>> Hello, and welcome to the Penguin Prof Channel.
In today's episode I want to talk about cellular respiration.
Oh my gosh.
This is so ugly, isn't it?
Don't worry.
I've got some help for you.
We're going to look at the big picture here.
Overall, the equation -- you may
or may not have seen it already -- looks like this.
Most people think about breathing
when they see cellular respiration,
and it's actually related.
It is because of what the cells are doing that you actually need
to take in oxygen and why you produce carbon dioxide.
So it is related to the respiratory system.
I'm going to make a lot of analogies with money
because people are familiar with finances a lot more
than cellular respiration.
Show me the money.
Oh, this is my favorite currency in the whole world.
Isn't it gorgeous?
That's the Yellow-Eyed Penguin from New Zealand.
Now, your options for getting money.
You can either make it yourself, be the boss.
Or you can get it from someone else and be an employee.
Ok. You're going to use this money, of course, to buy goods
and services, or you can save it for the future.
Now, of course, when we talk
about cells we're not talking about money.
We're talking about glucose.
Glucose is what cells need in order to do work.
You have a couple of options.
Some organisms make glucose all by themselves,
and we call those organisms autotrophs.
Auto means self.
Now there are a couple of different ways
that you can make your own carbohydrates.
You can use energy from the sun.
We call those organism photoautotrophs.
And some organisms use the energy from chemical bonds
which they break in order to make carbohydrates.
The other option if you can't make it yourself,
you got to get it from someone else.
And those organisms we refer to as heterotrophs.
Hetero just means other.
Either way you're going to use the glucose to do cell work
or you're going to store the glucose for the future.
Now, what we use the glucose for is shown here.
About 70 percent is just to stay alive, ok.
So here's the breakdown by organ.
A lot of people are surprised
that the liver has a bigger energy demand
than the brain does.
We got physical activity, and then the digestion
of the food itself also requires glucose.
Now, here's the thing.
The use of glucose doesn't directly fuel cell processes.
You need to convert it into something else first.
So you take the glucose, you run it through a series of reactions
and you convert into ATP,
and that's what cellular respiration is.
My financial analogy.
We have a penguin, of course, who works hard all day and wants
to have some ice cream after work.
He gets a paycheck.
Now he takes the paycheck and goes
to the ice cream truck, but you know what?
The ice cream truck won't take a paycheck.
They want cash, right.
You can't get ice cream with a paycheck.
So what the penguin has to do is, of course, go to the bank
and the bank does a conversion and gives the penguin cash money
in exchange for the paycheck, and now you can take your cash
and buy ice cream, right.
This is not a complicated idea yet, right.
So the analogy here is that the paycheck is the food
that you eat, the fuel molecules that we take in.
We cannot use the energy from the food directly.
They have to be processed.
All those nutrients get broken down and then converted mainly
by organelles in the cells called mitochondria,
which we're going to spend time looking at it today,
and then we're going to convert that energy
from the food molecules into ATP.
How good are we at doing this?
Well, if you take your paycheck and you go to a bank
and you have the bank do the conversion,
the bank should give you the exact same amount, right,
as your paycheck, unless you go to one of those, you know,
check cashing places where they charge fees and all that.
You don't want to go there.
The food that you eat gets converted
and if you had a hundred dollars' worth of food,
the amount of useable energy that you're going to get
out of it in the form of ATP is only about 40 bucks.
So we're about 40 percent efficient at breaking
down the energy from the food that we eat
and converting it into ATP.
Well, you might be wondering what happens
to the rest of it, right.
So you got about 60 bucks left.
That energy is lost as heat in the conversion process.
When you convert energy, you produce heat as a waste product.
And this was actually how cellular respiration was studied
in the beginning.
Of course, I have to talk about the beginning.
Antoine Lavoisier was the person who really is credited
for studying cellular respiration.
He studied so much stuff it's just ridiculous.
He's considered the father of modern chemistry.
He studied biology.
He also studied geology.
He did like the first geologic maps
of France among other things.
But what he did was he created a calorimeter,
and he used guinea pigs.
And so he had an outer chamber which he could keep
at a constant temperature with ice and then he had ice
in the middle chamber and then he put the guinea pig
on the inside.
And he found out that the guinea pig produced heat,
and he collected the water from the ice melt
in the middle chamber and he weighed it and he was able
to figure out what the guinea pig was doing in respiration
and he said it's combustion.
It's just like a candle burning.
Story gets a little bit more interesting.
He met a woman when he was 28.
She was only13.
She became a scientist in her own right.
But she was the daughter of a co-owner of the Ferme generale.
And I apologize for my pronunciation of French.
We'll get back to that in just a second.
She was really important in his work.
And that's her right there.
She's assisting him as he's looking at cellular respiration,
metabolic rate basically, in the humans.
The story does not end well.
Because of this organization
that his wife was involved with, he became involved.
These were like the tax collectors.
And these people, you know, during the revolution all
of them were executed.
He was tried and convicted and sent
to the guillotine at the age of 50.
Anyway, it's interesting to think
about how much more he could have discovered had he not
been executed.
By the way, the French Government did exonerate him one
and a half years later.
Here's the big picture.
In our cells we extract energy from carbohydrates
through a series of reactions beginning with glycolysis.
Glycolysis occurs in the cell cytoplasm.
And that is going to involve nine steps
which will break glucose, a six carbon sugar, into two molecules
of pyruvate or pyruvate acid, which are three carbons each.
Those molecules of pyruvate will go into the mitochondria.
And that's where we're going to spend the most of our time.
Mitochondria are like the energy transformers of the cell.
So we're going to see in a big picture sense how they extract
energy from carbohydrates.
The mitochondrion has two membranes, an outer membrane
and an inner membrane which is thrown into folds in order
to increase surface area.
The space inside the inner membrane is called the matrix,
the mitochondrial matrix and that's
where the citric acid cycle is going to take place.
There is also the space between the inner and outer membranes,
and we call that space the inter-membrane space,
not to surprisingly.
Now, the matrix, like I said, is where the citric acid cycle --
it's also called the tricarboxylic acid cycle
or TCA -- that's where this beautiful piece
of work is going to take place.
Oh geez. Don't worry.
Don't worry.
I'm just going to show you the highlights of this cycle.
We're not going to look at any of the details.
First thing, it's cycle, ok.
It goes round and round.
The second thing is that pyruvate gets groomed.
It gets chemically altered
into something called Acetyl-CoA before it enters the cycle.
So I point this out because a lot of students
when they look at, you know, the order of events, glycolysis
and then the citric acid cycle
and then the electron transport chain,
and they say what happened to pyruvate, you know,
because pyruvate doesn't actually enter the cycle.
It gets groomed.
It gets chemically altered, and it turns
into this thing called Acetyl-Coenzyme A.
So that's what happened to it.
The third thing to notice is bonds are being broken
from these carbohydrates, and the carbons
that are released are combined with oxygen
to form carbon dioxide, and this is the source
of the CO2 that you exhale.
Finally, most of the high energy bonds
from what was originally glucose but became pyruvate
and then Acetyl-Coezyme A as it enters the cycle,
most of those high energy bonds get oxidized, ok.
And if you remember if something gets oxidized,
somebody else got to get reduced.
I've got a Redox video.
I'll put that link in the description box below.
So who's getting reduced are these electron carriers.
So every time you see, for example, NAD+ going to NADH,
that is the addition of electrons
to NAD reducing it to NADH.
So what's happening is that the energy
from the original carbohydrate is being transferred
to these electron carriers, and they are going
to leave the citric acid cycle, and they're going to interact
with some proteins on the inner mitochondrial membrane
in order to generate ATP.
So here we are again in the mitochondria,
and you see where the citric acid cycle,
or the tricarboxylic cycle, is occurring
and pyruvate being fed in.
I'm not showing you all the inputs and outputs, ok.
And now we're going to see where the electrons
that are being carried
by the electron carriers, where they go.
And where they go is a series of proteins built
into the inner mitochondrial membrane called the Electron
Transport Chain.
Ok. So we've got NADH, FADH2, the NADs and FADs right there,
right, carrying the electrons that they picked
up in the citric acid cycle, and they're going to take them
to the proteins and the electron transport chain up here.
So the source of the electrons will be the electron carriers
and as the electron carriers give up their electrons,
the electron carriers get oxidized and the proteins
in the chain get reduced.
So this first protein will pick up the electrons
and then transfer it to this guy
and then the electrons transfer here.
And they kind of play a game of hot potato as it were.
And as the electrons move
through these different proteins, a little bit
of energy is given off each time and that energy is used
to do work and that work is to pump protons
into the inter-membrane space.
That's what the energy gets used for.
So ok, all of these proteins have names.
Some of them are sort of stationary,
some actually are mobile, they move around.
But the bottom line is what these things do is they --
they have increasing electron affinity.
So they want the electrons more than the protein just upstream.
So the electrons are moving through these regions
and a little bit of energy gets given off at various points
and that energy is used to create a proton gradient.
So this inter-membrane space gets packed with protons.
And where the electrons finally end up,
the final electron acceptor after the electrons pass
through all these proteins, is oxygen.
And that's why oxygen is needed for aerobic respiration
because that's where the electrons end up.
And when electrons are added to oxygen,
you are going to make water.
It's called metabolic water production.
So I'm going to clean this up a little bit
and show you the proton gradient in the inter-membrane space.
This is again created by the electron transport chain.
Notice the direction of the gradient.
That is where the protons want to go.
However, that inter-membrane is not permeable to protons.
So they cannot pass through it.
They are unhappy.
And this disequilibrium is the source of the energy
that we're going to use to make ATP.
The analogy is a hydroelectric power plant.
So this is going to be analogous
to how the electron transport chain pumps protons
into the inter-membrane space.
These protons are going to find that there is a magic space
that they can fit through.
They're going to flow down their gradient
in the direction they want to go just like water flows
down through the turbine because of gravity.
And instead of a turbine that converts that movement
into electricity, we're going
to have a protein called the ATP synthase, which is going
to convert the energy from the protons going in the direction
that they want to go into ATP.
Let's go back and check this out.
So here is the ATP synthase, and in the center there is a core
where the protons fit.
And as the protons flow in the direction
of the gradient now they're going in the direction they want
to go, the cell harnesses that energy and uses it
to phosphorylate ADP to make ATP.
How efficient is it?
Where if you have oxygen present and you run aerobic respiration,
if you break down the bonds in one molecule
of glucose you can generate up to 32,
some textbooks will say 34, molecules of ATP.
Compare that to anaerobic respiration.
If there is no oxygen present you stick that same glucose
into the mix and what you get, only two ATPs.
So you can see how much more efficient the aerobic
respiration pathway is compared to the anaerobic respiration.
So aerobic respiration is like 16 times more efficient
at stripping the energy from carbohydrates and converting
that energy into useable ATP.
You might imagine how back
in the day anaerobes were outcompeted by organisms
that could run this new more efficient method
of extracting energy.
Eukaryotic cells got much bigger.
We evolved into multi-cellular organisms,
and it just went up from there.
So the overall view of cellular respiration,
the inputs, glucose and oxygen.
We run glycolysis where we split glucose
into two molecules of pyruvate.
You get a little bit of ATP out of that.
By the way, you get two molecules of ATP just from that.
And if you don't have any oxygen, that's all you get.
If you do have oxygen, you can run the citric acid cycle.
And, again, the big deal about the citric acid cycle is this is
where most of the high energy bonds
from what was once glucose get broken,
and where the energy goes is to reduce electron carriers.
Now where the electron carriers take their electrons is the
electron transport change.
And so most of the ATP produced in aerobic respiration comes
from chemiosmosis, the production
of ATP using that gradient.
What comes out where the atoms end up is carbon dioxide
and water and the energy from what was glucose long,
long time ago gets converted into ATP.
Oh my goodness.
I told you it wouldn't be so painful.
As always, I hope that was helpful.
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