Overview of Signaling Molecules - Online Tutor, Practice Problems & Exam Prep

Extracellular signaling molecules are crucial for cell communication in multicellular organisms. Key classes include steroid hormones, gases like nitric oxide, neurotransmitters such as acetylcholine, peptide hormones like insulin, and eicosanoids. These molecules can signal over varying distances: endocrine signals travel long distances via the circulatory system, paracrine signals act locally, and autocrine signals target the same cell. Upon binding to receptors, signals are transmitted either through second messengers or by recruiting multiple proteins, leading to diverse cellular responses.

Types of Molecules

Hello, everyone. In this lesson, we are going to be doing an overview of extracellular signaling molecules.

So, we know that signals are incredibly important for cells because cells in a multicellular organism have to communicate with one another to get the job done. Whatever the job may be, we need to send signals between ourselves to communicate between the cells. Because they're all part of one organism, they need to talk to each other. And the way that they're going to talk to each other is they're going to utilize extracellular signaling molecules, and there are a ton of extracellular signaling molecules. We're not going to go over all of the different classes of extracellular signaling molecules, but we're going to go over some important ones that you are going to go over in your class. But there are more specific molecules as well, but those are probably going to be saved for a later, more in-depth class that may specifically be dealing with signaling molecules.

So, there are many types of extracellular signaling molecules, and they have numerous different functions. Generally, if there's a specific function that needs to be completed, there is going to be a specific extracellular signaling molecule for that function. Some examples of signaling molecules are going to include the ones in this box. So, we have steroid hormones, gases, neurotransmitters, peptide hormones, growth factors, and eicosinoids. That one's always hard for me to say, Eicosinoids.

Steroid hormones are going to be things like testosterone, estrogen. These are going to be hormones that are going to be made out of lipids. And because they are made out of lipids, they can simply diffuse through the lipid bilayer into the cell. They have really easy entry into the cell because they are made of lipids, which the plasma membrane is also made of lipids. Steroid hormones just slip right on through, and they are going to have nuclear receptors. They have receptors that bind to the steroid hormones inside of the nucleus. That's because steroid hormones commonly cause gene transcription to begin. For example, estrogen hormones will go through the plasma membrane into the nucleus, bind to the receptor, and then trigger whatever genes to express that deal with perhaps female development for estrogen.

We can also have gases. You usually don't think of gases as signals. Usually, we think of gases like oxygen and carbon dioxide as products and waste products of the cellular reactions, but gases are very commonly utilized as signaling molecules as well. Specifically, one great example is Nitric oxide, NO. Nitric oxide can easily diffuse through the plasma membrane because it is so small, and it can easily travel throughout the body, and it is going to do a whole bunch of things. It can dilate blood vessels. It can change the blood pressure. It's utilized for a lot of things like that to make the blood flow more evenly and things like that. So it actually does signal to cells to tell these cells to do something generally with blood flow. So Nitric oxide is a gas that is also a signaling molecule.

Neurotransmitters are also another big class of extracellular signaling molecules, which I know you've probably heard of before. Neurotransmitters are going to be things like Acetylcholine, Epinephrine, Dopamine. There's a whole bunch of different ones as well. And basically, neurotransmitters are chemical signals that travel from one neuron to the next to transmit a message. So they're going to travel from one neuron to the next; that's why they are neurotransmitters. They are transmitters between neurons, and these are going to be chemical signals.

Now, we're also going to talk about peptide hormones, which unlike steroid hormones that are made of lipids, peptide hormones are going to be made of peptides. Now, because they're not made of lipids, peptide hormones cannot diffuse through the plasma membrane, so they are going to have plasma membrane receptors, and growth factors are going to be the same way. Some great examples of peptide hormones and growth factors are insulin, endorphins, which is going to be utilized for lactation whenever a baby is around; insulin obviously deals with your pancreas and sugar concentrations, and then growth hormones as well are going to be utilized as extracellular signaling factors to tell different areas of the body to grow.

Finally, we're going to have eicosinoids. Eicosinoids are going to be lipids, but they're going to be lipids that cannot get through the plasma membrane. They're going to be lipids that have a plasma membrane receptor, and these are going to be signals for a lot of things dealing with the immune system like inflammation, fever, allergies, immune response, a whole bunch of different things. But that's going to be what eicosinoids are.

Now that we've gone over some examples of different classes of molecules, let's look at some examples of their functions. What's the point of all these signals? Well, basically, what the point of all these signals is, is we have to move a message. Right? We have to relay the signals inside the cell or outside of the cell. So you're going to have signaling molecules as scaffolds to bring signaling proteins together. Once one signal is activated, it's going to activate a whole bunch of other signaling proteins as well. When all the signaling proteins come together, the action is going to happen. That switch is going to be flipped and that thing, whatever they are signaling for, is going to begin. So, basically, once one signal turns on and all those other signals are activated, the action will begin. Whatever the cell is told to do, it will begin to do. Also, these signaling molecules are great for transducing signals into a different form. This is going to be something like changing the signal from an extracellular signal to an intracellular signal. So if an extracellular signal binds to its receptor on the outside of the cell, then that receptor will trigger an interior signal inside the cell to begin signaling inside the cell. That's the transduction of the signal or changing of the signal. And, really, really important is amplifying the signal through signaling cascades. We'll talk more about this later. Amplifying the signal is very important. Cyclic AMP is going to be a great example of a messenger that amplifies the signal. Basically, it makes the signal really giant, and there's a whole bunch of signaling molecules that are transmitting the same signal. And this is going to cause big changes in the cell. Also, of course, signaling molecules are utilized to spread other signals, anchor other signals, or modulate other signals, depending on what the cell needs to do. So that's a basic overview generalization of what signaling molecules are going to accomplish.

One more thing before we end our lesson is that signaling molecules act over different distances. Depending on the type of signal you're working with will tell you how far that signal can go. There are signals that work really long distances in the body, then there are some that work only locally in the same tissue, but not the whole body. And then there are some signals that really don't go very far whatsoever. So, there are different types of signals that have different distances which they can relay signals. So, the first one we're going to talk about is the endocrine molecules. The endocrine molecules are the hormone molecules. Your hormone system is also called your endocrine system. And these are going to be the really big, bad, awesome signaling molecules because they are incredibly long distance, and that's because they utilize the circulatory system to transmit that signal. Hormones are carried through the circulatory system, through the blood to get to really far distant cells. So a lot of cells can be communicated with at one time. Your endocrine system is your long-distance signaling system. Long-distance, remember that's very important. And remember, these are going to be molecules like testosterone and estrogen, which can affect the entire body. So they have to be long-distance signals.

Then, we have our paracrine system. The paracrine molecules are going to be close proximity or local mediators. So, while endocrine is like the long-distance signals, paracrine is kind of, you know, just local short distance signaling, not very far. And these are going to be things like neurotransmitters and growth factors. Neurotransmitters travel from one cell to the next. They don't go very far. Neurotransmitters, one neuron to the next and that's all that they do. So they are very localized in that particular area.

Last but not least, we have the autocrine which can be kind of confusing to some students, it confused me as well. These are going to be molecules that are sent from the cell to the same cell. So autocrine means self-signaling. These are molecules that are actually being made by one cell. Let me show you what that looks like. So, let's say that this is our cell, and it makes this signal. Well, this signal is going to be utilized to signal the same cell. Which sounds kind of odd, but it is utilized for certain purposes. Eicosanoids are going to be utilized in this manner. They are going to be autocrine molecules for the most part. So, autocrine is the most local, most short distance signaling because basically the cell is signaling to itself.

So, we have endocrine, paracrine, and autocrine, and this is going to be one of the diagrams that will show you this. You can see endocrine right here is being utilized to do the really long distance from one cell all the way to the other. It's the longest distance signaling. And then, we are going to have the paracrine system, which just goes from one cell to the next, kind of like neurons do. And then, we're going to have the autocrine, which goes from this cell back to itself. So, one cell is signaling to itself, which is kind of funny to understand, but it is important for the cell to get cellular things done whenever it has to signal to itself.

So, endocrine, paracrine, and autocrine, you are going to need to know those and you're going to need to know the basic functions of signaling molecules and the basic classes of signaling molecules. Okay, everyone. Let's go on to the next topic.

Cell Response

Okay, so now we're going to talk about the cell response to signaling molecules. So once a signaling molecule reaches a cell, what happens? And what does the cell do with that information? Right. So, let's say a signaling molecule has come in and it has activated a receptor; it's binding to that receptor. And now the receptor has to tell the cell what to do. So how does it do that? Well, it chooses one of two ways. The first is that it takes that signal and it says I am giving it to another protein and that passes that on to the rest of the cell. So it says I am activated right now and I am going to activate something else. And then it's not going to be my problem. I am going to let that other protein deal with it. So what that means is that a receptor takes that signal and then it transmits it to another nearby enzyme, and that enzyme is going to be called a second messenger because it wasn't the first messenger, right? The first one to receive the signal, it got that signal from the receptor. So it's the second messenger. That second messenger can go on to activate other messengers on down the cell. But it's going to keep signaling away from the receptor. So the first way is the receptor doesn't deal with it; it passes it off to another protein. The second way is when a receptor is activated, it wants to be all up in all the business and wants every type of interaction it can get. So what it does is it sends out these kinds of signals and all these proteins come in and attach onto that receptor and then they themselves are activated. So you get this whole scaffold of proteins surrounding this receptor, all of them become activated. They go on to cause more signaling. So the second way is that the receptor acts like a recruiting station or a train station where all these proteins are coming in getting activated and then going out into the cell. So when a receptor is activated, it either passes it off to something else or it's really all involved. And all these proteins come in and bind. Now you can imagine that the cell has to have a variety of different ways of responding to signals. It has to be able to recognize a variety of signaling molecules and pathways. So this is a huge network. There are so many hundreds, thousands even, of signaling molecules that bind differently to different receptors. And then once that receptor is activated, it can either send that off to something else or act as a recruiting hub. So there are so many proteins that are involved in any type of cell signaling response. And not only that, if you take one cell and a different type of cell and expose them to the same signaling molecule, they can have widely different responses. So an example of this is acetylcholine, now, do you remember what acetylcholine is, a neurotransmitter? And if you take a cardiac muscle from the heart and you take skeletal muscle from any of your other muscles and you expose those cells to acetylcholine, they are going to respond differently. The heart is going to decrease its contractions while the skeletal muscle is going to increase its attractions. So that same molecule, probably binding to similar proteins, causes very different reactions because of all the different proteins that come in and signal once the signaling molecule has gotten into the cell and the cell is processing that signaling pathway. So just to show you an image of these methods of receptor signaling, we have the first method where the receptor does not want to be involved. So what happens is a signaling molecule comes in, maybe activates this receptor, and then this receptor says not my problem, it's yours now. Okay. And so that second messenger, that signaling enzyme, goes on, and it takes that signal to the rest of the cell and does whatever it's going to do to activate it and allow the cell to respond. The second pathway is if we have a signaling molecule come in, it starts recruiting other proteins. It may even recruit another of itself. So a copy of itself to dimerize. It can do that. It often does that. And then once this happens, we get a variety of proteins coming into this train station of this recruiting station, all of them are going to become activated and they are going to go and signal further down, or they are going to travel away and do something else. There are so many different actions that these proteins can take inside the cell to allow the cell to respond to that signaling molecule. Okay, so with that, let's move on.

Which of the following is not a classification of signaling molecules?

• A. Autocrine
• B. Paracrine
• C. Endocrine
• D. Lyoncrine

Signaling molecules work only through signaling cascades.

• A. True
• B. False

Here’s what students ask on this topic:

What are the main types of extracellular signaling molecules?

The main types of extracellular signaling molecules include steroid hormones, gases, neurotransmitters, peptide hormones, and eicosanoids. Steroid hormones, such as testosterone and estrogen, are lipid-based and can easily diffuse through the cell membrane. Gases like nitric oxide (NO) can also diffuse through membranes and are involved in processes like blood vessel dilation. Neurotransmitters, such as acetylcholine and dopamine, transmit signals between neurons. Peptide hormones, like insulin, are protein-based and bind to receptors on the cell surface. Eicosanoids, which are lipid-based but cannot diffuse through the membrane, are involved in immune responses like inflammation and fever.

How do signaling molecules transmit signals over different distances?

Signaling molecules transmit signals over different distances through three main mechanisms: endocrine, paracrine, and autocrine signaling. Endocrine signaling involves hormones that travel long distances via the circulatory system to reach target cells throughout the body. Paracrine signaling involves local mediators that act on nearby cells, such as neurotransmitters and growth factors. Autocrine signaling involves molecules that act on the same cell that produced them, often seen in immune responses with eicosanoids. Each type of signaling ensures that the appropriate cells receive the necessary signals to perform their functions.

What happens when a signaling molecule binds to a receptor on a cell?

When a signaling molecule binds to a receptor on a cell, it can trigger one of two main pathways. In the first pathway, the receptor activates a second messenger, which then transmits the signal to other parts of the cell. In the second pathway, the receptor acts as a recruiting station, attracting multiple proteins that bind to it and become activated. These proteins then propagate the signal further into the cell. The specific response depends on the type of receptor and the signaling molecule involved, leading to diverse cellular outcomes.

What are the differences between steroid hormones and peptide hormones in terms of signaling?

Steroid hormones and peptide hormones differ significantly in their signaling mechanisms. Steroid hormones, such as testosterone and estrogen, are lipid-based and can easily diffuse through the cell membrane. Once inside the cell, they bind to nuclear receptors and often initiate gene transcription. In contrast, peptide hormones, like insulin, are protein-based and cannot diffuse through the cell membrane. They bind to receptors on the cell surface, triggering a cascade of intracellular events through second messengers or protein recruitment. These differences result in distinct pathways and cellular responses for each type of hormone.

How do cells amplify signals received from signaling molecules?

Cells amplify signals received from signaling molecules through signaling cascades. When a signaling molecule binds to a receptor, it can activate a second messenger, such as cyclic AMP (cAMP). This second messenger then activates multiple downstream proteins, each of which can activate additional proteins, creating a cascade effect. This amplification ensures that a small initial signal can produce a significant cellular response. The process allows cells to respond robustly to even low concentrations of signaling molecules, ensuring efficient communication and coordination within the organism.