|The electron transport chain is composed of four protein cytochrome complexes which are embedded in the inner membrane of the mitochondrion along with two mobile carriers (mobile carrier Q and C) that shuttle electrons through the ETC. The ETC is where the greatest amount of ATP is synthesized.
NADH+H+ arrives from Stage II of carbohydrate metabolism or Stage III (TCA cycle) to the ETC and immediately oxidizes to NAD+ with its protons (hydrogen ions) going into the matrix and its electrons (e-) going to cytochrome complex 1. As the electrons arrive on cytochrome complex 1 the complex immediately goes through redox (reduction and oxidation). This reaction creates a proton pump within the cytochrome, pumping 4 protons from the matrix through the cytochrome into the intermembrane space. The electrons now transfer to mobile carrier Q and NAD+ returns to its original source.
FADH2 arrives from the TCA cycle to the ETC and goes directly to citochrome complex 2. FADH2 oxidizes to FAD with its protons going into the matrix and its electrons going to complex 2. FAD returns to the TCA to pick up more hydrogens. Mobile carrier Q picks up all of the electons on complex 2 and shuttles the electrons to cytochrome complex 3. The electrons are transferred to cytochrome complex 3 and it immediately goes through redox (reduction and oxidation). This creates a proton pump, pumping 4 protons from the matrix through cytochrome complex 3 directly into the intermembrane space of the mitochondrion. (NOTE: there are many mobile carrier Q enzymes present).
The electrons are now shuttled to cytochrome complex 4 by mobile carrier C. As the electrons transfer onto cytochrome complex 4 it immediately goes through redox. This creates a proton pump, pumping 2 protons from the matrix through cytochrome complex 4 directly into the intermembrane space of the mitochondrion.
The electrons have no place to go from cytochrome complex 4. So, an oxygen atom, which has a very strong attraction for electrons, picks up a couple electrons (from cytochrome complex 4) and two protons from the matrix to form water. In essence, the FINAL acceptor of the electrons at cytochrome complex 4 is oxygen (which forms water with the protons and electrons it accepts into its structure). This is actually 20% of the water our bodies use (the rest we must intake from our fluids and foods) and it is referred to as metabolic water. (because it is made in metabolism)
The many protons in the intermembrane space have created a hydrogen ion concentration (charge) that the cell does not like. All cells prefer homeostasis with their organelles. Therefore, ATP synthase (a special protein embedded in the inner membrane) pumps several protons back into the matrix. As the protons go into the matrix energy is liberated to phosphorylate ADP with a Pi, thus synthesizing ATP. The ATP will leave the mitochondrion and go where it is needed for energy in the cells.
Nicole Willburn, Exercise Physiology Student in Spring 2004
Class, CLICK HERE to see a fabulous graphic of the ETC.