OK.
So when I introduced the endocrine system,
I mentioned that hormones can be classified
by where they function.
And 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 it's 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 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 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 going to be excreted, proteins and polypeptides
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 the 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 too,
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 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 predrew that as well.
I'm going to 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 going to label those A, B, C, and D.
And what 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 oftentime, 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 effecting a change in that cell that's
going to result in the transcription
and the translation of new proteins
and new products inside the cell.
And I'm going to 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,
A, is that they're made up of one amino acid.
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 thyrosine 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.