Electron transport chain or ETC is a series of complexes which helps to transfer electrons from the donors to the acceptors. Reduction and oxidation (in a simultaneous manner) reactions and proton transfer across the membrane are the procedures by which the electron transfer becomes possible. ETC is a complex of membrane-embedded protein (peptides, enzymes) and different kinds of organic molecules. An electrochemical proton gradient is created by the electron transport chain. This electrochemical proton gradient triggers the ATP synthesis. This electron transport chain generally is composed of four complexes. These complexes are designated as complex I, complex II, complex III, and complex IV. In case of eukaryotes, these ETC complexes are found in the inner mitochondrial membrane. On another side, plasma membrane is the place at which ETC components are found in case of the prokaryotes.
The movement of the electrons through the electron transport chain takes place from a higher energy level to a lower energy level, or, we can say that, the movements take place from the molecules having less electron affinity to the molecules having higher electron affinity. During these downhill electron transfers, energy is released. This released energy is then used by several protein complexes to pump protons. The protons are pumped from the mitochondrial matrix to the intermembrane space. This whole process leads to the formation of the proton gradient.
The sources of all electrons (that enter into the ETC) are NADH and FADH2 molecules (which come from different stages of the cellular respiration processes: glycolysis, pyruvate oxidation and the citric acid cycle).
Complex I is NADH:ubiquinone oxidoreductase. It is quite large in size. NADH is a very good electron donating molecule in redox reactions. The electrons of the NADH are present at a high energy level. Because of this reason, two electrons are removed from the NADH and directly transferred to complex I. Flavoprotein (a protein with attached flavin mononucleotide, FMN, a prosthetic group) is the molecule which first receives the electrons in the complex I. After receiving the electrons, those electrons are passed to another protein (Fe-S proteins) inside the complex I by the FMN. After that, the electrons are again transferred to a small mobile carrier ubiquinone. Energy is released during the movement of the electrons. And this energy is used by the complex to pump the protons from the matrix to the intermembrane space.
Complex II is succinate dehydrogenase. The additional electrons in complex II are delivered into the quinone pool (Q) which are originated from the succinate and transferred to Q, that is ubiquinone via FAD. There are four protein subunits present in Complex II. These are as follows: succinate dehydrogenase; succinate dehydrogenase [ubiquinone] iron-sulfur subunit, mitochondrial; succinate dehydrogenase complex subunit C and succinate dehydrogenase complex subunit D. Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD). Like NADH, FADH2 is not good enough for donating electrons. So, we can say that, the electrons of the FADH2 present at a lower energy level. Because of this reason, the electrons cannot be transferred to complex I. Instead of that, those are transferred to complex II.
Movement of the electrons from the NADH and FADH2 takes place exactly via the same route. Ubiquinone (Q) is the molecule at which the electrons are passed by both of the complex I and II. After accepting the electrons, Q is then reduced to QH2. After that, travelling through the membrane, the electrons are delivered to complex III.
(For Part-II check our future blog posts on Biology)