- Let's delve into the world of mitochondria
which are probably my favorite organelle.
So let's just have a little review of what mitochondria are
and then we can delve a little bit deeper
into their structure.
So let's just think about a cell
and not just any cell, but a eukaryotic cell.
So that's the cellular membrane
and when people say a eukaryote or a eukaryotic cell
they most typically say, "Oh! That must have its nuclear DNA
"in a membrane-bound nucleus." and that would be true,
so let's draw our membrane-bound nucleus.
That's our nuclear membrane.
You have your DNA in here,
so let's draw some DNA.
But when we talk about eukaryotic cells,
we're not just talking about a membrane-bound nucleus,
we're also talking about other membrane-bound organelles
and in a close second place for a membrane-bound structure
that is very important to the cell
would be the mitochondria.
So let's draw some mitochondria right over here.
So I'll talk a little bit more about
what these little squiggly lines that I'm drawing
inside of the mitochondria are
and this is actually a little bit more
of a textbook visualization, as we'll learn
in a few minutes or seconds that we now have more
sophisticated visualizations of what's actually going on
inside of a mitochondria, but we haven't actually
answered all of our questions,
but you might have already learned that,
so let me make it clear, these are mitochondria.
That's the plural.
If we're just talking about one of them,
we're talking about a mitochondrion.
That's the singular of mitochondria.
But you might have already learned,
some time in your past or in another Khan Academy video,
that these are viewed as the ATP factories for cells.
So let me right it this way.
So ATP factories.
A-T-P factories and if you watched the videos on ATP
or cellular respiration or other videos,
I'd repeatedly talk about how ATP is really the currency
for energy in the cell that when it's in its ATP form
you have adenosine triphosphate.
If you pop one of the phosphate groups off,
you pop one of the P's off, it release energy
and that's what your body uses to do all sort of things
from movement to thinking to all sorts of things
that actually go on in your bodies,
so you can imagine mitochondria are really important
for energy, for when the cell has to do things.
And that's why you'll find more mitochondria
in things like muscle cells, things that have to use
a lot of energy.
Now before I get into the structure of mitochondria,
I wanna talk a little bit about its fascinating past
because we think of cells as the most basic unit of life
and that is true, that comes straight out of cell theory,
but it turns out the most prevalent theory
of how mitochondria got into our cells
is that at one time the predecessors,
the ancestors to our mitochondria,
were free, independent organisms, microorganisms.
So they're descendent from bacterial-like microorganisms
that might have been living on their own
and they were maybe really good at processing energy
or maybe they were even good at other things,
but at some point in the evolutionary past,
they got ingested by what the ancestors of our cells
and instead of just being engulfed and being torn to shreds
and kind of being digested and eaten,
it was like, "Hey, wait, if these things stick around,
"those cells are more likely to survive
"because they're able to help process glucose
"or help generate more energy out of things."
And so the cells that were able to kind of live
in symbiosis have them kind of give a place
for the mitochondria to live or the pre-mitochondria,
the ancestor mitochondria, those survived
and then through kind of the processes of natural selection,
this is what we now associate,
we now associate eukaryotic cells
as having mitochondria,
so I find this whole idea of one organism being inside
of another organism in symbiosis even at the cellular level,
that's kind of mind-boggling, but anyway,
I'll stop talking about that and now let's just talk
about the present, let's talk about
what the actual structure of mitochondria are.
And I'll first draw kind of a simplified drawing
of a mitochondion and I'll draw a cross section.
So, I'm gonna draw a cross section.
So if we were to kind of cut it in half.
So what I've drawn right over here
this would be its outer membrane.
This is the outer membrane right over here
and we label that.
Outer membrane.
And all of these membranes that I'm gonna draw,
they're all going to be phospholipid bilayers.
So if I were to zoom in right over here,
so let me, if I were to zoom in,
we would see a bilayer of phospholipids.
So you have your
hydrophilic heads facing outwards,
hydrophilic heads facing outwards
and your hydrophobic tails facing inwards.
So.
You see something just like that,
so they're all phospholipid bilayers.
But they aren't just phospholipids.
All of these membranes have all sorts of proteins imbedded,
I mean cells are incredibly complex structures,
but even organelles like mitochondria have a fascinating,
I guess you would say sub-structure to them.
They themselves have all sorts of interesting proteins,
enzymes imbedded in their membranes
and are able to help regulate what's going on
inside and outside of these organelles.
And one of the proteins that you have in the outer membrane
of mitochondria, they're called porins
and porins aren't found only in mitochondria,
but they're kind of tunnel proteins,
they're structured so they kind of form a hole
in the outer membrane.
So I'm drawing them the best that I can.
These are porins
and what's interesting about porins is they don't allow
large molecules to pass through passively,
but small molecules like sugars or ions can pass passively
through the porins.
And so, because of that, your ion concentration
and well, I should actually say,
your small molecule concentrations tend
to be similar on either side of this membrane,
on either side of this outer membrane.
But that's not the only membrane involved
in a mitochondrion.
We also have a inner membrane.
I'll do that in yellow.
We also have a inner membrane
and I'm gonna draw it with a textbook model first
and then we'll talk a little bit about,
since we think this model is not quite right,
but in this, so we have this inner membrane,
inner membrane,
and this inner membrane has these folds in it
to increase their surface area
and the surface area is really important
for the inner membrane because that's
where the processes of the electron transport chain
occur across, essentially, these membranes.
So you want this extra surgace area
so you can essentially have more of that going on.
And these folds have a name.
So if you're talking about one of them,
if you're talking about one of these folds,
you're talking about a crista,
but if you're talking about more than one of them,
you would call that a cristae, cristae.
Sometimes I've seen the pronunciation of this
as cristae, cristae or cristae, that's plural for crista.
These are just folds in the inner membrane
and once again the inner membrane is also
a phospholipid bilayer.
Now inside of the inner membranes,
so between the outer membrane and the inner membrane
you could imagine what this is gonna be called.
That space is called the intermembrane space,
not too creative of a name, intermembrane space
and because of the porins,
the small molecule concentration
of the intermembrane space and then outside
of the mitochondria,
out in the cytosol,
those concentrations are gonna be similar,
but then the inner membrane does not have the porins
in it and so you can actually have a different concentration
on either side and that is essential
for the electron transport chain.
The electron transport chain really culminates
with hydrogen, a hydrogen ion gradient
being built between the two sides
and then they flow down that gradient through a protein
called ATP synthase which helps us synthesize ATB,
but we'll talk more about that maybe in this video
or in a future video,
but let's finish talking about the different parts
of a mitochondrion.
So inside the inner membrane you have
this area right over here is called the matrix.
It's called, let me use this in a different color,
this is
the matrix
and it's called the matrix 'cause it actually has
a much higher protein concentration,
it's actually more viscus than the cytosol
that would be outside of the
mitochondria.
So this right over here is the matrix.
When we we talk about cellular respiration,
cellular respiration has many phases in it.
We talk about glycolysis.
Glycolysis is actually occurring in the cytosol.
So glycolysis can occur in the cytosol.
Glycolysis.
But the other major phases of cellular respiration.
Remember we talk about the citric acid cycle
also known as the Krebs cycle,
that is occuring in the matrix.
So Krebs cycle
is occuring in the matrix
and then I said the electron transport chain
which is really what's responsible for producing
the bulk of the ATP, that is happening through proteins
that are straddling the inner membrane
or straddling the cristae right over here.
Now we're just done.
Probably one of the most fascinating parts of mitochondria,
we said that we think that they are descendent
from these ancient independent lifeforms
and in order to be an ancient independent life form,
they'd would have to have some information,
some way to actually transmit their genetic information
and, it turns out, mitochondria actually have
their own genetic information.
They have mitochondrial DNA
and they often don't just even have one copy of it,
they have multiple copies of it
and they're in loops very similar to bacterial DNA.
In fact, they have a lot in common
with bacterial DNA and that's why we think
that the ancestor to mitochondria that live independently
was probably a form of bacteria or related to bacteria
in some way.
So this is, this right over there,
that is the loop of mitochondrial DNA.
So all the DNA that's inside of you, the bulk of it,
yes, it is in your nuclear DNA, but you still have
a little bit of DNA in your mitochondria
and what's interesting is your mitochondrial DNA,
your mitochondria, are inherited, essentially,
from your mother's side, because when a egg is fertilized,
a human egg has tons of mitochondria in it
and I'm obviously not drawing all of the things
in the human egg.
It obviously has a nucleus and all of that.
The sperm has some mitochondria in it,
you could imagine it needs to be able
to win that very competitive fight
to get to fertilize the egg,
but the current theory is all or most of that
gets digested or dissolved once it actually gets
into the egg.
And anyway, the egg itself has way more mitochondria,
so the DNA in your mitochondria is
from your mother or is essentially from your mother's side
and that's actually used, mitochondrial DNA,
when people talk about kind of an ancient Eve
or tracing back to having kind
of one common mother,
people are looking at the mitochondrial DNA,
so it is actually quite, quite fascinating.
Now I said a little bit earlier,
and you know, obviously, it has its own DNA
and then because it has its own DNA
it's able to synthesize some of its own RNA,
its own ribosomes, so it also has ribosomes here.
But it doesn't synthesize all of the proteins
that are sitting in mitochondria.
A lot of those are still synthesized by
or encoded for by your nuclear DNA
and are actually synthesized outside of the mitochondria
and then make their way into the mitochondria,
but mitochondria are these fascinating, fascinating things.
They're these little creatures living in symbiosis
in our cells and they're able to replicate themselves
and I don't know, I find all of this mind boggling.
But anyway.
I said that this was the textbook model
because it turns out, when you look
at a micrograph, a picture of mitochondria,
it seems to back up this textbook model
of these folds, these cristae just kind of folding in,
but when we've been able to have more sophisticated
visualizations it actually turns out
that it's not just these simple folds
that the inner membrane essentially hooks
into the matrix and it turns out it has
these little tunnels that connect the space
inside of the cristae to the intermembrane space.
So I like to think about this because it makes you realize,
you know, we look in textbooks and we take these things
like mitochondria for granted, like, "Oh yeah, of course.
"That's where ATP factories are,"
but it's still an area for visualization research
to fully understand exactly how they work
and even how they are structured
that this Baffle Model where you see these cristae
kind of just coming in and out of the different sides.
This is actually no longer the accepted model
for the actual visualization, the structure of mitochondria.
Something more like this, something more where
you have this cristae junction model
where you have, if I were to draw a cross section
where this is the,
I drew the outer membrane and the inner membrane,
I'll just draw has these little tunnels
to the actual space inside of the cristae.
This is actually now the more accepted visualization,
so I want you to appreciate
that when in Biology, you read something in a textbook
you kind of say, "Oh, people have figured all
"of this stuff out," but people are still think about,
"Well, how does this structure work?
"What is the actual structure?" and then,
"How does it actually let this organelle,
"this fascinating organelle do all of the things
"that it needs to do?"