide bonds really pull this class together and unify them. And so right in between the carbon and the nitrogen here, and the carbon and the nitrogen here, I'll draw some arrows.
These are the peptide bonds that I was referring to, these carbon-nitrogen bonds. And so they can be small and they can be large, but these links of amino acids that are used as chemical messengers to signal effects in the body are called proteins and polypeptides. And one example is insulin.
Insulin is a relatively large hormone, and so it's a protein hormone. OK, so we've got proteins and polypeptides, and then the second major type of hormones are steroids. And when we hear steroids, I don't know, the first thought that comes to my mind are a bunch of athletes getting in trouble with their regulating committees. But steroids are actually one of the major types of hormones used in our body to communicate. And so there are a lot of steroids in our body.
But steroids come from lipids. And the major lipid that these steroids come from is cholesterol. And because they come from cholesterol, steroids have a really characteristic structure that all of them share.
And so I went ahead and I pre-drew that as well. I'm gonna fade that in. And so this is kind of the characteristic steroid backbone.
And so you can see there are four ring structures here, and these rings are made of carbon atoms, and so there are three cyclohexane rings, or six-membered carbon rings, and one cyclopentane ring. And I'm gonna label those A, B, C, and D. And with this characteristic structure, comes a really characteristic way of signaling a cell. And so, unlike proteins and polypeptides whose receptors are on the cell surface, steroids, because they're made of lipids, have a really easy time passing through the cell membrane and their receptors are located inside the cell.
And so steroids usually go all the way inside of the cell to signal the receptor as primary messengers. They're actually doing the signaling. And often time their receptors are located either in the cytoplasm or all the way in the nucleus, but steroids typically go in and their effect goes all the way down to the transcription and translation level of proteins. And so as primary messengers, they're going inside the cell and they're affecting a change in that cell that's gonna result in the transcription and the translation of new proteins and new products inside the cell.
And I'm gonna do a video on how these steroids actually affect the cell as well. But for now I want you to be thinking of steroids as one of the major hormones that are in our body, not just a means for athletes getting an edge on the competition. And so some examples of big steroids in the body are those that come from the adrenal cortex, like cortisol and aldosterone. and those hormones that come from the gonads, like the sex hormones, testosterone and estrogen and progesterone. And so we've got steroids, and we've got proteins and polypeptides, and the third major type of hormones by structure are tyrosine derivatives.
And tyrosine derivatives come from the amino acid tyrosine. And you might have caught on that I said these come from tyrosine, which is an amino acid. And I told you earlier that protein and polypeptide hormones are made of amino acids and so you might ask yourself why do these get their own major class if these are also made of an amino acid and what makes them really special is that they're made of one amino amino acids, so one amino acid, tyrosine, is manipulated to make these hormones, and B, these hormones that are derived from tyrosine end up being able to sometimes act like proteins and polypeptides and sometimes act like steroids, so they really get their own class.
And an example of tyrosine derivatives in the body are those that come from the thyroid gland, like T3 and T4, or triiodothyronine and thyroxine. that stimulate metabolism and these tyrosine derivatives act really similarly to steroids. And then another example of tyrosine derivatives are catecholamines.
And catecholamines are those hormones that are produced in the adrenal medulla that are involved in our fight or flight responses like epinephrine and norepinephrine. And these tyrosine derivatives act really similarly to peptides by binding on the outside of the cell and releasing those secondary messengers inside the cell. And so the thyroid hormones that are tyrosine derivatives act like steroids.
And the catecholamine tyrosine derivatives act like proteins and polypeptides. But it's important to remember that they form their own unique class because they're all derived from the amino acid tyrosine. And because I did it for proteins and polypeptides and steroids, I went ahead and I drew in what tyrosine looks like. So that's tyrosine. And that's the amino acid that this class of hormones is derived from.
And so I know it's hard to make learning these types of hormones fun, but maybe at least we can let our minds blow up a little bit over the fact that the structure of these hormones dictates almost everything we think or do, from fear to hunger to urinating and pushing babies out, all of our responses to the world around us are signaled by hormones.