When I say the word hormone, what comes to mind?
For many people, sex hormones are the first thing that they think about.
The hormones involved in sexual differentiation.
Hormones of course have much greater roles than that in sexual differentiation,
and that will be the topic, what I’ll be talking about in this lecture.
So the multicellular organisms have unique needs in responding to the environment that they’re in.
That those needs relate to for example, the food status.
We can think of, “I’m hungry, I go out and I want to get some food to eat”.
At the cellular level within the body, the individual cells have the same needs.
And they have to communicate with the rest of the body about their needs to get nutrients.
Cells in the body and the body as a whole also have to be able to respond to dangers.
We’ve all been in the circumstance for example, where we’re walking
alone at night and we think we hear noises or something to that effect.
Our bodies response to those dangers is also mediated by hormones.
And we look at the process of growth, we forget for example, that we start
out as a single cell organism, and today we have trillions of cells.
That growth had not only occurred but it to be managed properly.
My left arm is the same length as my right arm.
How did that happen, was that by chance?
No, in fact that was controlled by hormones.
And last, we have to be able to respond to injury.
If we break our leg, it’s really nice to be able to heal, but
the healing doesn’t happen in the absence of other things.
Hormones of course help to mediate that process.
So when we think about what hormones do, what they’re really doing is coordinating efforts.
If we think about an organism as an army, and the individual cells in the organism are soldiers.
Then getting those soldiers to be able to respond properly to the things that
they find and fight the battles that the organism has to find is important.
Hormones help to coordinate that effort as we shall see.
Now one of the challenges that happen with hormones is, actually
occurs at the level of the structure of an individual cell. Individual cells are
comprised of membranes. And those membranes are themselves
comprised of individual lipids. Those lipids are what we call
phospholipids and those phospholipids have an orientation like which you see on the screen here.
So the lipid bilayer is the cell’s protection against the rest of the world.
When you look at the structure of lipid bilayer, something about the way in
which the molecules organize prevent the movement of things into the cell.
This can be good and this can be bad.
The lipid bilayer contains individual phospholipids.
Phospholipids have a phosphate attached to them and those phospholipids have charges.
So when you look at a lipid bilayer for example, the top and the bottom are what we
call the outside of the lipid bilayer, and that’s the part that interacts with water.
Those parts are polar, positively negatively charged things.
Between those two layers are a non-polar or hydrophobic layer.
Now this combination of the positively charged, and negatively charged outside and
the non-charged inside is what makes it hard for molecules to cross that barrier.
As I said, that can be good in some cases and bad in others.
In the case of hormones, hormones have to either be able to navigate that barrier
by themselves which a few do, or more commonly they have to interact with receptors.
And these receptors are proteins that are embedded in the lipid bilayer of that cell.
We can see lipid bilayers spontaneously form.
Their structure and their chemistry allows them to actually form the membrane of a cell without even any effort.
There’s no enzymatic effort required for example to make a lipid bilayer.
Now hormones are molecules as I said that coordinate the efforts of the organism.
And they have to work with the restrictions of the lipid bilayer.
Hormones are made in one part of the body and move through the
bloodstream to another part of the body to exert their effects.
So for example, our adrenal glands release hormones called epinephrine.
Epinephrine’s also known as adrenaline.
So when we get in that dangerous situation, the adrenal gland releases
epinephrine and it travels to our muscle cells and to our liver cells.
And at those points it exerts its effects, telling the muscles cells, “Hey, let’s run and get out of here,” and
telling the liver cells, “Hey, let’s leave some glucose in the body so those muscle cells have some energy".
Now hormones are what we call in the overall system, a first messenger.
So first messengers are the very-- and start of a signal.
That signal is going to cause a response at the-- when it reaches the target cell.
Now hormones comes in a variety of forms, and I’ve put a few hormones on the screen here.
We’ve all heard of thyroid hormone.
Thyroid hormone of course is produced by the thyroid gland and has iodines as you can see attached to it.
It’s about the only molecule in the body in fact that has iodines.
Epinephrine as I’ve already mentioned is also known as adrenaline and
is used to help us in what we call the fight or flight response.
Epidermal growth factor's a very different molecule, in fact it’s a protein.
Some hormones are proteins and epidermal growth factor helps the body to regulate its growth.
And last we have a sex hormone, progesterone.
Progesterone is important as a precursor of other hormones and exerts its effects relative to pregnancy.
Now hormones as I said, travel in blood and they exert their effects through receptor proteins.
These proteins are, as I said, embedded in the lipid bilayer of individual cells.
Most hormones bind to receptors in the cell membrane but not all of them.
Some hormones like the steroid hormones can actually cross the lipid bilayer on their own.
Most molecules can’t do that but steroid hormones can.
And when they do, they bind to receptors that are on the inside of the cell.
So the steroid hormone receptors are mostly inside the cell whereas most
other hormone receptors are on the outside or the membrane of the cell.
Now a few steroid hormone receptors have been discovered in recent
years that actually are in fact located in the membrane of the cell.
And I’ll briefly mention them later, but most are located inside.
Most receptors convey their message through what are called second messengers.
And we’ll talk about those.
These are small molecules that are found inside of the cell.
And a thing that surprises people when they study the process of
hormone signaling is that there’s usually very many steps to the process.
Now what very many steps give is flexibility for the cell to respond to many different kinds of messages.
But it makes it for students, pretty hard to learn because there’s a lot of things to remember.