Resource Information

These resource, while free, are not openly licensed so they may not be altered from the original form.  Here I have supplied a link to the resources and a transcript of the video.  You can use the transcript as a basis to create your own worksheet, quiz, or as an additional resource for students who are differently-abled. 

I find it most helpful to play the video to the class and stop it every minute or two to further elaborate what Mr. Anderson is explaining.  You can highlight sentences in the transcript you would especially like the students to take notes on, and pause the video while you write the notes on the board.  This demonstrates note-taking skills for the students in real-time.  You can also pause the video as the answers to the worksheet questions appear, to give the students time to answer the questions.

This can also be given as a pre-lecture assignment to be done independently.  This works especially well if students have 1:1 computers and can do the assignment before class.  It also makes an excellent substitute lesson plan.

Positive and Negative Feedback Loops Video Description

Paul Andersen explains how feedback loops allow living organisms to maintain homeostasis. He uses thermoregulation in mammals to explain how a negative feedback loop functions. He uses fruit ripening to explain how a positive feedback loop functions. He also explains what can happen when a feedback loop is altered.

Link to Resource:

http://www.bozemanscience.com/positive-and-negative-feedback-loops

Bozemanscience Resources

Positive & Negative Feedback Loops Concept Map

Positive & Negative Feedback Loops Slideshow

Education Resources

Positive & Negative Feedback Loops Review Worksheet by Winnie LItten

Transcript

"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 you how fast you're going down at 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 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 you would experience if you were to see this sign as a challenge. So you

see you're going 30 miles per hour. So you realize I could go 39 or maybe 51. And eventually

get a ticket or break the sign. 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. An example I'll give you is temperature

regulation in mammals. In a positive feedback loop you move away from that target set point.

So you amplify that. An example I'll 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 have alterations in that feedback loop. And that can lead to

tragedy. An example would be diabetes. It's a problem in creating insulin or sensing that

insulin. And so that's just a problem in a feedback loop. And it can 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 environment

by just maintaining the same as the pond 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 or 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. A hairless cat is going to maintain it's body temperature. And so it

does that through feedback loops. It's going to maintain it's blood glucose level. It does

that through feedback loops. It's going to maintain the osmolarity of it's blood. It

does that through feedback loops. Now hairless cat doesn't have hair and so it's actually

hard to maintain it's internal body temperature and so you don't want to let them go outside

on their own. They'll lots of times put a coat on so they can maintain that.

Now there are kind of two life strategies.

In this picture it will 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 snake itself is what's

called an ectotherm. And what that means is that their internal temperature is going to

be the same as their external temperature. So the snake is about the same as 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 of 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 fahrenheit.

And so we use a negative feedback loop to maintain that. So let's say we put a thermometer

in this kid's mouth and the temperature all of a sudden increases. Now you actually sense

that, the area where we sense that is on the roof or our mouth in an area apart of, an

extension of the brain called 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 molecule

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're going to 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 maybe goose bumps. And these are kind of crazy goose bumps. But we're going

to get goose bumps right here and what goose bumps do, is if we had hair it'd actually

stand the hair up on end but it also kind of pulls your skin in. It's kind of like taking

a coat and pulling it in. It's conserving that heat. We're also going to vasoconstrict.

In other words we're going to shut off those capillary shunts and we're going to hold 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 heat is going to increase until

we hit that target set point. And so right now in fact, throughout the whole day your

whole body is 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 attractive so birds and humans are going to come and

eat the apple and spread the seeds somewhere else. But how do they maintain that? Well

they communicate. And it's kind of hard to understand how a plant could communication

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 apples 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 ripened 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 apples 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 of the fruit on the tree are ripe at the same exact time. Another famous

example of a positive feedback would be loop would be in childbirth. The pressure of the

head on the cervix of the mother actually causes contractions which pushes more pressure

on the cervix which causes more contractions and so eventually the baby is born. And so

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 there is 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 levels, the amount of glucose that is 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, its major

job is to empty digestive enzymes into the small intestine when there's food there. And

so we can break it, break it down or absorb that, digestive absorption. But the 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 and

the alpha cells are just maintaining, they're 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 of 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 too 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's going to be created by

the alpha cells in the pancreas. And so now when the blood glucose level goes too far

down, glucagon in 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 it dances like right next to it. It's a 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 this 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 statistic kind of 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 you are a Type I diabetic. Type I diabetic, the problem with

type I 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 I diabetes, or sometimes

called child onset diabetes, you have destroyed these cells. It's usually 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 to secrete and so blood glucose keeps going

up and going up and going up. And so 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. So here are some things

that happen. It increases the blood pressure. It can eventually effect the eyes, you have

nausea, vomiting. A lot of the ones in blue here are for type I diabetics. But it eventually

can lead to putting you in a coma or actually death. So if you're a type I diabetic that's

just a mistake in the beta cells in the pancreas so we can't make insulin. If you're a type

II 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 II diabetics,

the cells in your body just stop recognizing that insulin and as a result 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, and 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 an 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 3% of the people in the US or worldwide have diabetes. But 90-95% of that is not a

mistake in the beta cells, it's actually tied to our diet, type II diabetes. And this is

a chronic disease. Once you get diabetes you have it 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 negative and so I hope that's helpful."