Understanding the Electron Transport Chain

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

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

one through four.

Complexes one, three, and four directly pump

protons from the matrix into the intermembrane space.

Complex two does not directly pump protons, but it does promote

proton pumping in complexes three and four.

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 one.

A by-product of sugar metabolism called NADH

deposits two high-energy electrons in complex one,

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 one harnesses this energy across all the redox centers

and uses it to pump protons.

The last redox center in complex one donates two electrons to a

coenzyme Q molecule.

Complex two is similar to complex one in two important ways.

First, high-energy electrons also enter complex

two

via a by-product of sugar metabolism, although here the molecule

is FADH2.

Second, complex two also transfers electrons

between

several redox centers before donating them to coenzyme Q.

One major difference, however, is that complex two does not use the energy

liberated to pump protons.

Coenzyme Q molecules from complexes one and two

donate their electrons to complex three.

One electron is recyclable and can re-enter complex three later,

but the other passes through two redox centers before reaching

cytochrome C.

Cytochrome C carries the electron to complex four.

The electron transport chain ends in complex four, 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.