RL: All eukaryotic cells, from yeast to those that make up the human body,
contain membrane-bound organelles with specialized functions.
Mitochondria are double-membraned organelles
that harness most of the energy that cells need to grow and reproduce.
Nearly all of this energy comes from reactions that take place
at the inner mitochondrial membrane.
One of the key roles of this membrane is to act
as a barrier to positively charged particles
called protons, thus allowing a concentration
gradient to be maintained where the intermembrane space has
far more protons than the matrix.
The membrane also contains a large protein complex called the F1F0 ATP
synthase, which uses the proton gradient to drive
the synthesis of ATP molecules.
These ATP molecules ultimately provide the energy
for most of the cell's reactions.
Just as man-made power plants produce electrical energy
by using the flow of wind, water, or steam to rotate a turbine,
the synthase makes ATP by using proton flow
from one side of the inner membrane to the other to rotate protein subunits.
If there is no proton gradient, synthase subunits stop rotating,
and the cell can quickly become starved of the energy and die.
Therefore, the protein complexes and small molecules
that establish this gradient and maintain it
play an essential role in the life of the cell.
At the heart of this system are four protein complexes
numbered I through IV.
Complexes I, III, and IV directly pump protons
from the matrix into the intermembrane space.
Complex II does not directly pump protons,
but it does promote proton pumping in complexes III and IV.
Proton pumping requires energy, and the four protein complexes
get this energy by transferring electrons
through a series of coupled reactions.
This linked process of electron transport
is why the four complexes are collectively referred
to as the electron transport chain.
Let's focus on complex I. A byproduct of sugar metabolism
called NADH deposits two high-energy electrons
in complex I, where they are passed along a chain of redox centers.
Redox centers are clusters of atoms that have
different affinities for electrons based on their unique atomic configurations.
Let's closely consider a pair of redox centers
to reveal two reasons why an electron moves from the top redox center
to the bottom.
First, the bottom redox center has higher affinity than the top one.
Second, the distance between these adjacent redox centers
is ideal for an electron jump to occur, which explains why electrons typically
don't bypass the bottom redox center.
A small amount of energy is released each time
an electron is passed between redox centers.
Complex I harnesses this energy across all the redox centers
and uses it to pump protons.
The last redox center in complex I donates two electrons
to a coenzyme Q molecule.
Complex II is similar to complex I in two important ways.
First, high-energy electrons also enter complex II
via a byproduct of sugar metabolism, although here the molecule is FADH2.
Second, complex II also transfers electrons between several redox centers
before donating them to coenzyme Q. One major difference, however,
is that complex II does not use the energy liberated to pump protons.
Coenzyme Q molecules from complexes I and II
donate their electrons to complex III.
One electron is a recyclable and can re-enter
complex III later, but the other passes through two
redox centers before reaching cytochrome c.
Cytochrome c carries the electron to complex IV.
The electron transport chain ends in complex IV,
where a series of reactions involving four electrons
converts a molecule of oxygen to two molecules of water.
The proton gradient is strengthened because four protons from the matrix
are incorporated into water molecules, and another four
are pumped into the intermembrane space.
In the absence of oxygen, the electron transfer comes to a halt,
meaning that ATP synthesis also stops.
Indeed, the reason we breathe oxygen is so
that it can serve as the final electron acceptor
at the end of the electron transport chain.
In this animation, we have explored each protein complex in isolation,
but in reality, they are very densely packed.
Together, they effectively make the entire surface
of the inner mitochondrial membrane a giant cellular power plant.