Introduction to sustainability

WEBVTT
Kind: captions
Language: en

00:00:00.340 --> 00:00:02.420
- [Instructor] Let's talk
about sustainability.

00:00:02.420 --> 00:00:04.370
You've probably come across
the word "sustainable"

00:00:04.370 --> 00:00:05.790
at some point in your life.

00:00:05.790 --> 00:00:08.240
If I decided to continue to
talk for the rest of this video

00:00:08.240 --> 00:00:10.067
without taking a breath,
you might tell me,

00:00:10.067 --> 00:00:12.380
"Mia, that's just not sustainable."

00:00:12.380 --> 00:00:13.280
In this scenario,

00:00:13.280 --> 00:00:16.770
which I obviously would never
even dream of attempting,

00:00:16.770 --> 00:00:18.980
I'd be depleting the oxygen in my lungs

00:00:18.980 --> 00:00:20.550
without replacing it.

00:00:20.550 --> 00:00:23.230
Using resources faster than
they can be replenished

00:00:23.230 --> 00:00:24.760
is not sustainable.

00:00:24.760 --> 00:00:27.920
But what does
"sustainability" really mean?

00:00:27.920 --> 00:00:32.660
Well, the word "sustain" means
"to maintain" or "to hold,"

00:00:32.660 --> 00:00:34.930
so sustainability is
the ability of something

00:00:34.930 --> 00:00:37.800
to be maintained over a period of time.

00:00:37.800 --> 00:00:41.040
In environmental science, when
we talk about sustainability,

00:00:41.040 --> 00:00:43.450
we're talking about the
ability of the Earth's systems

00:00:43.450 --> 00:00:46.660
to survive and adapt to changing
environmental conditions

00:00:46.660 --> 00:00:49.470
and maintain the health of ecosystems.

00:00:49.470 --> 00:00:51.470
It's like a delicate balance

00:00:51.470 --> 00:00:55.350
where each process doesn't take
too much or produce too much

00:00:55.350 --> 00:00:58.210
so that all the other
processes can keep on going.

00:00:58.210 --> 00:01:00.760
And theoretically, when
all these processes

00:01:00.760 --> 00:01:04.860
are doing their jobs, this
balance could last forever.

00:01:04.860 --> 00:01:08.110
But some things that humans
do can disrupt this balance.

00:01:08.110 --> 00:01:10.570
Let's take a look at an example.

00:01:10.570 --> 00:01:12.770
In the early years of the United States,

00:01:12.770 --> 00:01:15.770
enslaved and free farm
workers in the American South

00:01:15.770 --> 00:01:18.980
planted or were forced to plant cotton,

00:01:18.980 --> 00:01:21.080
a plant that needs to use the nitrogen

00:01:21.080 --> 00:01:22.940
in the soil to survive.

00:01:22.940 --> 00:01:24.250
But when they planted cotton

00:01:24.250 --> 00:01:26.780
in the same fields year after year,

00:01:26.780 --> 00:01:29.260
the cotton plants would
deplete the nitrogen levels

00:01:29.260 --> 00:01:30.280
in the soil.

00:01:30.280 --> 00:01:34.100
This constant depletion of
nitrogen was not sustainable

00:01:34.100 --> 00:01:36.360
and the cotton crops suffered.

00:01:36.360 --> 00:01:38.010
In the early 20th century,

00:01:38.010 --> 00:01:41.340
agricultural scientist and
inventor George Washington Carver

00:01:41.340 --> 00:01:43.550
suggested that farmers could alternate

00:01:43.550 --> 00:01:47.360
between growing cotton and
growing nitrogen-fixing plants

00:01:47.360 --> 00:01:49.243
like peanuts or sweet potatoes.

00:01:50.200 --> 00:01:52.370
This is a more sustainable way of farming,

00:01:52.370 --> 00:01:55.320
as the nitrogen-fixing plants
would replace the nitrogen

00:01:55.320 --> 00:01:57.770
that the cotton plants
took out of the soil.

00:01:57.770 --> 00:01:59.420
Since then, we've discovered a way

00:01:59.420 --> 00:02:01.550
of putting nitrogen back into the soil

00:02:01.550 --> 00:02:04.610
using chemically-synthesized fertilizers.

00:02:04.610 --> 00:02:06.870
But sustainability is more complicated

00:02:06.870 --> 00:02:09.490
than just replacing depleted resources.

00:02:09.490 --> 00:02:11.600
When farmers use too much fertilizer

00:02:11.600 --> 00:02:14.550
or use poor methods of
fertilizer application,

00:02:14.550 --> 00:02:18.160
all that fertilizer can run
off into neighboring waterways.

00:02:18.160 --> 00:02:20.040
This excess nitrogen in the water

00:02:20.040 --> 00:02:22.920
could cause a lot of algae to grow.

00:02:22.920 --> 00:02:25.830
The thing about algae is that
when it grows too rapidly,

00:02:25.830 --> 00:02:27.740
it also dies rapidly,

00:02:27.740 --> 00:02:30.130
and the microbes that decompose the algae

00:02:30.130 --> 00:02:32.340
hog the available oxygen in the water,

00:02:32.340 --> 00:02:34.470
which makes it difficult
for other organisms

00:02:34.470 --> 00:02:36.320
living in the water to breathe.

00:02:36.320 --> 00:02:37.650
So, when the fertilizer runoff

00:02:37.650 --> 00:02:39.780
causes too much algae to grow,

00:02:39.780 --> 00:02:42.470
it creates an imbalance in the ecosystem

00:02:42.470 --> 00:02:44.210
and harms the other organisms

00:02:44.210 --> 00:02:46.890
that rely on the dissolved
oxygen in the water,

00:02:46.890 --> 00:02:49.040
so the addition of too much fertilizer

00:02:49.040 --> 00:02:51.240
is also unsustainable.

00:02:51.240 --> 00:02:52.640
But how could a farmer know

00:02:52.640 --> 00:02:55.490
whether they're using
fertilizer sustainably?

00:02:55.490 --> 00:02:58.793
Well, they could look out
for environmental indicators.

00:02:59.710 --> 00:03:01.750
Environmental indicators are basically

00:03:01.750 --> 00:03:03.217
when the Earth tells us,

00:03:03.217 --> 00:03:06.910
"Hey, there's something
unsustainable in this ecosystem."

00:03:06.910 --> 00:03:09.420
But instead of just telling
us that straight out,

00:03:09.420 --> 00:03:12.370
it tells us in a variety of clues.

00:03:12.370 --> 00:03:14.930
For our farmer, environmental
indicators can be things

00:03:14.930 --> 00:03:17.970
like the amount of dissolved
oxygen in the water,

00:03:17.970 --> 00:03:20.380
the biological diversity in the area,

00:03:20.380 --> 00:03:24.090
or even how much algae goes
in nearby water sources.

00:03:24.090 --> 00:03:27.090
The series of events triggered
by fertilizer pollution

00:03:27.090 --> 00:03:29.750
could deplete the amount
of oxygen in the water,

00:03:29.750 --> 00:03:31.330
so species that live in the water

00:03:31.330 --> 00:03:33.920
that require a lot of oxygen to survive

00:03:33.920 --> 00:03:36.000
could begin to die off.

00:03:36.000 --> 00:03:38.673
These species are called
indicator species.

00:03:39.810 --> 00:03:41.230
Because they're known to survive

00:03:41.230 --> 00:03:44.030
in very specific circumstances,

00:03:44.030 --> 00:03:46.310
spotting these species is an indication

00:03:46.310 --> 00:03:48.770
that there is a lot of
oxygen in the water.

00:03:48.770 --> 00:03:52.240
If these species are absent,
then that will be an indication

00:03:52.240 --> 00:03:53.810
that the pollution has reduced

00:03:53.810 --> 00:03:56.870
the amount of dissolved
oxygen in the pond.

00:03:56.870 --> 00:03:59.390
The farmer could then
change their behavior.

00:03:59.390 --> 00:04:02.580
In this way, environmental
indicators can guide humans

00:04:02.580 --> 00:04:04.910
to make sustainable choices.

00:04:04.910 --> 00:04:07.390
Another way humans can use
environmental indicators

00:04:07.390 --> 00:04:09.620
to make decisions about natural resources

00:04:09.620 --> 00:04:13.570
is by monitoring populations
and estimating their growth.

00:04:13.570 --> 00:04:16.270
Imagine that you have a
pond in your backyard.

00:04:16.270 --> 00:04:17.750
And let's imagine that you have searched

00:04:17.750 --> 00:04:20.290
for all of the correct
environmental indicators

00:04:20.290 --> 00:04:21.950
so you know that the fish in your pond

00:04:21.950 --> 00:04:23.723
are not suffering from pollution.

00:04:24.730 --> 00:04:27.120
One day, you decide to go fishing.

00:04:27.120 --> 00:04:29.770
It might be tempting to
fish as much as you can

00:04:29.770 --> 00:04:31.560
so that you can invite all of your friends

00:04:31.560 --> 00:04:34.200
and have a big happy fish feast.

00:04:34.200 --> 00:04:37.620
But if you decided to take all
of the fish out of the pond,

00:04:37.620 --> 00:04:40.280
then there wouldn't be
any left to reproduce,

00:04:40.280 --> 00:04:43.163
so your feast next year
would be pretty sad.

00:04:44.650 --> 00:04:47.900
Even if you just left a
couple of fish in the pool,

00:04:47.900 --> 00:04:49.660
they might not be able to reproduce enough

00:04:49.660 --> 00:04:52.010
to replenish the population.

00:04:52.010 --> 00:04:53.330
Other environmental factors

00:04:53.330 --> 00:04:55.160
might cause the small fish population

00:04:55.160 --> 00:04:56.960
to go down even further.

00:04:56.960 --> 00:05:00.480
The fish might catch
diseases, run out of food,

00:05:00.480 --> 00:05:03.133
or end up being some
other creature's feast.

00:05:04.010 --> 00:05:07.040
There's some maximum number
of fish that you could take

00:05:07.040 --> 00:05:09.050
so that there's enough
fish left in the pond

00:05:09.050 --> 00:05:12.000
to reproduce and replenish the population.

00:05:12.000 --> 00:05:15.223
This number is called the
maximum sustainable yield.

00:05:16.530 --> 00:05:19.270
Basically, you want to catch the most fish

00:05:19.270 --> 00:05:23.670
to have the maximum yield
while still being sustainable.

00:05:23.670 --> 00:05:26.370
If you fish more than the
maximum sustainable yield,

00:05:26.370 --> 00:05:27.710
then the fish will be captured

00:05:27.710 --> 00:05:29.650
more quickly than they can reproduce

00:05:29.650 --> 00:05:32.270
and the population will slowly decline.

00:05:32.270 --> 00:05:34.970
If you fish less than the
maximum sustainable yield,

00:05:34.970 --> 00:05:38.040
then the fish population
will grow exponentially

00:05:38.040 --> 00:05:39.980
until they reach the maximum capacity

00:05:39.980 --> 00:05:42.100
that the ecosystem can support.

00:05:42.100 --> 00:05:43.770
Some of the fish will eventually die

00:05:43.770 --> 00:05:45.420
from density-dependent factors

00:05:45.420 --> 00:05:48.170
like lack of food and disease.

00:05:48.170 --> 00:05:50.850
On a bigger scale than
your imaginary pond,

00:05:50.850 --> 00:05:53.520
natural resource managers try to estimate

00:05:53.520 --> 00:05:55.450
what the maximum sustainable yield will be

00:05:55.450 --> 00:05:57.030
each fishing season.

00:05:57.030 --> 00:05:58.590
This is why fishery managers

00:05:58.590 --> 00:06:00.520
regulate the amount of
certain types of fish

00:06:00.520 --> 00:06:02.840
each fisher is allowed to catch per year.

00:06:02.840 --> 00:06:06.113
This is to make sure that next
year we will still have fish.

00:06:07.060 --> 00:06:09.210
So, if we study the Earth's ecosystems

00:06:09.210 --> 00:06:12.020
and understand how we
impact the environment,

00:06:12.020 --> 00:06:14.560
then we can find ways to
interact with ecosystems

00:06:14.560 --> 00:06:15.800
more sustainably.

00:06:15.800 --> 00:06:19.093
And, really, that's what
environmental science is all about.


Heat transfer

WEBVTT
Kind: captions
Language: en

00:00:00.930 --> 00:00:02.240
- [Instructor] All right,
so I don't know about you,

00:00:02.240 --> 00:00:04.190
but I feel like talking about pizza.

00:00:04.190 --> 00:00:05.510
It's pizza night over here.

00:00:05.510 --> 00:00:07.900
I am smelling pizza as it's in the oven.

00:00:07.900 --> 00:00:09.240
It's on my mind.

00:00:09.240 --> 00:00:11.980
And I know we're supposed
to be talking about heat

00:00:11.980 --> 00:00:15.540
and thermal equilibrium, but
I think we can make this work.

00:00:15.540 --> 00:00:16.920
Now, if you're impatient like me,

00:00:16.920 --> 00:00:18.890
you probably put a slice
of pizza on your plate

00:00:18.890 --> 00:00:20.760
as soon as it comes out of the oven

00:00:20.760 --> 00:00:21.990
and you've probably noticed

00:00:21.990 --> 00:00:24.450
that the bottom of the plate warms up.

00:00:24.450 --> 00:00:26.900
So let's think about what's going on here.

00:00:26.900 --> 00:00:31.330
We have the temperature
of the pizza, T pizza,

00:00:31.330 --> 00:00:36.293
which is warmer than the
temperature of our plate, T plate.

00:00:37.650 --> 00:00:40.900
So, since the temperature
of the pizza is higher

00:00:40.900 --> 00:00:45.010
than that of the plate, the
plate starts to warm up.

00:00:45.010 --> 00:00:47.350
The pizza's temperature,
on the other hand,

00:00:47.350 --> 00:00:48.803
will actually start to lower.

00:00:49.960 --> 00:00:52.310
And this will happen
anytime the temperature

00:00:52.310 --> 00:00:53.940
of object one isn't the same

00:00:53.940 --> 00:00:55.590
as the temperature of object two.

00:00:57.500 --> 00:00:59.610
We know temperature is how we express

00:00:59.610 --> 00:01:01.360
that something is hot or cold.

00:01:01.360 --> 00:01:03.120
What does hot or cold mean?

00:01:03.120 --> 00:01:04.970
Temperature is actually the measure

00:01:04.970 --> 00:01:08.380
of the average kinetic energy
of the particles in an object.

00:01:08.380 --> 00:01:11.420
So let's digest that for a
second and break it down.

00:01:11.420 --> 00:01:12.920
So temperature, temperature

00:01:15.340 --> 00:01:19.093
is the average kinetic
energy of particles.

00:01:20.730 --> 00:01:25.023
And kinetic energy, as we
know, is 1/2 mv squared.

00:01:27.090 --> 00:01:28.640
So when the pizza is warm,

00:01:28.640 --> 00:01:32.520
its particles are moving
faster than if it was cold.

00:01:32.520 --> 00:01:34.760
And when particles and objects are moving,

00:01:34.760 --> 00:01:36.420
they're actually vibrating around.

00:01:36.420 --> 00:01:38.930
So what's happening in the hotter object

00:01:38.930 --> 00:01:42.210
is that the particles are vibrating faster

00:01:42.210 --> 00:01:44.270
than when the object is colder,

00:01:44.270 --> 00:01:47.570
and that's what it
means to be hot or cold.

00:01:47.570 --> 00:01:49.650
This type of kinetic energy,

00:01:49.650 --> 00:01:54.650
the motion of particles, is
also known as thermal energy.

00:01:56.440 --> 00:01:58.267
Now, you might be wondering,

00:01:58.267 --> 00:02:03.267
"If temperature is the measure
of kinetic energy in objects,

00:02:03.970 --> 00:02:05.870
and we have the pizza on the plate

00:02:05.870 --> 00:02:08.090
with their temperatures changing,

00:02:08.090 --> 00:02:10.160
does this mean there's some sort

00:02:10.160 --> 00:02:13.170
of energy transfer going on here?"

00:02:13.170 --> 00:02:15.100
And you would be spot on.

00:02:15.100 --> 00:02:18.430
Heat is the transfer of
energy between objects

00:02:18.430 --> 00:02:20.040
at different temperatures.

00:02:20.040 --> 00:02:22.000
This is a really important concept,

00:02:22.000 --> 00:02:24.740
so let's write that definition out.

00:02:24.740 --> 00:02:28.880
Heat is the transfer

00:02:28.880 --> 00:02:33.067
of energy between objects

00:02:35.280 --> 00:02:40.003
at different temperatures.

00:02:41.890 --> 00:02:43.840
As you can see with our pizza and plate,

00:02:43.840 --> 00:02:47.540
heat is transferring from the
hot pizza to the cold plate,

00:02:47.540 --> 00:02:50.750
from the hot object to the cold object.

00:02:50.750 --> 00:02:52.060
Why is that?

00:02:52.060 --> 00:02:55.060
Well, let's go ahead and
break down our problem

00:02:55.060 --> 00:02:57.480
and really dig into what's happening

00:02:57.480 --> 00:02:59.440
with everything in the system.

00:02:59.440 --> 00:03:00.570
In real life,

00:03:00.570 --> 00:03:03.450
a lot of energy is also
going to be transferred

00:03:03.450 --> 00:03:07.040
from the pizza into the air
around it, but for this problem,

00:03:07.040 --> 00:03:09.800
we're going to simplify it
to just look at the pizza

00:03:09.800 --> 00:03:10.633
and the plate.

00:03:12.040 --> 00:03:14.610
So we said that the
higher the temperature,

00:03:14.610 --> 00:03:17.460
the more all the particles
that make up the object

00:03:17.460 --> 00:03:18.960
are moving and vibrating.

00:03:18.960 --> 00:03:22.313
So let's go ahead and draw
that for the pizza particles.

00:03:23.240 --> 00:03:26.430
We're gonna use yellow
to represent the pizza,

00:03:26.430 --> 00:03:28.910
and I'm going to give the pizza particles

00:03:28.910 --> 00:03:32.480
a longer velocity vector
than those of the plate.

00:03:32.480 --> 00:03:33.593
And I'm going to assume that the particles

00:03:33.593 --> 00:03:36.780
in the pizza and the
plate have the same mass.

00:03:36.780 --> 00:03:39.610
That way, we can ignore the
mass part of kinetic energy

00:03:39.610 --> 00:03:41.350
and simplify the situation.

00:03:41.350 --> 00:03:43.600
Now, we can just focus on the velocities

00:03:43.600 --> 00:03:45.890
of all the particles in the system.

00:03:45.890 --> 00:03:48.760
When you put the slice
of pizza on the plate,

00:03:48.760 --> 00:03:51.090
the particles at the
surface of the objects

00:03:51.090 --> 00:03:53.010
will come into contact.

00:03:53.010 --> 00:03:56.020
So let's look at what happens
when these faster particles

00:03:56.020 --> 00:03:58.850
from the pizza collide
with the slower particles

00:03:58.850 --> 00:04:00.250
on the surface of the plate.

00:04:01.240 --> 00:04:02.810
When the particles collide,

00:04:02.810 --> 00:04:05.890
kinetic energy transfers
between the particles.

00:04:05.890 --> 00:04:08.130
Since the hotter object's particles,

00:04:08.130 --> 00:04:11.020
in this case, the pizza, have more energy,

00:04:11.020 --> 00:04:13.140
they're able to give some of that energy

00:04:13.140 --> 00:04:15.400
to the colder object's particles,

00:04:15.400 --> 00:04:18.410
which is why he always
transfers from the hotter object

00:04:18.410 --> 00:04:19.803
to the colder object.

00:04:20.880 --> 00:04:23.620
When the hotter object's
particles transfer some

00:04:23.620 --> 00:04:26.220
of their energy to the
colder object's particles,

00:04:26.220 --> 00:04:29.380
the hotter object particles
have now lost some energy

00:04:29.380 --> 00:04:30.490
and slow down.

00:04:30.490 --> 00:04:33.633
So we can show that by using
a smaller velocity vector.

00:04:34.780 --> 00:04:35.990
At the same time,

00:04:35.990 --> 00:04:38.580
the colder object's
particles have gained energy

00:04:38.580 --> 00:04:40.240
and sped up.

00:04:40.240 --> 00:04:42.020
So now this particle in the pizza

00:04:42.020 --> 00:04:44.690
is at a different velocity
than its neighboring particle

00:04:44.690 --> 00:04:45.660
in the pizza.

00:04:45.660 --> 00:04:48.110
So when they collide, guess what?

00:04:48.110 --> 00:04:50.040
We transfer energy again.

00:04:50.040 --> 00:04:52.950
The faster particle will give
some energy to the slower one,

00:04:52.950 --> 00:04:55.250
and in the process, it slows down.

00:04:55.250 --> 00:04:57.390
And this domino effect carries through

00:04:57.390 --> 00:04:59.370
to all the particles in the system,

00:04:59.370 --> 00:05:01.270
in the plate and in the pizza,

00:05:01.270 --> 00:05:04.493
until all the particles have
the same kinetic energy.

00:05:05.340 --> 00:05:07.610
And when the particles in both objects

00:05:07.610 --> 00:05:09.590
have the same kinetic energy,

00:05:09.590 --> 00:05:12.630
energy will no longer transfer
between the two objects

00:05:12.630 --> 00:05:15.220
and the system has reached
a state of stability,

00:05:15.220 --> 00:05:17.670
and we call this thermal equilibrium.

00:05:17.670 --> 00:05:19.520
You know when you leave your
pizza on the counter too long

00:05:19.520 --> 00:05:21.100
and you come back and it's cold?

00:05:21.100 --> 00:05:22.773
If you were to actually
measure the temperature

00:05:22.773 --> 00:05:25.230
of the pizza and the plate,
they're going to be the same.

00:05:25.230 --> 00:05:29.590
And we now know that
heat transfer only occurs

00:05:29.590 --> 00:05:32.430
when the temperature is
different between objects.

00:05:32.430 --> 00:05:34.550
So when the temperature is the same,

00:05:34.550 --> 00:05:36.210
no heat is transferring,

00:05:36.210 --> 00:05:39.300
and this is what is defined
as thermal equilibrium.

00:05:39.300 --> 00:05:41.319
So thermal equilibrium,

00:05:41.319 --> 00:05:46.120
equilibrium occurs when
there is no heat transfer

00:05:46.120 --> 00:05:46.953
in the system.

00:05:48.810 --> 00:05:50.780
So it turns out pizza
is actually pretty good

00:05:50.780 --> 00:05:54.210
for learning about heat transfer
and thermal equilibrium.

00:05:54.210 --> 00:05:55.530
And if you don't mind,

00:05:55.530 --> 00:05:57.820
that pizza and the other
room is calling my name,

00:05:57.820 --> 00:05:58.830
and personally,

00:05:58.830 --> 00:06:01.740
I prefer to eat it before it
reaches thermal equilibrium

00:06:01.740 --> 00:06:02.573
with my plate.


The photoelectric effect

WEBVTT
Kind: captions
Language: en

00:00:00.380 --> 00:00:01.580
- [Instructor] The photoelectric effect

00:00:01.580 --> 00:00:03.500
is another one of these
cool things in physics

00:00:03.500 --> 00:00:05.710
that sounds like it should be sci-fi,

00:00:05.710 --> 00:00:09.350
but actually describes an
everyday phenomenon around us.

00:00:09.350 --> 00:00:12.880
This and related effects are
used for all sorts of things,

00:00:12.880 --> 00:00:16.780
like solar panels and cameras.

00:00:16.780 --> 00:00:19.320
And the name itself is the biggest hint

00:00:19.320 --> 00:00:21.270
into what's going on.

00:00:21.270 --> 00:00:26.203
Photo, photon, electric, electron.

00:00:27.250 --> 00:00:30.310
So what is the photoelectric effect?

00:00:30.310 --> 00:00:33.960
It's the emission of
electrons from a metal

00:00:33.960 --> 00:00:37.140
that has absorbed
electromagnetic radiation

00:00:37.140 --> 00:00:39.463
like light over a certain frequency.

00:00:40.370 --> 00:00:43.300
A particle of light is called a photon,

00:00:43.300 --> 00:00:45.460
and when a photon has enough energy,

00:00:45.460 --> 00:00:48.380
it can actually knock an electron free.

00:00:48.380 --> 00:00:51.180
What happens is that the photon collides

00:00:51.180 --> 00:00:53.833
with the metal surface,
hitting an electron.

00:00:55.020 --> 00:00:56.540
When these particles collide,

00:00:56.540 --> 00:00:58.580
some of the energy of the photon is used

00:00:58.580 --> 00:01:00.390
to dislodge the electron.

00:01:00.390 --> 00:01:04.140
That electron is then shot
out of the metal, or emitted.

00:01:04.140 --> 00:01:06.960
The rest of the photon's
energy is then transferred

00:01:06.960 --> 00:01:08.433
to the emitted electron.

00:01:09.890 --> 00:01:12.457
We know that energy
transfers between objects

00:01:12.457 --> 00:01:14.400
and is conserved in a system,

00:01:14.400 --> 00:01:17.760
so maybe this photoelectric
effect sounds simple,

00:01:17.760 --> 00:01:20.290
but that's actually why
it's such a big deal,

00:01:20.290 --> 00:01:21.280
so big that it's

00:01:21.280 --> 00:01:24.980
what Albert Einstein
received his Nobel Prize for.

00:01:24.980 --> 00:01:27.060
Why was this such a big deal?

00:01:27.060 --> 00:01:29.270
By understanding the photoelectric effect,

00:01:29.270 --> 00:01:31.220
we also learned a lot about

00:01:31.220 --> 00:01:33.630
the fundamental properties of light.

00:01:33.630 --> 00:01:36.690
Today, we know that light
can behave as both a particle

00:01:36.690 --> 00:01:40.140
and a wave, but that wasn't always known.

00:01:40.140 --> 00:01:41.810
When this effect was discovered,

00:01:41.810 --> 00:01:45.540
it was clear that light
had wave-like properties,

00:01:45.540 --> 00:01:48.900
but the photo electric effect
was a big piece of evidence

00:01:48.900 --> 00:01:51.383
that light could also
behave as a particle.

00:01:52.580 --> 00:01:56.600
It also showed that the
energy of those particles

00:01:56.600 --> 00:01:59.423
was related to the frequency of the light.

00:02:01.260 --> 00:02:03.960
So how did scientists figure all this out?

00:02:03.960 --> 00:02:05.350
Well, let's start by thinking about

00:02:05.350 --> 00:02:07.860
how to make a metal emit an electron.

00:02:07.860 --> 00:02:10.510
This sounds an awful lot like work,

00:02:10.510 --> 00:02:13.470
and we need energy to perform work.

00:02:13.470 --> 00:02:16.230
It turns out that electrons
have what is called

00:02:16.230 --> 00:02:18.113
a binding energy.

00:02:19.400 --> 00:02:23.090
Binding energy, which I'm
going to refer to as Eb,

00:02:23.090 --> 00:02:24.990
refers to the amount of energy needed

00:02:24.990 --> 00:02:28.920
to pull an electron away from
the atom that it's orbiting,

00:02:28.920 --> 00:02:30.590
or that it's bound to,

00:02:30.590 --> 00:02:32.693
and hence the name binding energy.

00:02:33.820 --> 00:02:36.810
Electrons are happy to
remain where they are,

00:02:36.810 --> 00:02:39.570
unless something gives them enough energy

00:02:39.570 --> 00:02:41.350
to overcome this binding energy

00:02:41.350 --> 00:02:43.200
and bounce them out of place.

00:02:43.200 --> 00:02:47.270
So the energy hitting an
electron has to be greater

00:02:47.270 --> 00:02:50.053
than its binding energy to cause emission.

00:02:52.090 --> 00:02:55.360
Scientists had observed
the photoelectric effect,

00:02:55.360 --> 00:02:58.640
but they wanted to know how
it worked, because, well,

00:02:58.640 --> 00:03:00.400
that's what scientists do.

00:03:00.400 --> 00:03:01.630
So over the years,

00:03:01.630 --> 00:03:04.480
various scientists designed
experiments that tested

00:03:04.480 --> 00:03:06.680
the effects of different types of light

00:03:06.680 --> 00:03:08.620
on different materials.

00:03:08.620 --> 00:03:10.320
And let's look at what they found.

00:03:11.820 --> 00:03:14.340
They found that if a light
was shining on a metal

00:03:14.340 --> 00:03:16.920
and the metal wasn't emitting electrons,

00:03:16.920 --> 00:03:20.580
changing the intensity of the
light didn't change things,

00:03:20.580 --> 00:03:23.150
but they did notice that
raising the frequency

00:03:23.150 --> 00:03:24.083
of the light did.

00:03:24.950 --> 00:03:26.680
So, for example,

00:03:26.680 --> 00:03:29.440
if you set up an experiment
where you're shining a lamp

00:03:29.440 --> 00:03:33.180
on a piece of metal and it's
not ejecting any electrons,

00:03:33.180 --> 00:03:35.720
shining two of those lamps on the metal

00:03:35.720 --> 00:03:37.400
won't change anything.

00:03:37.400 --> 00:03:39.520
But if you replace the bulb

00:03:39.520 --> 00:03:42.570
with one that emits a
higher frequency of light,

00:03:42.570 --> 00:03:44.290
this might change things.

00:03:44.290 --> 00:03:47.320
And this was a big hint
that light might be behaving

00:03:47.320 --> 00:03:48.333
as a particle.

00:03:49.350 --> 00:03:51.160
Why is that?

00:03:51.160 --> 00:03:52.900
If this was a normal wave,

00:03:52.900 --> 00:03:56.060
we would expect that by sending
more energy at the metal,

00:03:56.060 --> 00:03:58.350
by increasing the intensity,

00:03:58.350 --> 00:04:01.630
electrons would gain enough
energy to be emitted.

00:04:01.630 --> 00:04:03.470
But since that didn't happen,

00:04:03.470 --> 00:04:06.630
it was clear something else was going on.

00:04:06.630 --> 00:04:09.020
And that eventually led to the idea

00:04:09.020 --> 00:04:11.950
that light was composed of photons.

00:04:11.950 --> 00:04:13.320
And these photons need

00:04:13.320 --> 00:04:18.320
to individually transfer
enough energy to an electron

00:04:18.330 --> 00:04:20.840
that it can bounce out of place.

00:04:20.840 --> 00:04:24.740
So by changing the frequency of a light,

00:04:24.740 --> 00:04:27.453
we can change the energy of a photon.

00:04:28.980 --> 00:04:32.250
Once this energy is higher than that

00:04:32.250 --> 00:04:34.840
of the binding energy
we talked about earlier,

00:04:34.840 --> 00:04:37.140
it can knock an electron loose.

00:04:37.140 --> 00:04:40.600
Any extra energy then
contributes to the energy

00:04:40.600 --> 00:04:42.283
of the emitted electron.

00:04:44.390 --> 00:04:47.190
This is a big deal and enables us

00:04:47.190 --> 00:04:49.160
to do some pretty cool things.

00:04:49.160 --> 00:04:53.420
So let's go back to one of
the examples at the beginning,

00:04:53.420 --> 00:04:54.930
solar panels.

00:04:54.930 --> 00:04:57.060
They have these things called photo cells,

00:04:57.060 --> 00:04:59.620
which we're pretending
I'm drawing right now,

00:04:59.620 --> 00:05:01.490
and these photo cells use a version

00:05:01.490 --> 00:05:04.680
of the photoelectric effect
to generate electricity.

00:05:04.680 --> 00:05:08.420
When sunlight hits the
cells, they emit electrons.

00:05:08.420 --> 00:05:09.610
The cells are designed

00:05:09.610 --> 00:05:14.180
so that these electrons moving
around generate a voltage,

00:05:14.180 --> 00:05:17.210
and voltage can power devices.

00:05:17.210 --> 00:05:19.450
Now we can see why the
photoelectric effect

00:05:19.450 --> 00:05:21.070
is such a big deal.

00:05:21.070 --> 00:05:24.370
Not only did it revolutionize
our understanding of physics,

00:05:24.370 --> 00:05:28.223
but it and similar effects are
used in tons of technology.


Solar Energy

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:03.880
- [Narrator] The sun is
about 93 million miles away,

00:00:03.880 --> 00:00:05.580
which means it takes about eight minutes

00:00:05.580 --> 00:00:07.700
for light from the sun to reach Earth,

00:00:07.700 --> 00:00:09.330
but it's still close enough for us

00:00:09.330 --> 00:00:11.550
to take advantage of solar energy.

00:00:11.550 --> 00:00:13.260
And why wouldn't we want to?

00:00:13.260 --> 00:00:15.840
After all, solar energy is renewable

00:00:15.840 --> 00:00:18.390
and essentially inexhaustible.

00:00:18.390 --> 00:00:20.780
It's also a clean energy source.

00:00:20.780 --> 00:00:23.690
So more and more we're
turning to solar energy

00:00:23.690 --> 00:00:25.920
to directly warm our homes

00:00:25.920 --> 00:00:29.440
or indirectly generate electricity.

00:00:29.440 --> 00:00:31.890
The first type of solar
heating we'll talking about

00:00:31.890 --> 00:00:33.720
is called passive.

00:00:33.720 --> 00:00:36.390
As the name suggests, passive happens

00:00:36.390 --> 00:00:38.180
without much work on your end.

00:00:38.180 --> 00:00:40.180
In my mind it's the equivalent of you

00:00:40.180 --> 00:00:41.820
just lying out in the sun,

00:00:41.820 --> 00:00:43.770
which is why throughout the world,

00:00:43.770 --> 00:00:48.100
this method is the most widely
accessible and inexpensive.

00:00:48.100 --> 00:00:51.580
Some examples of how homes
can be passively heated

00:00:51.580 --> 00:00:53.790
are through the actual materials

00:00:53.790 --> 00:00:55.630
that the home is made out of.

00:00:55.630 --> 00:01:00.630
So materials like stone, concrete, adobe,

00:01:00.640 --> 00:01:03.600
they absorb a lot of the
sun's heat during the day

00:01:03.600 --> 00:01:06.950
and then slowly release it at night.

00:01:06.950 --> 00:01:09.810
If we use this adobe house as an example,

00:01:09.810 --> 00:01:12.780
the thick walls and
small sunken in windows

00:01:12.780 --> 00:01:15.910
keep the house sheltered
from the sun during the day.

00:01:15.910 --> 00:01:19.490
And then at night, the clay
material that it's made out of

00:01:19.490 --> 00:01:23.610
releases that heat energy that
it stored throughout the day.

00:01:23.610 --> 00:01:27.990
So this particular home is
located in Santa Fe, New Mexico

00:01:27.990 --> 00:01:32.160
and a lot of homes that are
in desert areas like this

00:01:32.160 --> 00:01:34.750
are made out of clay and for good reason.

00:01:34.750 --> 00:01:37.150
Because during the day,
in desert locations,

00:01:37.150 --> 00:01:40.780
it can get very hot and that
clay will absorb the heat,

00:01:40.780 --> 00:01:43.520
keeping the inside of
the house pretty cool.

00:01:43.520 --> 00:01:47.470
And then at night desert
areas can get very, very cold.

00:01:47.470 --> 00:01:49.620
And so that clay will release the heat

00:01:49.620 --> 00:01:53.100
and keep the house moderately comfortable.

00:01:53.100 --> 00:01:55.340
Or in the Northern hemisphere,

00:01:55.340 --> 00:01:58.700
some homes are built with
South facing windows,

00:01:58.700 --> 00:02:00.460
which increases the number of hours

00:02:00.460 --> 00:02:02.810
that sunlight comes into the home.

00:02:02.810 --> 00:02:05.690
So these are ways to
passively warm a home,

00:02:05.690 --> 00:02:07.370
but there may be situations

00:02:07.370 --> 00:02:10.420
in which you wanna just block
the sun's heat and light

00:02:10.420 --> 00:02:12.440
to cool down the home instead.

00:02:12.440 --> 00:02:16.240
For example, the color of the
roof can make a big difference

00:02:16.240 --> 00:02:20.270
in how much heat does or
doesn't get stored in the home.

00:02:20.270 --> 00:02:21.960
Painting the roof a light color

00:02:21.960 --> 00:02:24.650
will reflect the sun's heat energy,

00:02:24.650 --> 00:02:26.880
much like choosing to wear a white T-shirt

00:02:26.880 --> 00:02:30.230
instead of a black shirt
on a hot summer day.

00:02:30.230 --> 00:02:34.600
There's also a specialty type
of roof called a green roof

00:02:34.600 --> 00:02:37.290
where plants are grown on top of the home

00:02:37.290 --> 00:02:40.440
to provide an additional
layer of insulation.

00:02:40.440 --> 00:02:42.880
Now, I haven't personally been on one,

00:02:42.880 --> 00:02:46.090
but I can only imagine in the spring time,

00:02:46.090 --> 00:02:48.690
if there are flowers
planted in the green roofs,

00:02:48.690 --> 00:02:52.490
they must smell and look amazing.

00:02:52.490 --> 00:02:55.030
And, of course, you can also create shade

00:02:55.030 --> 00:02:57.300
by planting trees around the home

00:02:57.300 --> 00:03:00.380
or adding awnings above the windows.

00:03:00.380 --> 00:03:03.040
To use solar energy in a different way,

00:03:03.040 --> 00:03:06.840
we can move from passive
techniques to active ones.

00:03:06.840 --> 00:03:09.730
For example, some homes have big panels

00:03:09.730 --> 00:03:13.230
attached to the roofs
called solar collectors.

00:03:13.230 --> 00:03:14.880
These solar collectors work

00:03:14.880 --> 00:03:17.400
by heating up a fluid inside of them

00:03:17.400 --> 00:03:21.700
and then use pumps or fans
to circulate that fluid

00:03:21.700 --> 00:03:24.360
and transfer heat into the home

00:03:24.360 --> 00:03:27.970
to warm up the air or water in the home.

00:03:27.970 --> 00:03:30.500
So the use of these pumps or fans

00:03:30.500 --> 00:03:32.190
to move the heat into the home

00:03:32.190 --> 00:03:36.800
is what makes this active
heating rather than just passive.

00:03:36.800 --> 00:03:39.860
A different alternative
if you happen to have

00:03:39.860 --> 00:03:42.020
a very large area of space

00:03:42.020 --> 00:03:47.020
is a concentrated solar power
system or CSP for short,

00:03:48.210 --> 00:03:50.620
which is basically a lot of reflectors

00:03:50.620 --> 00:03:54.000
that concentrate that solar energy.

00:03:54.000 --> 00:03:56.600
One specific type uses mirrors

00:03:56.600 --> 00:04:01.430
to reflect that solar
energy onto a single tower.

00:04:01.430 --> 00:04:04.320
That energy can then
be used to boil water,

00:04:04.320 --> 00:04:08.800
create steam, and ultimately
generate electricity.

00:04:08.800 --> 00:04:13.750
The largest CSP in the United
States is in the Mojave Desert

00:04:13.750 --> 00:04:17.260
very near California's border with Nevada.

00:04:17.260 --> 00:04:20.870
It's called the Ivanpah
Solar Power Facility.

00:04:20.870 --> 00:04:22.920
You'll see here that there are three sets

00:04:22.920 --> 00:04:25.440
of concentric circles with mirrors

00:04:25.440 --> 00:04:29.753
that are reflecting heat
energy onto individual towers.

00:04:30.620 --> 00:04:34.270
And there are hundreds
of thousands of mirrors.

00:04:34.270 --> 00:04:38.940
The plant takes up 3,500 acres of land

00:04:38.940 --> 00:04:43.880
and can power more than
100,000 homes in California.

00:04:43.880 --> 00:04:45.760
The downsides of these systems though

00:04:45.760 --> 00:04:49.040
is that they do cost a lot
of money and space up front

00:04:49.040 --> 00:04:52.150
and if the weather's bad
they don't work as well.

00:04:52.150 --> 00:04:54.400
There are also stories a few years back

00:04:54.400 --> 00:04:56.640
where there are hundreds of birds

00:04:56.640 --> 00:04:59.530
quite literally being burned up

00:04:59.530 --> 00:05:03.230
as they flew between the
mirrors and the towers.

00:05:03.230 --> 00:05:06.500
So you can imagine the
enormous amount of heat energy

00:05:06.500 --> 00:05:09.190
that's being reflected off these mirrors.

00:05:09.190 --> 00:05:12.570
On the upside, these reflectors
can be cheaper to maintain

00:05:12.570 --> 00:05:16.390
than other technologies and
can make use of large stretches

00:05:16.390 --> 00:05:20.243
of desert that are already
empty all over the world.

00:05:21.120 --> 00:05:25.060
And finally, one other way to
harness the power of the sun

00:05:25.060 --> 00:05:27.510
is through photovoltaic cells.

00:05:27.510 --> 00:05:31.350
If we break down that big
word, photo meaning light,

00:05:31.350 --> 00:05:35.930
and volt meaning a unit of electric force,

00:05:35.930 --> 00:05:38.920
then photovoltaic becomes using light

00:05:38.920 --> 00:05:40.980
to produce electric force.

00:05:40.980 --> 00:05:43.670
This idea dates back to 1905

00:05:43.670 --> 00:05:46.330
when Einstein first
published a paper about it.

00:05:46.330 --> 00:05:48.590
He then went on to win the Nobel Prize

00:05:48.590 --> 00:05:52.320
for this concept of the
photoelectric effect

00:05:52.320 --> 00:05:54.800
that when light shines on a metal,

00:05:54.800 --> 00:05:57.220
it causes electrons to be ejected

00:05:57.220 --> 00:05:59.950
and generates an electric current.

00:05:59.950 --> 00:06:03.410
Photovoltaic cells are
used in solar farms,

00:06:03.410 --> 00:06:06.450
on the roofs of houses,
and you might even have

00:06:06.450 --> 00:06:09.580
your very own solar powered calculator.

00:06:09.580 --> 00:06:13.160
You'll notice these types of
calculators work best outside

00:06:13.160 --> 00:06:14.660
or in a bright room,

00:06:14.660 --> 00:06:18.640
which can definitely be a
disadvantage in some cases.

00:06:18.640 --> 00:06:21.230
Another disadvantage is that sometimes

00:06:21.230 --> 00:06:24.050
you'll have to install
large amounts of these

00:06:24.050 --> 00:06:27.980
to generate enough
electricity for say a home,

00:06:27.980 --> 00:06:30.020
which can get expensive,

00:06:30.020 --> 00:06:33.730
and you also have to be sure
you're not moving anytime soon.

00:06:33.730 --> 00:06:36.500
On the other hand, installing
solar panels on a home

00:06:36.500 --> 00:06:40.380
reduces electricity costs
and your carbon footprint.

00:06:40.380 --> 00:06:42.300
So there you go, lot's of ways

00:06:42.300 --> 00:06:44.270
to harness the power of the sun,

00:06:44.270 --> 00:06:46.513
both its heat and its light.


The Indefinite Article

WEBVTT
Kind: captions
Language: en

00:00:00.320 --> 00:00:02.020
- Hello Grammarians,

00:00:02.020 --> 00:00:03.860
we've talked a little about the difference

00:00:03.860 --> 00:00:07.163
between these special adjectives
'a,' and 'an,' and 'the,'

00:00:08.230 --> 00:00:11.250
also known as the articles.

00:00:11.250 --> 00:00:12.823
I want to go a little deeper.

00:00:13.790 --> 00:00:17.110
Now we know that 'the'
is the definite article

00:00:17.110 --> 00:00:21.260
and 'a,' or 'an is the indefinite.

00:00:21.260 --> 00:00:23.170
If you're being non-specific in language,

00:00:23.170 --> 00:00:27.410
you'd use an indefinite article,
as in may I have an orange,

00:00:27.410 --> 00:00:29.220
if you want to talk about
one orange in particular,

00:00:29.220 --> 00:00:30.800
you'd use the definite article:

00:00:30.800 --> 00:00:33.220
may I have the orange.

00:00:33.220 --> 00:00:35.480
Now into this framework,
I'd like to introduce

00:00:35.480 --> 00:00:38.960
a complication into
that indefinite article,

00:00:38.960 --> 00:00:41.180
something to chew on,
something to think about.

00:00:41.180 --> 00:00:46.120
You'll notice I said the
indefinite article is 'a' or 'an,'

00:00:47.150 --> 00:00:48.745
well what's with that or?

00:00:48.745 --> 00:00:50.170
We use 'a' and when do we use 'an'?

00:00:51.010 --> 00:00:54.030
And the answer to that question
comes down to one thing

00:00:54.030 --> 00:00:56.170
and one thing only.

00:00:56.170 --> 00:00:58.593
The next sound to come out of your mouth.

00:00:59.820 --> 00:01:02.110
Let us take, for example, two apes,

00:01:02.110 --> 00:01:04.963
the orangutan and the bonobo.

00:01:05.860 --> 00:01:07.477
Orangutan starts with 'o,'

00:01:08.420 --> 00:01:10.320
bonobo starts with 'b.'

00:01:10.320 --> 00:01:12.500
Marvelous great apes, cousins to humans,

00:01:12.500 --> 00:01:14.900
treat them with love and respect.

00:01:14.900 --> 00:01:17.840
When you say a word that
begins with a consonant,

00:01:17.840 --> 00:01:20.810
which is to say any sound
that you make when your lips

00:01:20.810 --> 00:01:22.950
or your teeth or your tongue are touching,

00:01:22.950 --> 00:01:25.700
you say 'a.'

00:01:25.700 --> 00:01:26.683
A bonobo.

00:01:28.341 --> 00:01:31.070
The 'b' sound requires
my lips to come together

00:01:31.070 --> 00:01:32.800
and then pop apart.

00:01:32.800 --> 00:01:34.840
Ba, ba.

00:01:34.840 --> 00:01:37.190
When you say a word that
begins with a vowel sound,

00:01:37.190 --> 00:01:39.500
which is any sound that
you make with an open mouth

00:01:39.500 --> 00:01:43.617
and no teeth, lips, toungey
business, you say 'an.'

00:01:43.617 --> 00:01:45.910
An orangutan.

00:01:45.910 --> 00:01:48.680
Orangutan starts with an 'o' sound:

00:01:48.680 --> 00:01:52.960
oh, ah, ee, ahh, little vocal warmup.

00:01:52.960 --> 00:01:55.653
So you can see how this
is going to break down.

00:01:55.653 --> 00:01:59.190
Whatever sound comes after
the indefinite article

00:01:59.190 --> 00:02:01.030
is going to determine the shape

00:02:01.030 --> 00:02:03.880
the indefinite article takes.

00:02:03.880 --> 00:02:08.880
A pencil, an open door, a zebra,

00:02:09.280 --> 00:02:13.630
an extra pudding cup, a sailboat,

00:02:13.630 --> 00:02:17.420
an NBA player, a unicorn,

00:02:17.420 --> 00:02:20.720
wait, whoa, hold up.

00:02:20.720 --> 00:02:23.110
Do you notice something weird
about those last two examples?

00:02:23.110 --> 00:02:26.750
NBA player, well that
begins with 'n' doesn't it?

00:02:26.750 --> 00:02:29.400
And unicorn begins with 'u,'

00:02:29.400 --> 00:02:33.940
so why isn't it a NBA
player and an unicorn,

00:02:33.940 --> 00:02:37.060
because, and this is the
crucial, complicated,

00:02:37.060 --> 00:02:40.660
confusing part, it's not about the letter

00:02:40.660 --> 00:02:42.940
that the word begins with in spelling,

00:02:42.940 --> 00:02:46.053
it's about the sound that letter makes.

00:02:47.300 --> 00:02:50.809
So NBA doesn't begin with the 'nuh' sound,

00:02:50.809 --> 00:02:54.950
it's not nuh-BA, it's Eh, Ehn-BA.

00:02:54.950 --> 00:02:59.300
And unicorn doesn't begin with
an 'uh' or an 'ohh' sound,

00:02:59.300 --> 00:03:04.140
it begins with 'yuh,' yuh-icorn.

00:03:04.140 --> 00:03:07.840
It's not about the letter,
it's about the sound.

00:03:07.840 --> 00:03:12.840
'Eh' in NBA is a vowel
sound, so it's an NBA player.

00:03:13.890 --> 00:03:17.660
And 'yuh' in unicorn is a consonant sound.

00:03:17.660 --> 00:03:19.870
Notice how you lift your
tongue as you practice

00:03:19.870 --> 00:03:24.290
the difference between
un-icorn and une-icorn.

00:03:24.290 --> 00:03:28.090
- Ohh, yu, ohh, yu, so
it's a consonant sound.

00:03:28.090 --> 00:03:30.163
It's a unicorn.

00:03:31.440 --> 00:03:34.620
Same deal with words to
begin with silent 'h,'

00:03:34.620 --> 00:03:39.620
like herb, or heirloom, or hour.

00:03:39.670 --> 00:03:42.540
An hour had passed, ow,

00:03:42.540 --> 00:03:45.823
I'm going to start an herb garden, er,

00:03:46.800 --> 00:03:51.330
that cuckoo clock is an heirloom, air.

00:03:51.330 --> 00:03:52.630
Why does this happen?

00:03:52.630 --> 00:03:55.200
What's the difference
between 'a' and 'an?'

00:03:55.200 --> 00:03:58.710
The 'nuh' sound in 'an'
helps separate sounds.

00:03:58.710 --> 00:04:01.110
Here, listen to this incorrect example:

00:04:01.110 --> 00:04:03.860
for my snack today, I ate a apple.

00:04:03.860 --> 00:04:04.930
Sounds weird, right?

00:04:04.930 --> 00:04:07.308
One right after the other, ah, ah.

00:04:07.308 --> 00:04:08.410
And now listen to this:

00:04:08.410 --> 00:04:11.920
for my snack today, I ate an apple.

00:04:11.920 --> 00:04:15.140
The 'nuh' in 'an' is ind
of like a springboard

00:04:15.140 --> 00:04:17.470
from one vowel sound into the next.

00:04:17.470 --> 00:04:18.453
An-napel.

00:04:19.570 --> 00:04:21.680
And that's what I want you to
take away from this lesson,

00:04:21.680 --> 00:04:25.180
because it can be very
confusing and, well, indefinite.

00:04:25.180 --> 00:04:28.660
But think about the sound
that the word makes,

00:04:28.660 --> 00:04:30.783
not the letter that it begins with.

00:04:32.070 --> 00:04:37.070
An orangutan, a bonobo,
an NBA player, a unicorn,

00:04:38.050 --> 00:04:41.743
a you can learn anything, David out.


Pollution and human health

WEBVTT
Kind: captions
Language: en

00:00:00.960 --> 00:00:02.480
- [Narrator] Hey there friends,

00:00:02.480 --> 00:00:03.660
all of my life,

00:00:03.660 --> 00:00:05.900
I've struggled with asthma.

00:00:05.900 --> 00:00:08.300
And normally it doesn't
bother me too much,

00:00:08.300 --> 00:00:09.965
but when it's really cold outside,

00:00:09.965 --> 00:00:12.770
or if I've worked out really hard,

00:00:12.770 --> 00:00:15.480
my asthma symptoms get worse.

00:00:15.480 --> 00:00:16.360
When this happens,

00:00:16.360 --> 00:00:19.710
or in other words, when
I get an asthma attack,

00:00:19.710 --> 00:00:23.230
the airways in my lungs
become inflamed and swollen.

00:00:23.230 --> 00:00:26.100
And then also start to
narrow and fill with mucus,

00:00:26.100 --> 00:00:29.690
making it harder and harder
for me to breathe normally.

00:00:29.690 --> 00:00:31.440
However, it's not just the cold

00:00:31.440 --> 00:00:33.340
and CrossFit that trigger my asthma,

00:00:33.340 --> 00:00:35.510
but it's also air pollution.

00:00:35.510 --> 00:00:37.380
If I'm running around outside,

00:00:37.380 --> 00:00:39.100
or even just walking in areas

00:00:39.100 --> 00:00:42.020
where there's a lot of traffic
and cars driving around,

00:00:42.020 --> 00:00:44.840
my asthma will flare up immediately.

00:00:44.840 --> 00:00:45.860
But why?

00:00:45.860 --> 00:00:47.520
Well today, we're going to learn

00:00:47.520 --> 00:00:51.210
about the connections between
pollution and human health.

00:00:51.210 --> 00:00:53.659
This figure illustrates the
three main sources of pollution

00:00:53.659 --> 00:00:55.940
in our surrounding environment,

00:00:55.940 --> 00:00:59.320
namely air, water, and soil pollution.

00:00:59.320 --> 00:01:02.080
Exposure to these pollutants can in turn

00:01:02.080 --> 00:01:05.470
lead to a variety of
health issues and diseases.

00:01:05.470 --> 00:01:06.900
And today we're going to focus

00:01:06.900 --> 00:01:08.900
on air pollution in particular.

00:01:08.900 --> 00:01:11.140
Toxic gases, particulate matter,

00:01:11.140 --> 00:01:13.160
and chemicals released into the air

00:01:13.160 --> 00:01:15.230
can cause a variety of health issues,

00:01:15.230 --> 00:01:19.110
including respiratory and
cardiovascular illnesses,

00:01:19.110 --> 00:01:21.230
as well as headaches, fatigue,

00:01:21.230 --> 00:01:24.173
cancer, nausea, and skin irritation.

00:01:25.250 --> 00:01:27.520
Yet doctors and researchers

00:01:27.520 --> 00:01:29.400
are still learning about the connections

00:01:29.400 --> 00:01:32.190
between pollutants in our environment

00:01:32.190 --> 00:01:33.700
and human health issues,

00:01:33.700 --> 00:01:36.240
mainly because we humans

00:01:36.240 --> 00:01:39.190
collectively experienced
different levels of exposure

00:01:39.190 --> 00:01:42.510
to a variety of pollutants and toxins.

00:01:42.510 --> 00:01:43.830
So in short,

00:01:43.830 --> 00:01:47.250
it can be really hard to
pin down the exact cause

00:01:47.250 --> 00:01:48.500
of certain diseases

00:01:48.500 --> 00:01:51.970
because pollutants and
toxins can enter our bodies

00:01:51.970 --> 00:01:54.460
from the air, water and soil,

00:01:54.460 --> 00:01:56.100
as shown by this figure,

00:01:56.100 --> 00:01:59.120
which indicates the many
anthropogenic sources,

00:01:59.120 --> 00:02:02.490
that is to say sources
caused by human activities,

00:02:02.490 --> 00:02:05.730
of air, land, and water pollution.

00:02:05.730 --> 00:02:09.080
If we take a closer look of
air pollutants in particular,

00:02:09.080 --> 00:02:11.560
we see a cluster of industrial facilities

00:02:11.560 --> 00:02:13.360
releasing chlorofluorocarbons, CFCs,

00:02:15.230 --> 00:02:16.910
which are anthropogenic compounds

00:02:16.910 --> 00:02:20.700
that contribute to ozone
depletion in the stratosphere.

00:02:20.700 --> 00:02:24.860
We also see nitrous
oxides and sulfur dioxide,

00:02:24.860 --> 00:02:28.440
which can contribute to air
pollution as well as acid rain,

00:02:28.440 --> 00:02:32.330
and carbon dioxide, which
is a common greenhouse gas.

00:02:32.330 --> 00:02:36.223
So how do these common air
pollutants affect human health?

00:02:38.870 --> 00:02:41.090
One of them or well-studied connections

00:02:41.090 --> 00:02:43.780
between air pollution and human health

00:02:43.780 --> 00:02:47.440
is the link between
asbestos and mesothelioma.

00:02:47.440 --> 00:02:49.940
Many studies have shown
that the repeated act

00:02:49.940 --> 00:02:53.780
of inhaling or accidentally
swallowing asbestos

00:02:53.780 --> 00:02:56.900
is the most common cause of mesothelioma,

00:02:56.900 --> 00:02:58.210
which is a type of cancer

00:02:58.210 --> 00:03:00.130
that develops the lining of the lungs

00:03:00.130 --> 00:03:03.710
and sometimes in the
abdomen and heart as well.

00:03:03.710 --> 00:03:07.310
It might be a little difficult
to see in this chest x-ray,

00:03:07.310 --> 00:03:09.530
but this area is circled in orange

00:03:09.530 --> 00:03:11.120
shows affected lung tissue

00:03:11.120 --> 00:03:14.160
where mesothelioma has started to spread.

00:03:14.160 --> 00:03:15.710
Normally the lung tissue

00:03:15.710 --> 00:03:18.240
should appear fairly
transparent in x-rays,

00:03:18.240 --> 00:03:22.063
but it becomes opaque when
mesothelioma is present.

00:03:23.020 --> 00:03:25.660
Asbestos is a naturally occurring mineral

00:03:25.660 --> 00:03:27.410
composed of millions of fibers

00:03:27.410 --> 00:03:29.660
which bind together to create a light

00:03:29.660 --> 00:03:32.800
yet virtually indestructible material.

00:03:32.800 --> 00:03:34.890
The fiber structure of asbestos

00:03:34.890 --> 00:03:38.090
makes it a really effective
electrical insulator,

00:03:38.090 --> 00:03:39.700
and it's also heat resistant.

00:03:39.700 --> 00:03:43.880
So it was widely used in
construction and fireproofing.

00:03:43.880 --> 00:03:45.300
When I was growing up,

00:03:45.300 --> 00:03:48.020
my parents actually had
asbestos in their ceiling.

00:03:48.020 --> 00:03:51.120
And I also remembered that
both my elementary school

00:03:51.120 --> 00:03:52.380
and middle school,

00:03:52.380 --> 00:03:55.420
which were built in the late 1970s,

00:03:55.420 --> 00:03:58.900
had gymnasiums with asbestos ceilings.

00:03:58.900 --> 00:04:00.920
But as public health awareness

00:04:00.920 --> 00:04:03.780
about the dangers of asbestos has grown,

00:04:03.780 --> 00:04:07.440
the business of asbestos
removal has also grown.

00:04:07.440 --> 00:04:10.210
Although asbestos isn't
commonly used anymore

00:04:10.210 --> 00:04:12.860
in construction, some consumer products

00:04:12.860 --> 00:04:15.590
can still contain trace
amounts of asbestos,

00:04:15.590 --> 00:04:17.650
such as roofing materials.

00:04:17.650 --> 00:04:20.450
And sometimes when you're watching TV,

00:04:20.450 --> 00:04:22.920
you might see ads for asbestos abatement,

00:04:22.920 --> 00:04:24.470
which is the safe containment

00:04:24.470 --> 00:04:27.363
and removal of asbestos by professionals.

00:04:28.550 --> 00:04:31.200
The link between exposure to asbestos

00:04:31.200 --> 00:04:34.390
and mesothelioma has been heavily studied.

00:04:34.390 --> 00:04:38.840
And in turn, asbestos use has
greatly declined over time.

00:04:38.840 --> 00:04:40.540
Unfortunately though,

00:04:40.540 --> 00:04:43.030
symptoms or signs of mesothelioma

00:04:43.030 --> 00:04:46.860
may not appear until 20
or 50 years later or more

00:04:46.860 --> 00:04:50.210
after exposure to asbestos
with initial symptoms,

00:04:50.210 --> 00:04:53.280
including shortness of
breath and chest pain.

00:04:53.280 --> 00:04:55.180
However, over time,

00:04:55.180 --> 00:04:57.990
as we can see in the
lower part of this figure,

00:04:57.990 --> 00:05:00.470
asbestosis exposure can result in damage

00:05:00.470 --> 00:05:02.980
to and cancer of the pleura,

00:05:02.980 --> 00:05:05.120
which are membranes that cover the lungs

00:05:05.120 --> 00:05:08.120
and the inside of the
surrounding chest walls.

00:05:08.120 --> 00:05:10.600
These pleura, and even the diaphragm,

00:05:10.600 --> 00:05:12.140
which helps you breathe,

00:05:12.140 --> 00:05:15.650
can become scarred, damaged,
and hardened from asbestos.

00:05:15.650 --> 00:05:17.060
And of course,

00:05:17.060 --> 00:05:19.313
mesothelioma can develop too.

00:05:20.190 --> 00:05:21.350
What's even worse though

00:05:21.350 --> 00:05:24.150
is that mesothelioma
is generally resistant

00:05:24.150 --> 00:05:26.480
to radiation and chemotherapy treatments.

00:05:26.480 --> 00:05:31.130
So long-term survival and
cures are exceedingly rare.

00:05:31.130 --> 00:05:35.170
But mesothelioma can be
diagnosed and treated early

00:05:35.170 --> 00:05:38.030
through regular cancer
screenings and blood tests

00:05:38.030 --> 00:05:41.800
for those who may be most
vulnerable to this cancer.

00:05:41.800 --> 00:05:45.190
That is to say folks who
have had exposure to asbestos

00:05:45.190 --> 00:05:47.090
through their work like miners,

00:05:47.090 --> 00:05:49.670
factory workers, insulation, installers,

00:05:49.670 --> 00:05:51.283
and construction workers.

00:05:52.160 --> 00:05:53.930
So take a breath,

00:05:53.930 --> 00:05:55.340
take a sip of water.

00:05:55.340 --> 00:05:58.190
These are simple things that
we take for granted each day,

00:05:58.190 --> 00:06:01.990
yet pollution can threaten these
basic aspects of our lives.

00:06:01.990 --> 00:06:03.820
When my asthma starts to act up,

00:06:03.820 --> 00:06:06.920
I become acutely aware of
just how much pollution

00:06:06.920 --> 00:06:08.730
affects my day-to-day health.

00:06:08.730 --> 00:06:11.280
And thanks to regulatory legislation

00:06:11.280 --> 00:06:12.380
like the Clean Air Act

00:06:12.380 --> 00:06:14.780
and the Clean Water Act
in the United States,

00:06:14.780 --> 00:06:17.020
we have longer and healthier lives here.

00:06:17.020 --> 00:06:18.650
But this isn't the case everywhere,

00:06:18.650 --> 00:06:20.330
nor is it the case for everyone.

00:06:20.330 --> 00:06:22.800
So do your part and help to ensure

00:06:22.800 --> 00:06:24.763
safe air and water for all.


Mendelian inheritance and Punnett squares

WEBVTT
Kind: captions
Language: en

00:00:00.780 --> 00:00:02.960
- [Narrator] This is a
photo of Gregor Mendel,

00:00:02.960 --> 00:00:04.970
who is often known as
the father of genetics.

00:00:04.970 --> 00:00:07.360
And we'll see in a few seconds why,

00:00:07.360 --> 00:00:10.950
and he was an Abbot of
a monastery in Moravia,

00:00:10.950 --> 00:00:13.570
which is in modern day Czech Republic.

00:00:13.570 --> 00:00:17.860
And many people had bred plants
for agricultural purposes

00:00:17.860 --> 00:00:21.270
for hundreds, if not thousands
of years before Mendel,

00:00:21.270 --> 00:00:24.110
but he really gave us a
glimpse, gave us insights

00:00:24.110 --> 00:00:26.420
in how traits are really passed.

00:00:26.420 --> 00:00:29.020
And he did this through
his pea plant experiments

00:00:29.020 --> 00:00:32.230
that he conducted from 1856 to 1863.

00:00:32.230 --> 00:00:37.060
Over that time period, he
bred roughly 28,000 pea plants

00:00:37.060 --> 00:00:38.750
in order to get a better understanding

00:00:38.750 --> 00:00:40.970
of how they passed down different traits.

00:00:40.970 --> 00:00:44.090
And he studied things like
properties of the seeds,

00:00:44.090 --> 00:00:45.750
properties of the pea prods,

00:00:45.750 --> 00:00:48.990
and things like the height of the plant.

00:00:48.990 --> 00:00:52.610
And in his time, the mainstream theory

00:00:52.610 --> 00:00:56.830
was that if you bred a tall
parent with a short parent,

00:00:56.830 --> 00:00:59.050
you would get a medium offspring,

00:00:59.050 --> 00:01:01.160
but that's not what Mendel saw.

00:01:01.160 --> 00:01:04.350
When he bred tall pea plants
with short pea plants,

00:01:04.350 --> 00:01:06.630
all of the offspring were tall.

00:01:06.630 --> 00:01:08.610
But then when he self
fertilized those plants

00:01:08.610 --> 00:01:10.670
and plants have the interesting property

00:01:10.670 --> 00:01:12.950
that they can fertilize themselves.

00:01:12.950 --> 00:01:16.310
So the same plant can contribute
both the female gamete

00:01:16.310 --> 00:01:18.020
and the male gametes.

00:01:18.020 --> 00:01:20.580
In other words the same plant can be both

00:01:20.580 --> 00:01:23.820
the female parent and the male parent.

00:01:23.820 --> 00:01:26.870
Well, then he saw that
roughly there was a ratio

00:01:26.870 --> 00:01:28.710
of three to one, tall to short.

00:01:28.710 --> 00:01:29.970
Those weren't the exact numbers,

00:01:29.970 --> 00:01:33.280
but pretty close to three to one.

00:01:33.280 --> 00:01:35.830
And so there's a lot of really
interesting things here.

00:01:35.830 --> 00:01:37.600
First of all, at least for this trait,

00:01:37.600 --> 00:01:39.620
he didn't see any blending occur.

00:01:39.620 --> 00:01:43.240
And then the other thing is,
this short trait reappeared

00:01:43.240 --> 00:01:45.870
in this second generation.

00:01:45.870 --> 00:01:48.600
In order to explain these
results, he hypothesized

00:01:48.600 --> 00:01:51.140
that there are inheritable factors

00:01:51.140 --> 00:01:54.300
that are inherited from
an organism's parents

00:01:54.300 --> 00:01:56.280
and they're related to a specific trait.

00:01:56.280 --> 00:01:59.950
So in this case it would be around height.

00:01:59.950 --> 00:02:02.340
Now we know what he called
these inheritable factors.

00:02:02.340 --> 00:02:06.220
We now call genes, although
he did not use the term,

00:02:06.220 --> 00:02:08.930
and he also hypothesized
that these factors

00:02:08.930 --> 00:02:12.060
could come in different versions.

00:02:12.060 --> 00:02:15.010
Now, today we know that the
different versions of a gene,

00:02:15.010 --> 00:02:18.830
we call alleles, although
he did not use that term,

00:02:18.830 --> 00:02:22.010
but in this case, the versions
that we have at our disposal,

00:02:22.010 --> 00:02:23.670
you could have a tall height,

00:02:23.670 --> 00:02:27.700
which we can shorthand say,
capital T, capital T for tall,

00:02:27.700 --> 00:02:29.150
or you could have a short height,

00:02:29.150 --> 00:02:33.240
which I will use lowercase
t for, for a short height.

00:02:33.240 --> 00:02:36.830
Now generally speaking,
organisms will have two versions

00:02:36.830 --> 00:02:38.330
of their genes like this.

00:02:38.330 --> 00:02:41.680
For example, an organism
could have two tall alleles,

00:02:41.680 --> 00:02:44.240
or two short, or one of each,

00:02:44.240 --> 00:02:47.260
but when it produces its
gametes, the sex cells,

00:02:47.260 --> 00:02:50.280
so the sperm for a male
and the egg for a female,

00:02:50.280 --> 00:02:52.780
it will generally contribute
one of its two versions

00:02:52.780 --> 00:02:53.800
to its offspring.

00:02:53.800 --> 00:02:57.150
And this contribution of
one allele or the other

00:02:57.150 --> 00:03:00.540
is known as Mendel's law of segregation.

00:03:00.540 --> 00:03:03.520
And we can draw, what's
known as a Punnett square

00:03:03.520 --> 00:03:04.980
to depict this.

00:03:04.980 --> 00:03:08.300
So let me draw a little
bit of a grid here.

00:03:08.300 --> 00:03:11.040
And Mendel actually did not
invent the Punnett square,

00:03:11.040 --> 00:03:13.750
although he was thinking in these terms.

00:03:13.750 --> 00:03:17.580
It was actually invented by
Reginald Punnett in 1905.

00:03:17.580 --> 00:03:20.010
And this is useful to think
about the probabilities

00:03:20.010 --> 00:03:22.470
of various combinations based on what

00:03:22.470 --> 00:03:24.330
each parent could contribute.

00:03:24.330 --> 00:03:27.020
So let's say we're talking
about the tall plant,

00:03:27.020 --> 00:03:30.290
and let's say it has two tall versions.

00:03:30.290 --> 00:03:34.400
So it can contribute a
capital T or a capital T.

00:03:34.400 --> 00:03:36.800
And let's say that this
short plant over here

00:03:36.800 --> 00:03:39.060
has two of the short versions for now.

00:03:39.060 --> 00:03:41.680
So it could contribute
either a lowercase t,

00:03:41.680 --> 00:03:43.520
or a lowercase t.

00:03:43.520 --> 00:03:45.850
And so what are all the
possible combinations

00:03:45.850 --> 00:03:47.640
for its offspring?

00:03:47.640 --> 00:03:51.870
Well, in one scenario, you
could get this capital T

00:03:51.870 --> 00:03:55.060
from the male parent,
and this lowercase t,

00:03:55.060 --> 00:03:56.350
from the female parent.

00:03:56.350 --> 00:03:58.550
In another scenario, you
could get this capital T

00:03:58.550 --> 00:04:01.370
from the male parent, and a lowercase t

00:04:01.370 --> 00:04:03.200
from the female parent.

00:04:03.200 --> 00:04:05.920
In this scenario, and I know
these look very similar,

00:04:05.920 --> 00:04:09.870
a capital T from the male
parent and this lowercase t

00:04:09.870 --> 00:04:12.270
from the female parent, and
then last but not least,

00:04:12.270 --> 00:04:14.770
I know this looks repetitive,
you could get this capital T

00:04:14.770 --> 00:04:17.810
from the male parent and this lowercase t

00:04:17.810 --> 00:04:19.430
from the female parent.

00:04:19.430 --> 00:04:22.530
And the reason why in all of these cases,

00:04:22.530 --> 00:04:27.530
you see a tall plant, is
because the tall version,

00:04:28.060 --> 00:04:30.920
and he coined this term, is dominant.

00:04:30.920 --> 00:04:32.840
And once again, this
was all his hypothesis

00:04:32.840 --> 00:04:34.660
to explain his results.

00:04:34.660 --> 00:04:37.980
So this is dominant and
the short is recessive.

00:04:37.980 --> 00:04:40.370
So even if you have one of each,

00:04:40.370 --> 00:04:43.500
you're actually going to
show the dominant trait.

00:04:43.500 --> 00:04:45.410
We now call that your phenotype,

00:04:45.410 --> 00:04:48.010
what you show is going to be tall.

00:04:48.010 --> 00:04:50.250
Now, what's interesting
about this hypothesis

00:04:50.250 --> 00:04:54.410
is it seems to explain what
happens in the next generation.

00:04:54.410 --> 00:04:56.040
Just so a little bit of notation.

00:04:56.040 --> 00:04:57.610
The first generation is usually called

00:04:57.610 --> 00:04:59.450
the P generation for parental,

00:04:59.450 --> 00:05:01.760
and then the first generation of offspring

00:05:01.760 --> 00:05:04.330
is known as the F1, F for filial.

00:05:04.330 --> 00:05:07.260
And that comes filials,
which means sun in Greek,

00:05:07.260 --> 00:05:09.613
and then the generation
after that would be F2,

00:05:10.520 --> 00:05:12.600
that's just a little
bit of notation there,

00:05:12.600 --> 00:05:13.840
but let's think about what would happen

00:05:13.840 --> 00:05:17.600
at the F2 generation,
if you self fertilized,

00:05:17.600 --> 00:05:20.910
some of these characters right over here.

00:05:20.910 --> 00:05:25.180
Well, in that situation, let
me draw another Punnett square,

00:05:25.180 --> 00:05:27.480
on the male parents side,
you could contribute

00:05:27.480 --> 00:05:29.210
either your capital T version,

00:05:29.210 --> 00:05:31.800
which we now call your dominant allele,

00:05:31.800 --> 00:05:34.490
or you could contribute
the lowercase t version,

00:05:34.490 --> 00:05:37.300
and on the female parents side
and once again for a plant,

00:05:37.300 --> 00:05:39.520
you can have the same plant
that has both the male

00:05:39.520 --> 00:05:40.800
and the female parent.

00:05:40.800 --> 00:05:43.350
You could contribute the
dominant version, the capital T,

00:05:43.350 --> 00:05:46.200
or the recessive version, the lowercase t.

00:05:46.200 --> 00:05:49.370
Now we see something interesting
happen in the offspring.

00:05:49.370 --> 00:05:52.060
There is a one in four chance
you get both capital Ts.

00:05:52.060 --> 00:05:54.600
So capital T, capital T.

00:05:54.600 --> 00:05:58.790
There's also a one in four
chance that you get a lowercase t

00:05:58.790 --> 00:06:03.000
from the male parent up here,
and then you get a capital T

00:06:03.000 --> 00:06:05.050
from the female parent.

00:06:05.050 --> 00:06:09.500
There is another one in four
chance you get a capital T

00:06:09.500 --> 00:06:12.970
from the male parent, and a lowercase t

00:06:12.970 --> 00:06:14.460
from the female parent.

00:06:14.460 --> 00:06:16.330
And then there's a one in four chance

00:06:16.330 --> 00:06:18.350
that you get two lowercase ts,

00:06:18.350 --> 00:06:21.000
one each from the male and female parent.

00:06:21.000 --> 00:06:23.860
Now, if we accept the dominant
and recessive hypothesis,

00:06:23.860 --> 00:06:26.380
we would expect that plants
that got both capital Ts

00:06:26.380 --> 00:06:29.450
would be tall, but we would
also expect that these over here

00:06:29.450 --> 00:06:32.620
would be tall as well, because
the capital T is dominant.

00:06:32.620 --> 00:06:35.380
They would exhibit the tall phenotype.

00:06:35.380 --> 00:06:36.960
And then you would
expect probabilistically

00:06:36.960 --> 00:06:40.530
that one fourth of your plants
over time, would be short

00:06:40.530 --> 00:06:42.820
because they only have
the recessive alleles,

00:06:42.820 --> 00:06:45.980
the recessive traits in this situation.

00:06:45.980 --> 00:06:48.860
And that's actually what Mendel saw.

00:06:48.860 --> 00:06:50.890
Now what's amazing is that Mendel was able

00:06:50.890 --> 00:06:53.890
to figure this out without
knowing about chromosomes,

00:06:53.890 --> 00:06:56.280
without knowing all that we know today.

00:06:56.280 --> 00:06:58.280
Today we know this works because we have

00:06:58.280 --> 00:07:02.320
23 pair of chromosomes, and
each of those pairs have copies,

00:07:02.320 --> 00:07:05.590
have different versions
of usually the same gene,

00:07:05.590 --> 00:07:08.580
and that when meiosis occurs
and you have gamete formation,

00:07:08.580 --> 00:07:11.160
one member of each pair
will segregate randomly

00:07:11.160 --> 00:07:15.350
into the newly formed sex cell,
into the sperm or the egg.

00:07:15.350 --> 00:07:17.320
That's why this occur and we
go into some detail on that

00:07:17.320 --> 00:07:19.240
in other videos, but it's pretty cool,

00:07:19.240 --> 00:07:22.453
that Mendel was able to figure
this out in the 19th century.


Sources of genetic variation

WEBVTT
Kind: captions
Language: en

00:00:00.240 --> 00:00:01.820
- [Instructor] In this video,
we're going to talk about

00:00:01.820 --> 00:00:04.370
sources of genetic variation,

00:00:04.370 --> 00:00:09.010
which is key for evolution and
natural selection to happen.

00:00:09.010 --> 00:00:11.820
Just as a little bit of a
primer, natural selection,

00:00:11.820 --> 00:00:13.670
you can have a bunch
of different organisms

00:00:13.670 --> 00:00:16.260
with different genetics,
different genotypes,

00:00:16.260 --> 00:00:18.830
and they can express themselves
as different phenotypes.

00:00:18.830 --> 00:00:21.530
And I'll just do this as
different colored circles

00:00:21.530 --> 00:00:22.800
right over here.

00:00:22.800 --> 00:00:24.910
So there are all of these
different phenotypes,

00:00:24.910 --> 00:00:28.000
and I'm just expressing different
phenotypes of one trait.

00:00:28.000 --> 00:00:30.670
And then depending on what's
going on in the environment,

00:00:30.670 --> 00:00:33.500
some of these phenotypes
might be more favorable

00:00:33.500 --> 00:00:35.700
for survival and reproducing

00:00:35.700 --> 00:00:39.010
and therefore passing on those
genes to the next generation.

00:00:39.010 --> 00:00:42.470
And if you do that over many,
many, many, many generations,

00:00:42.470 --> 00:00:44.560
you can have a change in your gene pool

00:00:44.560 --> 00:00:48.090
because the genes that provide
the variants of phenotypes

00:00:48.090 --> 00:00:51.050
that are more successful will exist more.

00:00:51.050 --> 00:00:52.200
But an interesting question is,

00:00:52.200 --> 00:00:55.070
where does this variation come from?

00:00:55.070 --> 00:00:57.740
And there's several sources of it.

00:00:57.740 --> 00:01:00.490
So one of the key and probably
the most primitive version

00:01:00.490 --> 00:01:03.910
of genetic variation is mutation.

00:01:03.910 --> 00:01:08.890
Cells are incredibly accurate
when they are copying DNA,

00:01:08.890 --> 00:01:11.310
but there are going to be some errors.

00:01:11.310 --> 00:01:14.050
Now, most of these errors can oftentimes

00:01:14.050 --> 00:01:17.810
break the organism in some way
or might not matter at all,

00:01:17.810 --> 00:01:20.080
but every now and then,
some of these errors,

00:01:20.080 --> 00:01:22.600
either as an individual base pair change,

00:01:22.600 --> 00:01:25.840
or maybe cumulatively can
produce a different phenotype

00:01:25.840 --> 00:01:29.170
and potentially a phenotype
that has an advantage.

00:01:29.170 --> 00:01:31.910
And so this has always been the case.

00:01:31.910 --> 00:01:34.870
Now, another major source
of genetic variation

00:01:34.870 --> 00:01:37.240
is sexual reproduction.

00:01:37.240 --> 00:01:40.920
And to remind ourselves
of sexual reproduction,

00:01:40.920 --> 00:01:45.500
I will show you this diagram of meiosis.

00:01:45.500 --> 00:01:47.790
Now sexual reproduction is the process

00:01:47.790 --> 00:01:49.910
by which we form gametes.

00:01:49.910 --> 00:01:51.160
So for a male organism,

00:01:51.160 --> 00:01:52.810
that would be producing the sperm cells,

00:01:52.810 --> 00:01:54.430
or for a female organism,

00:01:54.430 --> 00:01:56.730
that would be producing the egg cells.

00:01:56.730 --> 00:01:58.970
This meiosis diagram is for an organism

00:01:58.970 --> 00:02:01.900
that has two pairs of chromosomes,

00:02:01.900 --> 00:02:05.750
while we know that human
beings actually have 23 pairs.

00:02:05.750 --> 00:02:07.670
But if we saw a diagram with 23 pairs,

00:02:07.670 --> 00:02:09.560
it would get very complicated, very fast,

00:02:09.560 --> 00:02:12.110
so the two pairs help us
understand what's going on

00:02:12.110 --> 00:02:15.150
and help us understand where
some of this genetic variation

00:02:15.150 --> 00:02:16.410
is going to come from.

00:02:16.410 --> 00:02:19.380
So I've already pre-labeled the
homologous chromosomes here.

00:02:19.380 --> 00:02:20.500
And just as a reminder,

00:02:20.500 --> 00:02:22.820
homologous chromosomes are ones

00:02:22.820 --> 00:02:25.400
that have the same genes on them.

00:02:25.400 --> 00:02:28.440
Now they could have different
versions of the genes on them,

00:02:28.440 --> 00:02:31.300
but they're fundamentally
coding for the same genes.

00:02:31.300 --> 00:02:36.300
You can view chromosomes as
really long stretches of DNA

00:02:36.610 --> 00:02:40.630
that has all been rolled
in and bunched in together,

00:02:40.630 --> 00:02:41.590
something like this.

00:02:41.590 --> 00:02:44.040
A human chromosome can
have on the order of

00:02:44.040 --> 00:02:46.930
100 million base pairs in it.

00:02:46.930 --> 00:02:49.590
Now, if you were to
straighten that string of DNA,

00:02:49.590 --> 00:02:51.180
if you were to unwind it,

00:02:51.180 --> 00:02:54.100
you would see different section's
code for different genes.

00:02:54.100 --> 00:02:55.900
So that might be one gene there,

00:02:55.900 --> 00:02:57.530
that might be another gene there.

00:02:57.530 --> 00:03:00.030
You might have one long
gene right over there.

00:03:00.030 --> 00:03:03.320
On average, the genes are about
27,000 base pairs in length

00:03:03.320 --> 00:03:05.670
but some of them could be
millions of base pairs.

00:03:05.670 --> 00:03:07.390
So on one of these chromosomes,

00:03:07.390 --> 00:03:10.800
you can actually have
thousands of genes being coded.

00:03:10.800 --> 00:03:13.010
And so if you were to pick this chromosome

00:03:13.010 --> 00:03:15.020
and this chromosome right over here,

00:03:15.020 --> 00:03:16.940
they would be coding for the same genes

00:03:16.940 --> 00:03:18.130
because they're homologous.

00:03:18.130 --> 00:03:19.760
But once again, they could
have different alleles,

00:03:19.760 --> 00:03:21.960
different versions of those genes on them.

00:03:21.960 --> 00:03:26.280
And similarly, this
chromosome and this chromosome

00:03:26.280 --> 00:03:27.430
are also homologous.

00:03:27.430 --> 00:03:30.390
They're also coding for the same genes.

00:03:30.390 --> 00:03:32.510
Now, as we go into meiosis,

00:03:32.510 --> 00:03:35.250
the first step is that the chromosomes

00:03:35.250 --> 00:03:37.220
are essentially going to copy themselves

00:03:37.220 --> 00:03:39.290
into two sister chromatids.

00:03:39.290 --> 00:03:43.250
So, for example, this one right over here

00:03:43.250 --> 00:03:47.120
has now copied itself and it
has that telltale X shape.

00:03:47.120 --> 00:03:50.020
But this side of this now chromosome,

00:03:50.020 --> 00:03:51.510
which we would call a chromatid,

00:03:51.510 --> 00:03:54.320
and this sister chromatid
should be identical.

00:03:54.320 --> 00:03:55.660
Now there might be some errors

00:03:55.660 --> 00:03:57.590
that got introduced through mutation.

00:03:57.590 --> 00:04:00.390
But if we don't assume mutation,
they would be identical.

00:04:00.390 --> 00:04:03.820
Likewise, this side and this side,

00:04:03.820 --> 00:04:06.850
assuming no mutations,
they would be identical.

00:04:06.850 --> 00:04:08.700
Now what's interesting about this

00:04:08.700 --> 00:04:11.280
is what happens in the next phase.

00:04:11.280 --> 00:04:15.340
In the next phase, you have
the independent assortment

00:04:15.340 --> 00:04:17.680
of homologous chromosomes.

00:04:17.680 --> 00:04:21.730
So as we said, this and
this might be coding

00:04:21.730 --> 00:04:24.770
for the same genes, it might
just have different versions.

00:04:24.770 --> 00:04:26.420
But as we go into this phase,

00:04:26.420 --> 00:04:30.130
as we do meiosis I, as it's
often known right over here,

00:04:30.130 --> 00:04:32.280
this blue chromosome could go here,

00:04:32.280 --> 00:04:35.750
while the homologous red
chromosome would go there.

00:04:35.750 --> 00:04:38.060
The purple chromosome is going here

00:04:38.060 --> 00:04:40.710
and the light blue
chromosome is going there.

00:04:40.710 --> 00:04:41.790
And this is really interesting

00:04:41.790 --> 00:04:44.380
because there's a lot of
different ways this could happen.

00:04:44.380 --> 00:04:47.390
In this situation, you have two pairs.

00:04:47.390 --> 00:04:50.330
Each of these intermediary
steps in meiosis

00:04:50.330 --> 00:04:53.070
could randomly have one from each pair.

00:04:53.070 --> 00:04:57.170
So just in this example,
you have two to the number

00:04:57.170 --> 00:05:00.950
of pairs combinations at
this stage right over here.

00:05:00.950 --> 00:05:02.520
Now this was only when we have two pairs.

00:05:02.520 --> 00:05:04.000
If we're talking about a human being,

00:05:04.000 --> 00:05:08.490
we're talking about two to the
23rd different combinations

00:05:08.490 --> 00:05:11.450
of which of the two homologous
chromosomes you get.

00:05:11.450 --> 00:05:13.610
So there's a lot of variation here.

00:05:13.610 --> 00:05:14.720
Now on top of that,

00:05:14.720 --> 00:05:17.240
some of y'all might have
noticed something interesting.

00:05:17.240 --> 00:05:19.090
If you just follow the colors here,

00:05:19.090 --> 00:05:22.670
it looks like a little chunk
of this chromosome got swapped

00:05:22.670 --> 00:05:24.530
with a little chunk of this chromosome.

00:05:24.530 --> 00:05:25.600
You could see it here.

00:05:25.600 --> 00:05:28.200
The red is now on the big blue X

00:05:28.200 --> 00:05:30.500
and the blue is now on the big red X.

00:05:30.500 --> 00:05:33.500
This is another source
of genetic variation

00:05:33.500 --> 00:05:35.863
and it is known as crossover.

00:05:36.940 --> 00:05:37.860
And what it does is,

00:05:37.860 --> 00:05:41.280
it can actually mix DNA
between chromosomes.

00:05:41.280 --> 00:05:43.210
Once again, these are
homologous chromosomes,

00:05:43.210 --> 00:05:45.500
they are encoding the same genes,

00:05:45.500 --> 00:05:47.620
but now alleles that were
sitting on the blue one

00:05:47.620 --> 00:05:49.360
could now sit with the rest of the red one

00:05:49.360 --> 00:05:51.340
and the alleles that was
sitting with the red one

00:05:51.340 --> 00:05:53.330
can now sit with the
rest of the blue ones.

00:05:53.330 --> 00:05:56.530
And crossover is actually
reasonably common during meiosis.

00:05:56.530 --> 00:05:59.220
So once again, it's
mixing things up even more

00:05:59.220 --> 00:06:01.890
than this two to the 23rd combinations.

00:06:01.890 --> 00:06:04.300
So a lot of variation that you can produce

00:06:04.300 --> 00:06:05.600
through sexual reproduction.

00:06:05.600 --> 00:06:08.800
And then as we go into this
last phase into meiosis II,

00:06:08.800 --> 00:06:11.180
and we're actually producing the gametes,

00:06:11.180 --> 00:06:14.480
if this meiosis is going
on in the gonads of a male,

00:06:14.480 --> 00:06:17.170
this would be the chromosomal
makeup of the sperm cells.

00:06:17.170 --> 00:06:19.520
If this is going on within the female,

00:06:19.520 --> 00:06:24.030
then this would be the DNA
makeup of the egg cells.

00:06:24.030 --> 00:06:25.590
And what you see, and
just to make it clear

00:06:25.590 --> 00:06:28.080
what's happened here is
that your sister chromatids

00:06:28.080 --> 00:06:30.260
have now spread apart, although
they're no longer identical,

00:06:30.260 --> 00:06:32.060
especially if you have the crossover.

00:06:32.060 --> 00:06:35.870
So for example, this one went over here

00:06:35.870 --> 00:06:40.260
and this one went over here as well.

00:06:40.260 --> 00:06:44.180
And then you have another
scenario where you have this one

00:06:44.180 --> 00:06:46.830
and this one ended up in this gamete,

00:06:46.830 --> 00:06:48.220
and we can go on and on.

00:06:48.220 --> 00:06:49.350
So actually you can have,

00:06:49.350 --> 00:06:51.160
especially if you consider crossover,

00:06:51.160 --> 00:06:55.530
more than two to the 23rd
possible combinations.

00:06:55.530 --> 00:06:58.180
Now two to the 23rd power is approximately

00:06:58.180 --> 00:07:01.840
a little bit more than
eight million combinations.

00:07:01.840 --> 00:07:03.530
And if do you want a little math trick

00:07:03.530 --> 00:07:05.180
for estimating powers of two,

00:07:05.180 --> 00:07:07.190
you can just recognize
that two to the 10th power

00:07:07.190 --> 00:07:08.970
is a little bit more than 1,000.

00:07:08.970 --> 00:07:10.780
So this is going to be two to the 20th,

00:07:10.780 --> 00:07:12.840
which is about a million,
and then two to the third,

00:07:12.840 --> 00:07:15.380
which is eight, so a little
bit more than 8 million.

00:07:15.380 --> 00:07:16.213
And once again,

00:07:16.213 --> 00:07:19.090
that's before considering
crossover and mutation,

00:07:19.090 --> 00:07:22.000
which is going to make the
combinations even larger.

00:07:22.000 --> 00:07:24.970
And I'll also point out
these are the combinations

00:07:24.970 --> 00:07:28.300
for the gametes, and
the gametes are haploid.

00:07:28.300 --> 00:07:31.870
They have half the DNA of a full organism.

00:07:31.870 --> 00:07:33.750
And so when the gametes combine,

00:07:33.750 --> 00:07:36.540
you're actually going
to have two to the 23rd

00:07:36.540 --> 00:07:40.110
times two to the 23rd combinations,

00:07:40.110 --> 00:07:42.210
just from sexual reproduction,

00:07:42.210 --> 00:07:44.880
and you'll have even more
from mutation and crossover.

00:07:44.880 --> 00:07:46.780
And so that's going to lead you

00:07:46.780 --> 00:07:51.690
to more than 70 trillion combinations

00:07:51.690 --> 00:07:53.953
just from these two parents.


Feedback in living systems

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:02.300
- [Instructor] So last
weekend, my family and I

00:00:02.300 --> 00:00:04.410
went out hiking in the desert.

00:00:04.410 --> 00:00:06.070
And as you can tell from these pictures

00:00:06.070 --> 00:00:09.410
I snapped along the way,
it was a gorgeous hike.

00:00:09.410 --> 00:00:13.340
We made our way to this lake
around a small canyon range

00:00:13.340 --> 00:00:15.430
and up and down this mountain trail.

00:00:15.430 --> 00:00:17.460
Now, all of this was really great,

00:00:17.460 --> 00:00:19.360
but there was just one problem.

00:00:19.360 --> 00:00:21.500
It got super hot.

00:00:21.500 --> 00:00:24.340
And because we were
exercising out in the hot sun,

00:00:24.340 --> 00:00:26.470
we started sweating buckets.

00:00:26.470 --> 00:00:29.580
And all I wanted to do after
a while was to find some water

00:00:29.580 --> 00:00:31.910
and shade as soon as possible.

00:00:31.910 --> 00:00:34.320
So finally, after we sat down to have

00:00:34.320 --> 00:00:35.940
a nice picnic in the shade

00:00:35.940 --> 00:00:39.160
and our sweat provided
some evaporative cooling,

00:00:39.160 --> 00:00:42.590
our bodies were able to cool
down without overheating.

00:00:42.590 --> 00:00:45.140
So you might be wondering
why did our bodies

00:00:45.140 --> 00:00:47.350
in our behavior respond that way?

00:00:47.350 --> 00:00:49.730
Why did we sweat and want to find shade?

00:00:49.730 --> 00:00:53.670
Well, the answer is our bodies
were protecting us from harm.

00:00:53.670 --> 00:00:55.810
The human body isn't able to function

00:00:55.810 --> 00:00:57.460
at too high of a temperature.

00:00:57.460 --> 00:01:00.580
So our bodies helped cool us
down through a combination

00:01:00.580 --> 00:01:04.220
of physiological and behavioral responses.

00:01:04.220 --> 00:01:07.087
Physiological responses
being the internal, chemical,

00:01:07.087 --> 00:01:09.270
and physical changes that our bodies

00:01:09.270 --> 00:01:11.160
carry out unconsciously,

00:01:11.160 --> 00:01:13.530
and behavioral responses being the actions

00:01:13.530 --> 00:01:17.490
we carry out consciously in
response to what our body needs.

00:01:17.490 --> 00:01:19.980
So in this case, the
physiological response

00:01:19.980 --> 00:01:22.130
would be sweating, which our body does

00:01:22.130 --> 00:01:23.860
in order to cool itself down.

00:01:23.860 --> 00:01:27.570
And in addition to sweating,
other physiological responses

00:01:27.570 --> 00:01:31.400
were also happening such as,
our blood vessels were dilating

00:01:31.400 --> 00:01:33.500
and we were getting thirsty.

00:01:33.500 --> 00:01:35.940
And the behavioral
responses were our attempts

00:01:35.940 --> 00:01:39.823
to find shade and get out of
the sun and to drink water.

00:01:40.780 --> 00:01:45.080
This tendency of an organism
to maintain internal conditions

00:01:45.080 --> 00:01:48.350
within an acceptable range despite changes

00:01:48.350 --> 00:01:52.840
in its external environment
is called homeostasis.

00:01:52.840 --> 00:01:54.700
And I'll write down our definition.

00:01:54.700 --> 00:01:59.700
So it's the tendency to
maintain internal conditions

00:02:00.060 --> 00:02:04.320
despite changes in external conditions.

00:02:04.320 --> 00:02:08.360
So homeostasis is incredibly
important because without it,

00:02:08.360 --> 00:02:12.170
we could have overheated
and been in real danger.

00:02:12.170 --> 00:02:15.300
So in other words,
homeostasis is necessary

00:02:15.300 --> 00:02:17.870
in order for organisms to survive.

00:02:17.870 --> 00:02:20.550
Now, you might also be
wondering how living things

00:02:20.550 --> 00:02:24.600
generally maintain this
homeostatic condition of theirs.

00:02:24.600 --> 00:02:28.500
And this usually involves
negative feedback loops.

00:02:28.500 --> 00:02:30.160
So let me draw this diagram for us.

00:02:30.160 --> 00:02:33.070
We have our stimulus, we have a detection

00:02:33.070 --> 00:02:34.953
and then a response.

00:02:37.400 --> 00:02:42.330
So in negative feedback, a
stimulus or a detectable change

00:02:42.330 --> 00:02:45.950
in internal conditions
triggers the body to carry out

00:02:45.950 --> 00:02:50.190
a response that will counteract
or oppose this change.

00:02:50.190 --> 00:02:53.990
So it'll bring conditions
back within an ideal range.

00:02:53.990 --> 00:02:56.100
And this is what is represented right here

00:02:56.100 --> 00:02:58.873
by this blocking symbol in the diagram.

00:02:59.890 --> 00:03:01.990
So going back to my family's hiking trip,

00:03:01.990 --> 00:03:04.590
we can say that the
stimulus was the increase

00:03:04.590 --> 00:03:06.920
in our body temperatures as a result

00:03:06.920 --> 00:03:09.110
of hiking in the hot desert.

00:03:09.110 --> 00:03:11.980
Our bodies detected that
our internal temperature

00:03:11.980 --> 00:03:15.220
was moving outside of
the acceptable range,

00:03:15.220 --> 00:03:18.410
which typically falls between 97.7

00:03:18.410 --> 00:03:23.410
to 99.5 degrees Fahrenheit or
36.5 to 37.5 degrees Celsius.

00:03:26.850 --> 00:03:28.130
And the cool thing is that,

00:03:28.130 --> 00:03:30.570
once our bodies detected the stimulus,

00:03:30.570 --> 00:03:33.230
they produced a response to counteract

00:03:33.230 --> 00:03:35.800
this change through negative feedback.

00:03:35.800 --> 00:03:38.970
We were hot, so we wanted
to become less hot.

00:03:38.970 --> 00:03:41.120
And in this case, the
negative feedback loop

00:03:41.120 --> 00:03:44.310
caused responses like sweating that helped

00:03:44.310 --> 00:03:45.890
cool our body temperatures down

00:03:45.890 --> 00:03:49.580
to the acceptable or the ideal range.

00:03:49.580 --> 00:03:51.280
It's also worth noting that our bodies

00:03:51.280 --> 00:03:54.870
can elicit negative feedback
mechanisms in response

00:03:54.870 --> 00:03:57.940
to our body temperature dropping too low.

00:03:57.940 --> 00:04:01.300
So if our body temperature
drops below the ideal range

00:04:01.300 --> 00:04:03.800
or our body temperature decreases,

00:04:03.800 --> 00:04:06.590
then the body counteracts
this change through responses

00:04:06.590 --> 00:04:09.850
like shivering and blood
vessel constriction,

00:04:09.850 --> 00:04:12.520
all with a goal of
helping to keep us warm.

00:04:12.520 --> 00:04:15.320
So negative feedback
mechanisms help cool us down

00:04:15.320 --> 00:04:19.760
when we get too hot or they
warm us up when we get too cold.

00:04:19.760 --> 00:04:23.330
So they help to keep our
body temperatures just right.

00:04:23.330 --> 00:04:25.890
And this process of
maintaining body temperature,

00:04:25.890 --> 00:04:28.660
otherwise known as thermoregulation,

00:04:28.660 --> 00:04:30.230
let me write that out for us,

00:04:30.230 --> 00:04:33.340
it can be seen in all
different kinds of organisms.

00:04:33.340 --> 00:04:36.120
You might've seen dogs
pant when they're hot

00:04:36.120 --> 00:04:39.030
or spotted lizards
sunbathing to stay warm.

00:04:39.030 --> 00:04:41.260
And these are all homeostatic responses

00:04:41.260 --> 00:04:43.960
that help keep the
organism's body temperature

00:04:43.960 --> 00:04:47.190
within the acceptable
range that we talked about.

00:04:47.190 --> 00:04:50.170
Another really awesome example
of a negative feedback loop

00:04:50.170 --> 00:04:53.940
is osmoregulation, specifically in salmon.

00:04:53.940 --> 00:04:55.810
And here's a picture.

00:04:55.810 --> 00:04:59.270
Now, salmon spend part of their
lives in freshwater streams

00:04:59.270 --> 00:05:02.540
and the other part of their
lives in salt-water oceans.

00:05:02.540 --> 00:05:05.800
So in fresh water, the salt
concentration of the water

00:05:05.800 --> 00:05:08.630
is lower than the salt
concentration you would find

00:05:08.630 --> 00:05:11.500
in the fish's internal body fluid.

00:05:11.500 --> 00:05:14.760
While in saltwater, the salt
concentration of the water

00:05:14.760 --> 00:05:18.930
is higher than this fish's
internal salt concentration.

00:05:18.930 --> 00:05:20.830
So this means that in fresh water,

00:05:20.830 --> 00:05:23.140
the fish will tend to absorb water

00:05:23.140 --> 00:05:25.290
and lose salts through their skin.

00:05:25.290 --> 00:05:28.340
Well, the opposite is true in saltwater.

00:05:28.340 --> 00:05:31.110
Any large change in a fish's internal salt

00:05:31.110 --> 00:05:33.440
or water levels could be fatal.

00:05:33.440 --> 00:05:35.810
So how exactly can salmon tolerate

00:05:35.810 --> 00:05:39.280
these extremely different
environmental conditions?

00:05:39.280 --> 00:05:42.960
Well, they also use negative
feedback mechanisms.

00:05:42.960 --> 00:05:45.560
So salmon have a negative feedback system,

00:05:45.560 --> 00:05:49.100
which detects changes in
internal salt concentrations

00:05:49.100 --> 00:05:52.380
and causes a response that
involves either taking up

00:05:52.380 --> 00:05:54.940
or excreting salt through the gills

00:05:54.940 --> 00:05:57.650
or having more or less dilute urine

00:05:57.650 --> 00:06:01.600
in order to reestablish ideal
internal salt concentrations.

00:06:01.600 --> 00:06:05.840
And this is otherwise
known as osmoregulation.

00:06:05.840 --> 00:06:08.320
So again, we have a
feedback loop that acts

00:06:08.320 --> 00:06:11.110
to oppose a stimulus, which in this case

00:06:11.110 --> 00:06:14.750
is the change in internal
salt concentrations.

00:06:14.750 --> 00:06:17.620
So now we know that homeostatic mechanisms

00:06:17.620 --> 00:06:20.250
usually involve negative feedback loops,

00:06:20.250 --> 00:06:22.720
but what about positive feedback loops?

00:06:22.720 --> 00:06:25.890
Well, many organisms actually
use positive feedback loops

00:06:25.890 --> 00:06:28.490
to bring processes to completion.

00:06:28.490 --> 00:06:31.590
So while negative feedback
loops dampen stimuli

00:06:31.590 --> 00:06:35.730
or oppose stimuli, positive
feedback loops do the opposite.

00:06:35.730 --> 00:06:38.450
They amplify stimuli.

00:06:38.450 --> 00:06:40.700
And as you can tell from this diagram,

00:06:40.700 --> 00:06:43.380
instead of having a blocking symbol here,

00:06:43.380 --> 00:06:45.210
we have an arrow to indicate

00:06:45.210 --> 00:06:48.000
the amplification of the stimulus.

00:06:48.000 --> 00:06:51.090
So in humans for instance,
a positive feedback loop

00:06:51.090 --> 00:06:53.060
is used for childbirth.

00:06:53.060 --> 00:06:54.940
So as you can see from this diagram,

00:06:54.940 --> 00:06:57.740
the stimulus in childbirth
comes from the baby's head,

00:06:57.740 --> 00:07:00.610
which presses against the cervix here.

00:07:00.610 --> 00:07:03.340
And this stimulates neurons in the cervix,

00:07:03.340 --> 00:07:06.070
which send a signal for
the brain to release

00:07:06.070 --> 00:07:09.200
a special kind of hormone called oxytocin.

00:07:09.200 --> 00:07:11.920
Now, oxytocin is responsible for causing

00:07:11.920 --> 00:07:15.151
the uterus to contract,
which as you might've guessed

00:07:15.151 --> 00:07:17.670
causes more pressure on the cervix

00:07:17.670 --> 00:07:19.660
which sends more neural signals,

00:07:19.660 --> 00:07:22.050
which releases more oxytocin.

00:07:22.050 --> 00:07:24.750
And this loop continues on and on

00:07:24.750 --> 00:07:28.070
all the way until the baby is born.

00:07:28.070 --> 00:07:31.050
So when the baby is born,
because the baby's head

00:07:31.050 --> 00:07:33.400
isn't pressing up against the cervix

00:07:33.400 --> 00:07:35.380
and the pelvic floor anymore,

00:07:35.380 --> 00:07:37.200
the neuron stops sending the signal

00:07:37.200 --> 00:07:38.720
and the brain stops triggering

00:07:38.720 --> 00:07:41.190
the release of so much oxytocin.

00:07:41.190 --> 00:07:44.593
So that's how the loop will
eventually come to an end.

00:07:45.950 --> 00:07:48.150
So to recap on what we've talked about.

00:07:48.150 --> 00:07:50.320
Today, we learned that organisms maintain

00:07:50.320 --> 00:07:53.660
their internal conditions
through homeostasis.

00:07:53.660 --> 00:07:55.060
And this is usually accomplished

00:07:55.060 --> 00:07:56.820
through negative feedback loops,

00:07:56.820 --> 00:08:00.420
which dampen or oppose
stimuli as we talked about

00:08:00.420 --> 00:08:04.000
with thermoregulation and osmoregulation.

00:08:04.000 --> 00:08:06.730
But on the other hand
as we saw in childbirth,

00:08:06.730 --> 00:08:10.200
positive feedback loops
work to amplify stimuli

00:08:10.200 --> 00:08:13.103
in order to bring processes to completion.