this video is part of the higher level topic c2.1 on chemical signaling and we'll be taking a deep dive into signaling molecules and receptors theme C is all about interaction and interdependence and in order for cells to interact with each other and really work together they have to be able to communicate and they're going to do that through signaling molecules and those signaling molecules are going to need receptors another name for the signaling molecule are lians okay so these lians are shown over here in red and they are going to act as signaling molecules and cells are going to pick up on those signals if these lians can bind with protein receptors so I would find these protein receptors embedded in the membrane of cells if they have them and they have on them a binding site for these lians now when this Lian binds with the prote protein receptor it causes a slight change in the shape of this receptor protein and that slight change in shape can cause this Cascade of events it can either cause some change in gene expression or it can kick off a series of cellular responses but there's something about the binding and the changing of the shape that makes stuff happen now if this is looking familiar to you and you're thinking enzymes great I love that because there are some things that are similar right so lians are specific to their receptors just the way that enzymes are specific to the substrates that they can act on the liand also remains unchanged so this is a little bit of a difference here right so in enzyme reactions I would expect the substrate to be changed by the enzyme but in this case the liand remains unchanged so the liand um is not being worked upon it's not being Chang Changed by this protein receptor the magic here happens when they are together okay so in that way they're just a tad bit different I would say the other difference I would maybe think about here when I'm comparing it with enzymes is that when an enzyme connects with a substrate it's like very quick enzyme changes substrate substrate uh those products are released and we all move on with our life here it's a little bit different here not only is the liand remaining unchanged but it can actually remain seated in and connected to this receptor for quite some time okay and so that that that cellular response can last um quite a long time so it's good to notice some similarities but keep our eye on some differences now you can imagine if I have more of these Lins more of these chemical messaging molecules I'm going to have a greater response okay and so this is related to a phenomenon that we call Quorum sensing and this is a change in behavior of a colony so a group of living organisms when it's population density reaches a certain threshold so you can imagine if I have more cells okay that are sending out more chemical messaging molecules then at some threshold of messenger molecules I'm going to have a change in activity so this is a great example of interdependence where if I just have a few cells sending out a few messenger molecules I'm going to get no great change but if I have a lot of them then I'm going to see this difference and there's a great example here um in this vibrio fisheri this bacteria that is bioluminescent and has a mutualistic relationship with the bobtail squid so in large enough numbers this um bacterium starts to bioluminous and that's again only going to happen when it reaches a certain population density you're going to notice a lot of divers in the types of chemical signaling molecules that cells employ so this will include things like hormones neurotransmitters cyto canines and calcium ions just to name a few and we'll start off by talking about hormones hormones come from endocrine glands and endocrine glands are Grands glands that secrete hormones directly into the bloodstream you may have already studied exocrine glands and they secrete non hormone substances and not into the bloodstream but through ducts endocrine glands we need to be thinking bloodstream and hormones and because they travel in the bloodstream they're going to go all throughout the body so how is it that we can control where those effects take place well it has to do with receptor proteins so only target cells are going to have the receptor protein capable of binding with that particular hormone the hormones that are in the bloodstream floating past non-target cells they don't have that receptor protein so even though that hormone is there it doesn't affect a change in this cell or in this tissue because it doesn't have the receptor protein this is a really interesting way of spreading messages here um hormone messaging can have a long effect time um some great examples here insulin glucagon these have to do with glue glucose homeostasis and then of course sex hormones like testosterone and estrogen um again we want to be thinking about target cells and receptors and how our cells utilize those to make sure that messages are only interpreted and acted upon by certain tissues next we'll talk about neurotransmitters as examples of chemical signaling molecules so these are much different from hormones they don't travel in the blood they only travel in a very small space between two neurons and that small space is called a synapse maybe you've already studied synoptic transmission but if not here it is okay so it's a very isolated effect whereas hormones are traveling all over the body this is isolated isolated to just that one tiny Junction between these two tiny nerves they also have very short effect time so these are a very rapid chemical message in um mechanisms and the reason for that is is because these neurotransmitters are very quickly removed from the synapse so we're going to notice neurotransmitters being released by one neuron and then they bind to another neuron but almost immediately after that message is passed these neurotransmitters here are either destroyed by enzymes or they are taken back up into this original neuron so very short messaging time here some great examples dopamine and serotonin we might find those in the brain those are great um neurotransmitters for different brain things and then acetylcholine is a neurotransmitter that can go between motor neurons and muscles but the mechanism here is all the same now let's move on to our next messenger these are cytokines and these are proteins that are going to be passed between nearby cells so this is really how cells in a tissue communicate with each other especially if they're not nerve cells right because nerve cells can use neurotransmitters so what this will look like is that a cell will receive some kind of stimulus okay from the environment and produce these cytokines those cytokines will bind with receptors on nearby cells which can produce some kind of effect and that effect can really vary depending on the binding site so different cells with different binding sites um may have a different outcome so some great examples interferon those are cyto kindes that are going to help with like inflammation processes and in our immune cells so if one cell is infected with a virus it can kind of like worn some cells nearby or kick off an immune response um or rrop potin can produce red blood cells so again these are chemical messenger molecules between two nearby cells and the fourth one calcium ions these are a bit of a weirdo because the first three that we talked about hormones neurotransmitters and cytokines these are all biological molecules okay so some kind of like protein or lipid something like that calcium ions are exactly what they sound like they're a calcium ion but still with the right receptor on the right target cell these calcium ions can do some pretty amazing things so two really great examples here one is with muscle fibers so when we have a muscle contraction calcium ions are going to really allow that mein head to bind with actin so if you haven't already studied musculature in the sliding filament theory here's a quick review um a protein called meios has to bind with another protein called actin because that is what slides muscles Inward and helps them to contract but almost all of the time there is a fiber covering up the bind ing site on this actin molecule and that's going to prevent the mein from binding well if we want a muscle to contract then our muscle cells need to secrete these calcium ions and the calcium ions bind to this um Protein that's covering the binding sites on actin and it moves it out of the way and that allows mein to bind to actin so before we can even start with a muscle cont raction it has to begin with a calcium ion secretion first then if we want to think about neurons again if you haven't studied synaptic transmission yet that's okay but we already know about neurotransmitters these neurotransmitters have to be released into the synapse that is going to be initiated by the influx of calcium ions into this preoptic neurons so calcium ions are going to have to move in here in order to get these neurotransmitters to be secreted out of the cell so it's a great example here of chemical signaling molecules having different effects um on different types of cells throughout the different topics on physiology I think you're going to find that hormones and neurotransmitters are talked about with much greater frequency so let's take a deeper dive into those two they evolve separately many times and we know this because they are so different they have such different forms they have different functions it's not possible that they all derived from the same chemical family however they do have some things in common um they are both very small molecules and they are soluble in water and we know that they have shapes that are compatible with their receptors so that's true of both hormones and of neurotransmitters no one expects you to be an organic chemist and understand what exactly all of these different uh chemical groups are but you are expected to have an appreciation for the diversity of different chemical structures that make up both hormones and neurotransmitters so hormones can consist of things like steroids like testosterone amines like melatonin or peptides like insulin and all of those are hormones um but they consist of very different chemical structures they are considered hormones because of the way that they work and where they come from in our body not necessarily what their structure is and we can say the same thing as neurotransmitters so neurotransmitters have the same mechanism and they all come from neurons but they have very very different chemical um structures so it could be like nitrous oxide it could be a glutamate it could be um dopamine here could be Esters like acetal col so again all of these are neurotransmitters but have very different chemical structure and that's really the main learning here in addition to their different um chemical structure their effects are also quite different if we're comparing hormones and neurotransmitters so again hormones can act over large distances because they are traveling through the blood um to a variety of target cells and again only target cells with that receptor are going to be affected by that hormone neurotransmitters on the other hand are going to have a much l more localized effect because they're only traveling the distance between two neurons so I know on this page or on the screen it looks like this distance and this distance are the same they're not so imagine a hormone being produced um in the pituitary gland but it really acts on the ovaries that's a lot of distance this we're talking about nanometers okay between to neurons so a much more localized effect here so now let's reframe our thinking let's think about another way to categorize these not in terms of their chemical structure or whether they're a neurotransmitter or a hormone or a cyto let's just go ahead and come up with a new categorization a simple one can they enter the cell or not because whether or not a chemical messenger molecule can enter the cell is going to dictate a lot about what kind of receptor protein that we need okay so if a signaling molecule can enter the cell then its receptor protein is going to be located inside the cell and that's what this word intra means inside so the receptor is intracellular intracellular receptor because a signaling molecule can enter the cell that means that the receptor is going to have to be covered in Hydro philic amino acids okay so this refers to the surface of the receptor and that is going to be very important because this receptor is floating around in the cytoplasm and it must be dissolved okay so this cytoplasm is made primarily up of water and so it's important that this receptor is soluble in water if it's going to be hanging out in this cell so a great example here here are things like steroid hormones there are other things that can do this this is just an example but this steroid hormone can enter the cell it's it is hydrophobic so it's going to enter the cell and bind with this intracellular receptor which is covered in hydrophilic amino acids and then it can go off and do its things and we'll talk about that later okay but just a little bit of a difference here that can be a little tricky the hormone itself is hydrophobic it's non-polar that's how it's able to get past these hydrophobic tails in the membrane the receptor is covered in hydrophilic amino acids because it must be dissolved in the solutions on the inside of the cell that will be very different for molecules that cannot enter the cell so something like non-steroid hormones like insulin let's say insulin can't get into the cell it just can't it's not hydrophobic it can't get past those Tails so it would not make sense for that receptor to be inside the cell because the molecule can't get in the cell so its receptor protein is going to be embedded in the plasma membrane and we call that a transmembrane protein it spans both sides of the membrane so the receptor has a part sticking outside of the membrane and a part sticking inside of the membrane a transmembrane protein and because it needs to be a transmembrane protein it needs to have hydrophilic and hydrophobic amino acids so remember these hydrophobic tails are hydrophobic and this part of that transmembrane protein needs to be covered in hydrophobic amino acids however on those um phosphate or phospholipids those heads are hydrophilic they are water loving so on a transmembrane protein those amino acids in that area also need to be hydril so there are some things to consider um when building these transmembrane proteins for molecules that cannot sit outside of the cell okay or that cannot enter the cell so some big implications there okay so just to sum things up here and I'll maybe try to color code these differently we're going to need transmembrane receptors for signaling molecules that cannot enter the cell for signaling molecules that can enter the cell I can have an intracellular receptor because it can enter the cell