Complex I: In the inner mitochondrial membrane, Nicotinamide adenine dinucleotide

(NADH) produced by glycolysis is oxidized (removes electrons) by the enzyme NADH

dehydrogenase. The enzyme removes two electrons from NADH and attaches them to an

electron carrier, ubiquinone. The transfer of these electrons reduces (adds electrons)

ubiquinone into ubiquinol. While this redox reaction is occuring, four hydrogen atoms

(protons) are pumped across the inner membrane to the intermembrane space. This

creates a proton gradient, which basically means there is a higher concentration of

protons outside the inner membrane (in the intermembrane space) than inside the

membrane (in the mitochondrial matrix). NADH binds to Flavin mononucleotide, reducing

NADH to NAD+ and reducing Flavin mononucleotide to FMNH2. Notice that NADH is

losing its negative hydrogen atom, resulting in the positive charge of NAD+. The two

electrons and two hydrogens taken from NADH are carried by FMNH2 (which is now

called an "electron carrier") to two Iron (Fe) atoms in Iron-Sulfur (Fe-S) centers located

within the complex. The hydrogens then act as protons and are pumped back into the

mitochondrial matrix, not the intermembrane space. The electrons in the two irons are

accompanied by two protons and transfered to ubiquinone (remember from the

begining?), which is also called "coenzyme Q." Ubiquinone then passes the electrons to a

new Fe-S center, releasing the two protons into the matrix. A new ubiquinone is given the

electrons and rests within the inner membrane, again pushing two protons to the matrix.

Complex II: Two electrons from the citric acid cycle are transfered to complex II,

powering the oxidation of the enzyme succinate (also from the citric acid cycle) into

fumarate. Fumarate then passes the two electrons to coenzyme FAD, which moves the

electrons to an Fe-S complex and then to ubiquinone. Complex II does not produce a

proton gradient because there is not enough free energy to pump protons into the

intermembrane space.

Complex III: Complex III recieves two electrons from the reduced ubiquinone from

complex's I and II. The electrons are passed through an Fe-S complex to cytochrome C,

an electron carrier, pumping four protons into the intermembrane space, two from

ubiquinone and two from cytochrome C. This creates another proton gradient.

Complex IV: Cytochrome C, which operates in the intermembrane space, transports one

electron at a time to complex IV. These electrons provide the energy needed to reduce

molecular oxygen to two molecules of water. Complex IV creates a proton gradient.