Hey there!  It's Professor S, and 
for the next five minutes or less I
want to talk to you one more time about
different groups of lipids. I want to
begin this video actually by  asking you a question
that i ask my students frequently:
Is human growth hormone a steroid?  
A very large number of students will say "Yes,
yes it is."  They've heard it 
mentioned in news reports about
doping in athletics and talking about
steroids.
In fact, the answer is no, it's very definitely not a steroid.
The steroids being discussed in those news reports
are what are called anabolic steroids,
which are signals that
make cells grow.  Well human growth
hormone is a signal molecule
and it is definitely anabolic 
-- it directs cells to grow --
but it's protein, it's not a steroid.  See
steroids are groups of lipids like this
four-ring structure next to me.
This four ring structure here is the
very definition of what it is to be a steroid:
three hexagonal and one pentagonal hydrocarbon rings.
All steroids have this conformational
arrangement.
Let's shrink this down for a second,
and i'll run through some examples just
to show you that all steroids have it.
Let's start with "the mother of all
steroids," cholesterol.
In the human body it's used to build
other steroids, which is why i call it
"the mother of all steroids." It's also
used in animal cells and some other
organisms to stabilize
the plasma membrane.  
If that doesn't make total sense to you,
I would strongly encourage you to maybe watch a video
that's maybe under five minutes
by maybe someone we know
on plasma membrane structure...  
Anyway, in addition to cholesterol we have cholic
acid, which in humans is a component 
of bile produced down in
the liver.  It's playing a critical role
in helping to digest fats in the diet.
Then we have a whole series of
steroids that are used as signal molecules.
These include all the various versions
of estradiol -- the estrogens.
This includes testosterone and progesterone 
-- most of the sex hormones if
not all of the sex hormones.
It includes other signals in the
human body like cortisol and 
aldosterone, which are playing roles
in regulating
blood sugar and stress and blood
pressure, and those those last two are
called corticosteroids because they're
produced in the adrenal cortex.
Now the takeaway here is that every
single one of those molecules i just showed you
has that same four-ring structure. They
vary a little bit in what hangs off of
it, but they all share that structure.
They're used to stabilize membranes,
they're used to help in digestion of other lipids,
and they're used as signal molecules. And
there's your steroids.
But there are a few other small groups
of lipids besides the steroids.
Take these three guys. We have thromboxane A2.
We have thromboxane B2, and we have
prostaglandin G2, or PGG2.
These three molecules are types of
lipids that are all derived
from arachidonic acid, and they're
collectively called eicosanoids.  All
eicosinoids are derived from the same fatty acid.
They have that unusual ring structure in them, and
these are important signal molecules
where they're used.  Eicosinoids are playing
important roles in processes like immune system
coordination and blood clotting, just to name a couple
examples.  Now this up here is paraffin. I'm
guessing you've heard of paraffin.
if you've heard of paraffin, you know
it's paraffin wax, and waxes are another group of lipids.
When you look at a molecule like paraffin,
you should notice it's nothing but
carbon and hydrogen. There's
nary an oxygen or nitrogen to be seen.
This is an extremely hydrophobic
substance.  In animals and plants and other
living organisms that produce waxes,
that's what waxes are being produced for.
They're extraordinarily hydrophobic,
and so they're being produced to form
really, really potent waterproof barriers.
If you do organismal biology and study
some basic botany, the Casparian strip in the roots of
plants is based on waxes.  So waxes are
another small group of lipids.
This last group i want to present is not
a particularly large one that gets
talked about
in the lipids chapters of introductory
biology textbooks.
But a couple of its members come up, 
and are really ones you should know anyway.
So if we start with beta-carotene here --
there's vitamin A -- it's an important
plant pigment.  And all the other members of 
this group are derived from vitamin A,
including this guy -- retinal -- which 
in humans and in other
vertebrates --  in the rod cells this is the photopigment
that detects light.  All the members of this group -- called
prenol lipids -- are derived from vitamin A.
Beta-carotene comes up with
photosynthesis, retinal comes 
up with vision and anatomy
and physiology, and so it's worth knowing
that this group is out there.  And so
there you have the last few groups of
lipids: the steroids,
the eicosanoids, the waxes and the prenols.
Big Al :  Has anyone seen the so-called talent?  What the hell is this? "Gone fishing!?!"
He doesn't even own a fishing rod or a
boat!  This is ridiculous!!
He never uses a script, he
expects me to do everything...
He's so hard to work with!
If you enjoyed that video, here a few
others you might enjoy.   Like and
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