Bozeman science: Positive and negative feedback loops

hi it's mr. Andersen and welcome to biology essentials video number 18 this is on positive and negative feedback loops if you've ever driven you've probably seen a sign like this this is a guilt sign so they put the speed limit up here and then they show how fast you're going down the bottom but you constantly get feedback as far as your speed goes so you will see that you're going a little fast and so you'll slow down but then you'll realize you're slowing down way too much and so then you'll speed up and if it works out well you'll hit that speed limit or that target set point now what I'm just showing you here is what's called a negative feedback loop in other words you'll dance around that point you'll speed up you'll slow down and eventually you hit that point so negative feedback loop brings you closer to the target set point a positive feedback loop boo you would experience if you were to see this sign as a challenge so you see you're going 30 miles an hour to realize that you get a ticket or break the sign and so that's a positive feedback loop it's when you're amplifying and moving away from that target set point so in this podcast I'm going to start by talking about homeostasis and that's our internal environment and to maintain a stable internal environment we use feedback loops with each of those you have a target set point in a negative feedback loop you're going to move above or below that but try to stay as stabilized and so you're as close to that as you possibly can be example I'll give you is temperature regulation in mammals um in a positive feedback loop you move away from that target set point so you amplify that example give you is fruit ripening why all the apples on a tree seem to ripen at the same exact time now with a feedback loop you can always have mistakes you can always of alterations in that feedback loop and that can lead to tragedy example be diabetes is a problem in creating insulin or sensing that insulin and so that's just a problem in a feedback loop and it can lad to lead to really bad things and so let's start with homeostasis homeostasis if we were to define what it is it's an internal stable environment and so if you live in a pond if you're a Paramecium like this and you live in a pond you let a lot of your environment just go and you maintain that in environment by just maintaining the same as the polland around you so you have this external pond and whatever the temperature of that is that's the temperature of the Paramecium now they'll regulate a few things like water concentration using contractile vacuoles but mostly they have a pond now as we move out of the pond error as we become more sophisticated we kind of bring that pond with us that internal pond now is what homeostasis is and so this is a hairless cat hairless cat is going to maintain its body temperature and so it does that through feedback loops it's going to maintain its blood glucose level it does that through feedback loops it's going to maintain the osmolarity of its blood it does that through feedback loops now hairless cat doesn't have hair and so it's actually hard to maintain its its internal body temperature and so you don't want to let them go outside on their own lots of times put a coat on so they can maintain that now there are kind of two life strategies in this picture it'll take you a second to figure out what it is this is a snake eating a rat and it's taken with a thermal image camera and so the the the snake itself is what's called an ecto therm and what that means is that their internal temperature is going to be the same as their external temperature so this is this the snake is about the same as you know whatever the counter that it's sitting on but the mouse is going to be an endotherm and so that is going to keep an internal temperature that's going to be constant now there's some advantages if you keep an internal constant temperature then all the metabolism will work at the same exact rate the problem with that is that the rat right here is probably going to eat a lot more than that snake just to maintain that body temperature so let's see how that works in a mammal and so in humans we use a negative feedback loop to maintain our constant body temperature and so our constant body temperature is around 37 degrees or about 98.6 degrees Celsius and so we use a negative feedback loop to maintain that so let's say we put a thermometer in this mouth and the temperature all of a sudden increases now you actually sense that the area where we sense that is on the roof of our mouth in an area part extension of the brain cut the hypothalamus and so we're going to sense that temperature change and so what's the first thing that your body does when you start to get too hot is that you're going to sweat and so what does that do well as these water molecules evaporate that's going to create evaporative cooling in other words they're going to carry a little bit of heat with them you'll also start to vasodilate in other words the blood is going to be carried towards the surface of the skin and so then just through convection we're going to start to lose more of that heat to our environment now another thing that would happen if we had fur which we don't really have is that that fur is going to lay really flat and the reason why is that then we can have more of that heat being lost through convection so what does that do to our temperature well it's going to drop but it might drop a little bit too far so then we're going to turn those things off we're going to quit sweating and we're going to quit vasodilating but now we're too cold so what do we do how do we keep ourselves warm well we're going to start to maybe get goosebumps and these are kind of crazy goosebumps but we're going to get goosebumps right here and what goosebumps do is if we had hair to actually stand the hair up on end but it also kind of pulls your skin in it's like taking a coat and kind of pulling it in its conserving that heat we're also going to vasoconstrict in other words we're going to shut off those capillary chanson we're going to cold that body that blood in towards the inside of our body to maintain that and as a result of that we're going to have less convection and our body is going to increase until we hit that target set point and so right now in fact throughout the whole day your body is just regulating your body temperature and it's doing that through a negative feedback loop again trying to maintain that target set point what happens if we want to go in the opposite direction well let's say we have this so we've got a fruit on a tree and we call that the target set point but let's say we want to move from fruit on a tree to fruit that is ripe now why do trees make ripe fruit I could talk about that for a long time what they're essentially doing is making it a track so birds and humans are going to come and eat the apple and spread the seed somewhere else but how do they maintain that well they communicate and it's kind of under hard to understand how plant can communicate but they communicate through a plant hormone called ethylene ethylene is just c2h4 and it's given off by ripe fruit in other words this gas is going to be given off by ripe fruit it's going to be picked up by apple's next to it and then it's going to cause them to ripen as well and so if you take one nasty over ripened apple and put it in a barrel of apples that aren't ripe at all they will all ripen as a result of that ethylene and so one bad apple can really spoil the whole lot but how is this an example of positive feedback what goes on is that that first apple will become ripe and so it's going to start giving off ethylene and that ethylene is going to be picked up by Apple's right next to it and they're going to create more ethylene which is going to create more ethylene which is going to create more ethylene and so through this positive feedback loop or amplification all of a sudden all the fruit on the tree are ripe at the same exact time another famous example of positive feedback would be loop would be in childbirth the pressure of the head on the cervix of the mother actually causes contractions would pushes more pressure on the cervix which causes more contractions and so eventually the baby is born and so we usually we see positive feedbacks when we want something to happen really really quickly it's not something that we maintain for a long period of time so what happens when something goes wrong or what happens when something there's a mistake a great example of a feedback loop in us so this is a negative feedback loop would be blood glucose levels and so the blood glucose level is the amount of glucose that's moving around the blood in your body where else could it be that glucose could also be taken in by the cells so they can do respiration we can get ATP from it or we could also store it in glycogen which is mostly going to be found in the liver and so we use two different hormones insulin and glucagon to do that and so this is your pancreas pancreas is going to it's major job is to empty digestive enzymes into the small intestine when there's food there and so we can break it but they break it down or absorb that digest and absorb it but they also have a dual purpose and one is to regulate the blood glucose level and so they have two types of cells in here they have beta cells and alpha cells and so the beta cells in the alpha cells are just maintain their just sensing the blood glucose level and so this is an example of a beta cell right here and so this is our glucose transport it takes glucose in this would be cellular respiration here and so if we have a lot of blood glucose outside the cell in a beta cell in the pancreas that's going to trigger an influx of calcium but more importantly it's going to increase the amount of insulin that it's giving off and so what that means is that when the blood glucose level is high insulin which I'll represent with this red kind of a dot is going to be secreted from the pancreas and that's going to move throughout your whole body and it's going to trigger cells in your body to take in that blood glucose and it's also going to tell your liver to store that as glycogen and so when your blood glucose level goes to high insulin is secreted and that's going to cause your blood glucose level to go down what happens when your blood glucose goes down so dangerously down well then your body is going to quit producing that insulin and it's going to start producing glucagon glucagon is going to be created by the alpha cells in the pancreas and so now when the blood glucose level goes too far down glucagon is added and that's going to increase the blood glucose level in other words it's going to free up glucose from the from glycogen in your liver it's going to increase the blood glucose so you can actually use that throughout your body now let's actually see how this works throughout the day so this would be in a typical person what we've got here is the glucose levels in red and so the glucose is going to increase but it's increasing three times a day and that's because you have breakfast lunch and dinner but if you look at this dance is like right next to it this is this dance between the blood glucose level which is red as it goes up the insulin level is going to increase as well what does that insulin do it causes the blood glucose to drop and then we're going to increase insulin again and so we get the cycle throughout the day of an increase in blood glucose increase in insulin and then we keep that at that level and so if you look at it our blood glucose levels throughout the day are going to maintain a fairly static kind of a position now if we were to eat smaller meals throughout the day that would probably help us keep that together but what happens if we have a mistake what if we have an alteration in this feedback loop let's say for example that you are type 1 diabetic type 1 diabetic the problem with type 1 diabetics is that they have beta cells that don't work so remember the beta cells inside your pancreas are secreting that insulin but if you have type 1 diabetes or sometimes called that child onset diabetes you have destroyed these cells it's usually there's a genetic component to it but it's mostly an autoimmune disease where you destroy the beta cells well what happens if you destroy the beta cells now we're going to have a feedback loop where the blood glucose comes up there's no insulin sista Creek and so blood glucose keeps going up and going up and going up and so your your cells aren't going to take in that blood glucose and so we have some nasty stuff that comes as a result of that and so here are some things that happen increases the blood pressure can eventually affect the eyes you have nausea vomiting a lot of the ones in blue here are for type 1 diabetics but it eventually can lead to putting you in a coma actually death so if you're a type 1 diabetic that's just a mistake in the in the beta cells in the pancreas so we can't make insulin if your type 2 diabetic that essentially means that you've had too much glucose throughout your whole life it usually is tied to lack of exercise obesity things like that type die of 2 diabetics the cells in your body just stop recognizing that insulin and as a result they keep they quit taking it in and so how could we solve that problem knowing what you now know well if you were to get insulin shots like an insulin shot right here or an insulin shot right here or an insulin shot right here an insulin I think was first synthesized or created in the 1920s but now we make it through molecular biology but if you could get insulin shots throughout the day then you can regulate that blood glucose or now they use a insulin pump which is going to administer different amounts of insulin throughout the day so it works almost as a feedback loop now this is a map of where diabetes is on our planet about three percent of the people in the US have a worldwide have diabetes but about ninety to ninety-five percent of that is not a mistake in the beta cells it's actually tied to our diet type 2 diabetes and this is a chronic disease once you get diabetes you have it for the rest of your life and if you look here where are we seeing the greatest incidence of diabetes it's just tied to our diet and so we're eating a high-fat diet and as a result of that the diabetes a high sugar diet corn syrup diet and as a result of that we're getting huge diabetic increases but the whole thing is tied to a mistake in a feedback loop and so that's feedback loops positive and feedback positive and negative and so I hope that's helpful