Hi. It's Mr. Andersen and welcome
to the Unit 4 review. In this review I'm going
to talk about homeostasis. Homeostasis at
the level of the body. So how do we keep our
temperature constant on the inside if we're
endothermic. Or homeostasis at the level of
population or even an ecosystems. So it's
a pretty diverse unit. And so I'll try to
tie it all together. So basically before we
start let's talk about what a feedback loop
is. A feedback loop is when I'm taking and
gathering information from the environment
and then I'm changing my behavior based on
that. And so basically if you see a sign like
this and it says that my current speed is
38, well, I better slow down. And let's say
it says that my speed is 18, well then I better
speed up. And so I'm responding to that feedback
that I'm getting from, in this case the speed
sign. Now feedback loops essentially come
in two different flavors. We've got negative
feedback loops and positive feedback loops.
And which one is most popular? For sure it's
going to be the negative feedback loop. So
how does a negative feedback loop work? Well
let's say that my temperature were to increase.
So let's say I were to get an increase in
temperature, what's going to happen? Well
I'm going to start to sweat. I''m going to
start to vasodilate. And that's going to cool
it down. Eventually my temperature goes down.
If it goes too far down then I'm going to
start to shiver. Maybe goosebumps. And maybe
I'm going to vasoconstrict. So hold that body
temperature close to my body. And so that's
a negative feedback loop. And basically you
know it's a negative feedback loop when we
dance back and forth around a set point. So
it may go above that. It may go below that.
But we're trying to stay right around that
one set point. And so almost all feedback
loops, in living systems, are going to be
negative feedback loops. Positive feedback
loops occur when we want to go in one direction
away from a set point. So if I see the speed
limit as a challenge, like if I'm going 38.
Now I'm going to go 58. Now I'm going to go
98, well that's a positive feedback loop.
Example. This is an apple here. It's going
to give off ethylene once it's ripe which
is going to pick up other apples which are
now going to produce ethylene. So we're going
to even have more ethylene. And pretty soon
we have so much ethylene that the whole tree
turns red at once. And so another example
of that might be childbirth. Where the head
of the baby on the cervix creates contractions.
Which puts more pressure on the cervix. And
so we really only have positive feedback loops
when we want to go in one direction as quickly
and as really as possible. Okay. So now let's
define a couple of other terms. And those
are behavioral and physiological response.
So what we do. If we do and have a behavioral
response, what that refers to is the whole
of the organism. A physiological response
is going to be a response within the body.
And so example. When it gets colder and colder
and colder some organisms, like a humming
bird will undergo torpor. Some will undergo
a form of hibernation and some will be true
hibernators. And so that's a behavioral response.
The whole organism is responding to that change.
In this case to the temperature. Now a true
hibernator you could pick them up and knock
them on a table, and they wouldn't even wake
up. So they're really out of it. But it's
a behavioral response. Or let's say the temperature
gets cold in the northern hemisphere, I could
fly to the southern hemisphere. Or migrate.
And so migration is another behavioral response.
I could just go back and forth as the climate
changes. Or excuse me, as the weather changes.
But again since the whole organisms is doing
it, we call that behavioral. Physiological
would be for example this runner here starts
to get hotter and so it's going to start to
sweat and vasodilate. So that would be a physiological
response. Let's talk about the important ones
that you need to know in AP Biology. And those
are going to be, we'll start with the blood
glucose. What happens with the blood glucose
level? Remember the way I remember it is by
B I and G A. And so basically, if the blood
glucose level increases, so if it goes up,
the beta cells of the pancreas are going to
secrete insulin. That tells the cells, hey,
take in more glucose. It allows that glucose
to get into the cell. That's going to lower
the blood glucose. And it's eventually going
to go down to the set point which is around
90 - 100 milligrams per milliliter, per 100
milliliters. And so then what happens if it
goes to low? So if it goes too low the blood
glucose, that's going to stimulate the alpha
cells, the A down here to secrete glucagon.
And glucagon is basically going to tell your
liver to breakdown glycogen and release the
glucose. And so the glycogen, which is a polysaccharide
in the liver is now going to breakdown to
glucose. And that's going to increase the
blood calcium, blood glucose level up to a
set point. Again a thing I forgot to mention
out here is that we could take blood glucose
and store it as glycogen in the liver. That's
another thing that's going to be a response
to insulin. Let's try another one. Thermoregulation.
So we sense that in the hypothalamus which
is in a portion of the brain right above your
mouth. And so basically what happens, if the
temperature goes up, that's going to stimulate
our body to create sweat. To vasodilate. So
the capillaries get broader. It brings more
heat towards the surface of the skin. And
that's going to drop our body temperature.
If it goes too low, then it's going to activate
those capillaries to vasoconstrict, pull the
body temperature close to the body. We're
going to shiver. Maybe we're going to get
goosebumps as we try to hold our hair up on
end. And that's going to raise the temperature.
Now what is this? It's a feedback loop. What
type? It's a negative feedback loop because
we're trying to stay as close to that core
body temperature as we possibly can. Let's
go to osmoregulation, because we do that as
well. So what's osmoregulation? That's basically
keeping the osmolarity in our blood the same.
So right here we're looking at an organ called
the kidney. And the kidney does essentially
two things. One thing it does is it filters
the blood. And the second thing is it regulates
osmolarity. So it osmoregulates. I'm running
out of room. So let's look at some of the
parts of it. So if we look at this functional
unit of the kidney, basically blood is going
to flow in this direction. It's eventually
going to enter into the glomerulus, where
it goes to a dead end. And essentially the
small little bits are going to be filtered
out. Once it gets filtered out it enters into
the filtrate. And that's going to eventually
on our way down the loop of Henle and through
the collecting duct. And it's eventually going
to go into the bladder and be on its way.
So we filtered out some of the small things.
But we may want to reabsorb some of those
and secrete some that we don't want. And so
it does this job of filtering the blood. But
the other thing that it does is it osmoregulates.
And so if you ever notice that your urine
will change color from really a yellow to
really really clear, well that's as a result
of your collecting duct. So basically what
we're doing is we're deciding do we have enough
water inside our body? If so, then we can
let some of that H2O go. If we don't have
enough of that water, then we're going to
keep that water. So we can regulate how much,
how permeable this is to water. And so we
can osmoregulate. Okay next thing I want to
talk about is biotic and abiotic factors.
Biotic means living factors. Abiotic are going
to be non living factors. So let's say we
reintroduce the wolf into Yellowstone Park.
That would clearly be a biotic factor. Let's
say that carbon dioxide levels on the planet
are increasing leading to global warming,
that would be an abiotic factor. Let's say
we're looking at how the population of snowshoe
hares is going to effect the lynx population.
That clearly would be responding to biotic
factors. Let's try this one. We've got, if
we ever have a surface and we have water flowing
over that surface, bacteria will create something
called a biofilm. So they'd be responding
abiotic factors in order to maintain homeostasis.
Okay. Homeostasis is also going to reflect
evolution. And so if we've got a worm and
a earthworm and a lion. And they all essentially
have the same method for getting rid of nitrogenous
waste. It's essentially a filtrate material
moving in and then filtering off the smaller
material. Well that suggests that this was
a problem that was solved and it's just continued
to be solved in the same way over time. And
so homeostasis shows this homologous structure.
Or this ability to excrete waste. If we look
at something like getting oxygen into our
body, well, if you're in water and you're
on land, you're facing different constraints.
And so if you are in the water, basically
it's really moist which is great for absorbing
oxygen, but there's not a lot of oxygen there.
And so the evolution of the gill with its
countercurrent exchange works efficiently
there. As organisms eventually went from fish
to amphibians and eventually to reptiles,
we moved on to land. That just didn't cut
it anymore. Because now we had this problem
of a moist environment that we had to bring
with us. We couldn't use that anymore. We
had the advantage of all of this oxygen, and
so the gills simply don't work anymore. And
so now we had a split. So we've got the split
to the gills and a split to the lungs. And
so that shows the homeostasis or that bifurcations
of that homeostatic mechanism shows evolution
as well. Just to different constraints. Now
sometimes we'll have huge disruptions. And
what that can do is it can disrupt homeostasis.
Example, brown tree snake was introduced into
Guam and essentially wiped out all of these
bird species. That's because it had evolved
to have a homeostasis that was reflective
of the organisms that lived there at that
time. Example. Physiological disruption could
be decrease in the amount of water can lead
to dehydration and some really severe consequences
as a result. Next thing I want to talk about
is plant and animal defenses. Remember in
plant and animal defense, it's different between
plants and animals in that plants show what's
called non-specific defense. That means that
if you're invading a plant, they'll essentially
destroy the cells and then harden the cells
around it so they can fight an infection.
But it doesn't matter what that is. It doesn't
matter what the invader is. They're going
to have the same response. This non-specific
response. We have non-specific response as
well. It's called skin. It's called macrophages
that eat anything that get past that skin
layer. But the interesting thing that we have,
when I'm talking about we I mean mammals,
is that we have a specific response as well.
And so the way that that works is that we've
got, let me grab a pen, we've got these macrophages
that will eat the material when it comes in
and then present that to the surface. And
so what does that mean? An antigen is an invader.
And we essentially have an infinite number
of antibodies that our body normally has.
So we've got a bunch of cells, we call these
B lymphocytes. Each of them have different
antibodies on its surface. And we literally
have almost an infinite number of shapes at
the end of these antibodies. And so we're
infected by an organism, we can basically
find the one where there's a perfect fit between
the antigen and the antibody. And then we
simply produce a lot of those B lymphocytes.
And so the way we do animal defense remember
is to sense the shape. T helper cell sits
right in the middle. It is going to transmit
that shape to the B cells so we can make more
antibodies and more memory B cells. So we
don't succumb to that same infection in the
future. But we also make killer T cells so
we can target cells that have been infected
already. Next thing as far as development
goes, there are three big terms on here that
I'd like you to understand, or have a good
remembrance of. And that would be differentiation.
So if we start up here. Basically when a cell
starts as a stem cell, it hasn't decided what
kind of a cell it's going to become. But when
it becomes a red blood cell or a neuron or
a typical just, we could say this is an epithelial
cell. How does it do that? Well it does that
by basically surpressing certain genes. And
so if it is a neural cell for example there
are going to be all these genes that make
a neural cell. Or a neuron. All the other
bits of that chromosome are essentially going
to wad up. And so they're methylated. And
so they can't function. And so once a cell
has decided what cell it's going to become,
then it has differentiated. And this is kind
of a one way boat. You don't go back from
a cell to a stem cell. How do cells figure
out which cell they're going to become? Well,
they're secreting these tissue specific proteins.
And so when you have a bunch of stem cells
together, they're communicating to the cells
around them telling them, hey, this is the
cell I'm going to become. So we call this
differentiation and cell growth. But what
happens if a cell dies? That process is called
apoptosis. And that's just as important. And
so it's not only the formation of the cells
that make this embryo, but it's the death
of the cells between the fingers that forms
those fingers. And apoptosis is really important
in development over all. We've got a set of
genes that control that. One other thing remember
that we talked about was the hox genes and
how the hox genes but body parts in their
right spot. And hox genes found in fruit flies,
mice and us are essentially the same thing.
Suggests that they're homologous. How do we
sense our environment? Well that is through,
if we're a plant, through phototropism. Essentially
a plant is going to grow towards the light.
And the way it does that is the auxin, which
is that plant hormone is going to move in
the stem away from the light. So it's going
to move in this direction. And that's going
to cause the cells on the dark side to move
towards the light. So that's day to day response.
Photoperiodism is when a plant using phytochromes
is figuring out how much night time are we
having. So they can figure out essentially
the season. Now we respond to our environment
as well. We use what are called circadian
rhythms. And so that's essentially our pineal
gland secreting melatonin. And so we can kind
of tell what time of the day it is. So right
now it's almost 2:00. And so I start to get
sleepy in the afternoon. Quorum sensing remember
is used by bacteria. It's a way to respond
to each other and their environment and make
sense of their environment. And then the last
thing that I want to leave you with is this
idea of how did all this come to be? How did
we get all of these organisms and ecosystems
and things responding to their environment?
That's essentially through natural selection.
In other words, how does a plant know that
it's a perfect time of the year to flower
using photoperiodism? Well they didn't just
figure that out. There were a bunch of those
that flowered here. Bunch of those that flowered
here. These ones flowered too late. And they
didn't make it. These ones flowered too early
and they didn't make it. So we have this perfect
bell shaped curve. And so a bower bird figuring
out this beautiful bower to attract a female
is simply going to be selected for. The better
it's selected through sexual selection, the
better that bower is going to become. And
it's fun to speculate. Like, how did pollination
come to be? How did that first bee transfer
pollen from that first flower? Well it was
probably an accident. But it gave advantage
to that. And through natural selection we
were able to develop that mechanism. And so
that's homeostasis. I know it's all over the
place, but it's really important. And I hope
that's helpful.
