M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Hormones get no respect. They’re one of the two major control systems of your entire body, but nobody ever pays attention to them. When I weep out an optical illusion in my class, kids are, "Oh show us another!" But I cause one little explosion to demonstrate the effects of adrenaline, and everybody gets all bent out of shape. The only time people talk about hormones is when they’re blaming them for their mood swings, or for the icky stuff that’s starting to grow on their body during puberty.
Now, the nervous system it’s really good at doing things like detecting sound or helping keeping you balanced. But the hormone system isn’t good for those sorts of things. The nervous system gets its wonderfulness, because it’s highly specific. It’s good for detecting danger. And let's say if somebody started to throw something at my face, I could detect it and send nerve signals to just the right muscle cells that are needed to cause the response. It does that by sending electric signals along the specific nerve cells that are responsible for that reaction.
Your hormone system on the other hand, it works by having a gland somewhere in your body release a chemical. That chemical washes through your circulatory system. It goes to every cell in your body that has the right receptor protein that matches up to that particular chemical. It will trigger off all sorts of reactions depending on which cell it hits. If I tried using my hormone system to respond to something flying towards my face, that all body response is pointless. Luckily the guy who’s throwing can’t hit worth a darn.
Now, on the other hand, for a long term, all body responses that take long periods of time, then the hormone system or endocrine system as it’s properly called, is the way to go.
Imagine using your nervous system to take care of things like growth. You would have to focus for 20 years for that. You get distracted by SpongeBob and oops after a while you wind up being a half a foot shorter than you should have been. Now, a lot of times the nervous system and hormone systems actually work hand in hand together, to influence each other's behavior, and to coordinate their actions.
An example of this is when your body releases adrenaline. When that does, you know this. It makes your muscles jittery, why? Because they’re becoming much more sensitive to the nerve signals sent by your brain. Or alternately, when you tan, that’s caused by your hormone systems. What’s happening is that when light enters your eyes, that sends signals to your Pituitary gland. If you get a high enough amount of light, the signals being sent to your pituitary gland, it’ll start releasing something called MSH, or Melanocyte-stimulating Hormone.
The Melanocytes are the cells in your skin that produce the melanin that I obviously can’t do very well. That melanin is what makes you tan. This is kind of amusing to me. Because I see kids trying or people at the beach trying to get a tan, what are they wearing? Sunglasses thus blocking the light that is needed to signal the need for a tan.
Now hormones work basically the same way. They're based on the same chemical signalling process that’s been used forever since there’s been more than one kind of cell in this planet. It’s the same basic process that goes on whether you’re a slim old amoeba, trying to send signals together for reproduction. Or you’re a plant trying to control where the roots grow and where the trunk goes, no matter what position the seed is in. I’m going to focus in on human hormones, but once you get this idea down, you’ll be able to handle plant hormones or even the chemical signalling that goes in between cells of the immune system.
Now before I get started with this, I’ll go into a process known as negative feedback. Negative feedback is the main way that your hormone system is controlled.
Once you get that down, we’re going to the hydrophilic hormones; the ones that are water soluble to talk about how they get their actions. Then we’ll discuss the hydrophobic hormones. The ones that are non-water soluble and what are their effects.
Like I said, negative feedback is the main way that the hormone systems are regulated. Before I get too far into it, I want to make sure that you understand that negative feedback in an underlying principle that governs a lot of Biology, from the cellular level to the whole ecosystem level. So it’s an important thing to understand for the AP Biology test.
Now what is negative feedback? Well, negative feedback is the idea that when there’s a change in the environment, or stimulus, your body will compensate. It will come up with a response to that. Now that response tends to help get rid of the original change, or compensate for it. Which ultimately ends up slowing down or even stopping response.
Now I can say that, but a lot of kids will just go huh? Let me give you an example that is not hormonal. Let me give you a couple of them. That will really help you understand this.
For example when you get cold, your brain detects I’m cold. So what does it do? It sends signals to your muscles and you start shivering. What does shivering do? Well, it generates heat. So the response to being cold is to make heat. What does the heat do? It gets rid of the cold. So you ultimately stop shivering. You stop doing the response, thus negating it. And that’s where the whole negative feedback of it comes up.
What about another example? Well, let’s suppose you have a sudden increase in the number of rabbits in an area. Well, more rabbits means more food for the wolves. They can have more babies. So you wind up having an increase, a response of more wolves in the environment. More wolves mean more dead rabbit. So the rabbit population starts to drop.
This causes the number of the wolves to start starving and so the response, the increase in wolves negates itself returning things back to normal.
Now I bring this up, because, recently the AP Biology essay has been having a lot of questions. Where they say give multiple examples from different areas of Biology of some concept. If they ask about negative feedback, just knowing what I said about the shiver response, or how predator and prey relationship keep things around some normal level, those two examples are enough for you to get a decent score in the essays. If you spend the time to think about it, you’ll look around the world, and you’ll be able to come up with lots of them. Thus getting yourself a really good score on those essays.
Now, what would be an example of a hormone system negative feedback loop? Well, the basic way this works is that, your hormones are all released from glands. That gland will detect drop in some chemical, say for example in your blood supply. That will trigger off to start releasing the hormone. This can also be detected by the change in the chemical properties sometimes detected by your brain. And then your brain will trigger the gland to release the hormone.
Well, if say the chemical got too high, you start releasing the hormone, the hormone will then trigger the chemical to go back down, and then the hormone levels will go back down again. Let me give you a very specific example of this. And that will help cement it in your mind.
Think back to Halloween. You go out Trick-or-Treating, you come back with a big old bag of candy. Now your mum gives you that very wise advice, "You should eat maybe five pieces of candy tonight, and then you do that every night and you’re going to have candy for a month." You nod at her very wisely, and say, "Yes, mum you’re right." You go up to your room, you get that bag and you strap it on and you start eating. Who cares about a wrappers, that’s just extra fiber in your diet.
Well, pretty soon your blood glucose level starts to spike really high. Your pancreas organ immediately underneath your stomach, that also produces intestinal enzymes. That pancreas has special cells and things called Islets of Langerhans, don’t blame me, they’re Dutch. Those special cells in the Islets of Langerhans called beta cells, start releasing insulin.
Insulin is a chemical also signal a hormone, that goes into your blood supply. It goes everywhere throughout your body. You've heard about it, because who people have problems with it are called diabetics. That insulin triggers every cell in your body, or most of them to start opening up special channels that allow glucose in from your blood supply. Not only that, your muscles and your liver, start opening up their channels and convert the glucose into a different polysaccharide called glycogen.
That glycogen pours into your muscles, gets into your liver, and that brings your glucose levels back down. Once the glucose levels get back down, the insulin being released by your beta cells in your pancreas, they start to go down too. So you wind up returning to normal.
Now some of you are pretty on the ball. You realize, "Hey what happens if the blood glucose levels get really low?" Well, you’re always releasing a certain amount of insulin. So you could possibly just stop releasing any insulin. But what if the glucose levels are still continuing to drop? This highlights something that is a basic principle of how most hormones actually work. It’s not just negative feedback, but you actually wind up having two different hormones that work in opposition to each other.
This idea of two different hormones working in opposition, is called antagonism, or antagonistic hormones. It’s kind of like how the gas and break pedal in your car. One makes the car go faster, one makes the car go slower. So working together you can maintain a constant speed, especially if you’re doing something like going down the hill.
So what’s the antagonistic hormone to insulin? Well, remember how I said there was beta cell? That kind of implies that there’s alpha cells. Alpha cells release a chemical called glucagon. What happens is. if your blood glucose levels get too low, the alpha cells in your pancreas release glucagon. That goes to your liver cells and that glucagon triggers the liver cells to start breaking part of the glycogen.
That glycogen becomes glucose, which is released into your blood supply. So that brings your glucose levels back up. So, working together, the insulin and glucagon, can go up and down to keep your glucose levels at some constant level. Thus making sure your brain has enough energy to run as well as the rest of your body. These levels that hormones will aim to keep everything at, are called homeostatic set points.
So you get in general how does insulin control blood glucose levels. But how does it actually cause its effects? It turns out that insulin is an example off a kind of hormone called a hydrophilic hormone. Now sometimes they can just call these water soluble hormones.
Now what is a water soluble molecule? It’s a molecule like an hydrogen bond, and because it can, it winds up being able to easily dissolve in the blood stream and be carried along by the blood plasma. So gland like pancreas will just dump a water soluble or hydrophilic hormone into the blood supply. That will just flow throughout the bloodstream.
Now however, because it’s polar it can’t cross the membrane to cause the effect in its target cells. Instead, it has to wind up binding to a particular receptor protein on the surface of the outside of the cell. Let’s take a look at that.
So here we see insulin. It binds to this molecule here which is an insulin receptor. That’s a specialized protein that has a matching side that perfectly matches the shape of insulin. Now the insulin stays on the outside. The insulin receptor on the other hand triggers off the creation of number two.
What’s number two? That’s some other chemical that’s inside the cell. That second chemical can trigger off a whole series of biochemical reactions. Whether it’s causing this glucose transporter to open up to allow glucose in, or can cause the creation of glycogen. Or all sorts of other things to occur like increasing the rate glycolysis. That’s why the second molecule is often called a second messenger. A lot of times people will just talk about second messenger proteins, or second messenger hormones.
Now what is the second messenger? There's many different chemicals that act as it, but the most common one is a second messenger molecule called Cyclic AMP, or Cyclic Adenosine Monophosphate. So that is the most common one that is used. Now how is that made? Well, let’s take a quick look at a YouTube video that shows the process, the steps that are involved in making CAMP or camp for short.
So here is our YouTube video. Let’s go ahead and make it larger. Now before we get things started, I want to go over some of these things so you can understand, because this is pretty short video. Right up here, this low yellow ball, that’s the insulin. This right here, the kind of yellowish thing that extends outside the cell, that’s the receptor. You can see it’s got a little cap. That’s the right shape to fit the ball of the insulin.
These other guys inside of the cell, those are the proteins embedded within the membrane, that ultimately at the green guy, cause the creation of the Cyclic AMP. For those of you who are really into names that’s called a Cyclic Adenylates.
So let’s go ahead and get it started. Here we see the insulin lands on the receptor, which causes a series of biochemical events. Then we’re going to have our Adenylate cyclase, the thing that makes Cyclic AMP turn that little blue guy there his GDP. And it eventually will trigger off the creation of our Cyclic AMP. That’s it.
So those Hydrophilic hormones they stay on the outside of the cell, binding to receptors. And those receptors trigger off that second messenger system, which can cause all sorts of effects. With the Hydrophobic hormones on the other hand, they get their effects by going into the cell and generally they’re limited to turning on or off genes inside the nucleus.
So let’s take a closer look at the hydrophobic hormones. In general, because they’re not water soluble, most of them are the steroid hormones. There are a few hydrophobic proteins out there protein hormones, but the most largest class of the hydrophobic ones are the steroid hormones. Now because they’re not water soluble, when a gland releases them, they need to actually be carried in the blood supply by a special protein carrier that helps them dissolve their way through the blood supply.
But once they get to their target cell, or target organ, then their non-polar nature takes over and they can actually penetrate into the cell. Let’s go ahead and take a look at a quick YouTube video that shows the effects of steroid hormones.
Now taking a look at this YouTube video, let’s go ahead and make it large, so I can properly explain to you what’s going on here.
So out here we can see outside the cell, and that’s where the hormones are going to be coming in. Now in this video they don’t show the steroid hormone being carried by the carrier protein that’s in the blood. But let’s assume it happens.
Now again here is the plasma or cell membrane. You don’t see a receptor protein there. Instead, you see the nucleus. Now just like with the hydrophilic hormones, you need to alternately have a receptor molecule, some kind of protein. But now it’s inside the nucleus and it’s going to going there to turn on or turn off some genes. So let’s go ahead and get this video started.
Here we’ll see the steroid hormone which you can recognize, because of its steroid core, the group of four interlocking rings.
That’s it coming on right about now. Here you see because it is hydrophobic, it goes straight through the plasma membrane. Notice the shape of the receptor, it properly matches up to the shape of that steroid hormone. Now, this combination of hormone plus receptor, will go to a particular gene recognizing its promoter and it’ll trigger transcription which is the creation messenger RNA.
That messenger RNA now will do what m-RNA does. It will leave the nucleus going through the nuclei membrane. And there in the cytoplasm it will start the creation of a new protein. That’s the protein that will carry out the effect of that steroid hormone, whether it’s growth of hair or increase muscle size or whatever. So there you see, that’s how hydrophobic hormones work.
To do a very quick review of the two different kinds of hormones, let’s take a quick look at a hydrophobic hormone. You can see, the big difference is where is the receptor protein? With a hydrophobic one, the receptor protein is here within the cell. While with the hydrophilic, they’re staying outside the cell. Now you may also realize that since the hydrophobic are going in and they’re turning on genes, that sometimes it can take a little bit longer for their reactions to be generated. While with the hydrophilic ones, the ones that may be triggering off an enzyme, that’s already made as opposed to causing the creation of a new enzyme, the hydrophilic ones are often a little bit faster.
Now on the AP Biology exam, if they’re going to ask any questions, they’re going to focus in on insulin and glucagon; the two that are controlling glucose levels. A second one might be the thing that controls Calcium which is Calcitonin, and PTH, which is short for Parathyroid hormone.
If there’s any others that they might ask questions about, it would be those that are controlling things like the water level in your body i.e. controlling your kidneys.
So spend maybe 15 minutes looking at the chapter summary on the hormone system that’s in your textbook, unless they have a nice little summary page. Go through that. Remember a few of the effects, and that way you can just spot the right answer in the multiple choice question. Or be able to vomit up a few of these names on the essay portion, because they’re not going to ask a big long thing about the hormones, outside of what I’ve gone over. Do that and you will be good to go.