RL: The work done by a cell is often not visible to the human eye.
So to make it more meaningful to us, we can consider the analogous work
we might do as humans.
So consider a boulder lying at the bottom of a hill.
We can imagine having to exert a lot of energy
to push that boulder to the top of the hill.
If we then release the boulder at any point
before getting it all the way to the top,
it will roll back downhill without needing any help from us.
Processes like a boulder rolling down a hill,
that happen without having to add any additional effort or energy,
are called spontaneous.
Processes like pushing a boulder up a hill,
that requires the input of energy, are called non-spontaneous.
Many cellular processes are spontaneous.
For example, in the last lesson, we described
how phospholipids self-assemble into bilayers
when dissolved in an aqueous solution such as water or the cytosol.
We can now further define this self-assembly
as being a spontaneous process.
On the other hand, when a cell has to perform a non-spontaneous process,
such as making a protein, it typically does so
by releasing the energy stored in a molecule called ATP.
Since ATP is used to power so many cellular functions,
it is vitally important for a cell to be able to produce large amounts of ATP.
With this in mind, let's examine one organelle in particular,
the mitochondrion, one of my favorites.
As we mentioned briefly in the last lesson,
the mitochondrion is the functional module
that allows cells to maximize the amount of ATP that
is generated from the energy contained in the food we and our cells consume.
Compartmentalization is crucial.
Here, we're looking at a three-dimensional transmission
EM of a cell, contrary to the 2D representation that we usually see.
The compartmentalization is clearer and more evident.
The role of the mitochondrion is quite specifically to synthesize ATP.
Each mitochondrion is surrounded by a double membrane structure made up
of separate outer and inner membranes.
The outer mitochondrial membrane contains porin channels.
These are channels that allow ions on most small molecules
to diffuse into the mitochondrion.
Macromolecules, such as proteins and RNA molecules,
are way too large to diffuse through the outer membrane.
The outer membrane is also extremely rich in enzymes associated
with mitochondrial lipid synthesis.
This is critical for the division of mitochondria-- indeed,
their reproduction-- because to create more mitochondria,
you need more membrane.
In contrast, the inner mitochondrial membrane
is highly folded into invaginations called cristae.
And it surrounds the innermost compartment
of the mitochondrion, called the matrix.
Cardiolipin is an abundant lipid in the mitochondrial inner membrane.
However, it is not present in the mitochondrial outer membrane,
which is primarily composed of regular phospholipids.
This helps to underscore just how different those two membranes are.
Unlike the outer membrane, the inner membrane
is highly impermeable to most small ions.
It is composed of three major classes of proteins that make up 76% of its mass.
The first class of proteins are those enzymes
critical for catalyzing a subset of reactions necessary for metabolism.
The second class are transport proteins that
help move metabolites, like pyruvates, from the intermembrane space
into the matrix of the mitochondria.
And the third class are a highly specialized set
of enzyme complexes called the F1F0ATPase that
directly synthesizes ATP.
Given the relative difficulty of moving molecules
across the inner mitochondrial membrane and into the matrix,
specific transporter proteins are required for moving small molecules
and ions across this membrane.
The mitochondrion's double membranes establish
two discrete and distinctive areas within the organelle.
We saw that the matrix is the volume enclosed by the inner membrane,
making it the innermost space within the mitochondrion,
while the volume between the two membranes
is called the intermembrane space.
The matrix contains hundreds of enzymes involved
in what's called the citric acid cycle.
And it also contains the mitochondrial DNA genome.
It is a far more viscous environment compared to the cytosol,
because it has a higher concentration of soluble proteins.
It plays a critical role in ATP synthesis,
and in the process of so-called oxidative phosphorylation.
The intermembrane space has a similar consistency as the cytosol.
However, the protein composition is very different due to size selectivity
of the outer mitochondrial membrane.
There are also proteins that are specifically
localized in the intermembrane space.
Cytochrome c is one such example of a protein concentrated in this region.