Mitochondria

- 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?"