Electron Transport Chain: The Basics
The electron transport chain (ETC) is a step in cellular respiration that occurs after the Krebs/citric acid cycle. The ETC also occurs in photosynthesis in the chloroplasts of phototrophic (light dependent) cells.
Where it Takes Place:
The ETC takes place in the mithochondria of eukaryotic cells, more specifically the inner mithochondrial membrane. In prokaryotic cells, electron transport occurs in the plasma membrane.
Hint: It is good to know that a hydrogen atom can be thought of as either a proton (H+) or an electron (H-), since it contains one of each.
If any of these terms seem unfamiliar, check the "Terms to Know" list at the bottom of the page.
Electron Transport in the mitochondria occurs in four steps at four different sites embedded in the inner membrane, protein complexes I-IV.
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 occurring, 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 transferred to ubiquinone (remember from the beginning?), 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 transferred 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 receives 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.
Electron Transport Chain
The overall point of the electron transport chain is not the production of two molecules of water. Although water is certainly essential in many biological functions, the profit is the proton gradient. As the electrons move throught the inner membrane through each of the four protein complexes, free energy is released. This energy powers the pumping of protons across the inner membrane into the intermembrane space, generating the gradient. This proton gradient can then be used in another process, oxidative phosphorylation, to generate ATP.
Terms to Know:
ATP: a molecule consisting of a 5-carbon pentose sugar, an adenine molecule, and three phosphate groups hydrolized to produce energy. Note that ATP consists of one more phosphate group than ADP
Electron: a basic particle of an atom (subatomic) consisting of a positive electrical charge
Enzyme: biological molecules that catalyze, or increase the speed of, biological reactions. Note that enzymes will not cause a reaction to take place if it wouldn't normally, it only causes the reaction to go faster.
Inner membrane: The mitochondria has two cell membranes, this is the membrane that surround the matrix but is surrounded by the outer membrane.
Intermembrane Space: the thick, viscous liquid between the inner and outer membranes of the mitochondria; basically the cytosol of the mitochondria.
Mitochondria: An energy producing organelle within eukaryotic cells and the site of the ETC; contains two cell membranes.
Matrix: the thick, viscous liquid surrounded by the inner membrane of the mitochondria; basically the cytosol of the mitochondria.
NADH: A reducing agent used in redox reactions to donate electrons to other molecules.
Outer membrane: The mitochondria has two cell membranes, this is the membrane that surrounds the entire cell.
Oxidation: the loss of an electron or gain of a proton/hydrogen atom by a molecule.
Protein Complex: A site of electron transport embedded in the mitochondrial inner membrane
Proton: a basic particle of an atom (subatomic) consisting of a positive electrical charge.
Proton Gradient: a source of energy resulting from a higher concentration of protons in the intermembrane space of a mitochondrial inner membrane that in the mitochondrial matrix (more protons outside than in).
Redox Reaction: a reaction in which one reactant is oxidized and one is reduced.
Reduction: the gain of an electron or loss of a proton/hydrogen atom by a molecule.