Question

What are the four complexes present in the ETC?

Answer 1

The electron transport chain (ETC) is the last step in aerobic respiration and the only phase of glucose metabolism that requires oxygen from the atmosphere. Electron transport is a relay race-like series of redox reactions. Electrons are swiftly transferred from one component to the next until they reach the chain's terminus, where they break down molecular oxygen and produce water.

Complete answer:
We will solve this question by first defining a complex in the ETC.
A complex is a structure made up of a core atom, molecule, or protein that is loosely linked to the atoms, molecules, or proteins that surround it. The electron transport chain is made up of four of these complexes (designated I through IV) and the mobile electron carriers that go with them.
1. Complex-I
To begin, two electrons are transported by NADH to the first complex. Complex I is made up of flavin mononucleotide (FMN) and an iron-sulfur enzyme (Fe-S). FMN is one of numerous prosthetic groups or co-factors in the electron transport chain, and it is produced from vitamin ${B_2}$ (also known as riboflavin). NADH dehydrogenase, a big protein with $45$ amino acid chains, is the enzyme in complex I. Complex I can pump four hydrogen ions from the matrix into the intermembrane gap across the membrane.
2. Complex-II
$\,FAD{H_2}$ is delivered straight to complex II, bypassing complex I. Ubiquinone is the molecule that connects the first and second complexes to the third (Q). The Q molecule is lipid soluble and travels easily across the membrane's hydrophobic core. Ubiquinone transfers its electrons to the next complex in the electron transport chain after being reduced to $Q{H_2}$ . Q gets electrons from complex I generated from NADH and electrons from complex II obtained from$\,FAD{H_2}$ , including succinate dehydrogenase. This enzyme and $\,FAD{H_2}$ create a tiny complex that bypasses the first complex and delivers electrons straight to the electron transport chain.
3. Complex –III
The third complex, also known as cytochrome oxidoreductase, is made up of cytochrome b, another Fe-S protein, Rieske centre ($2Fe - 2S$ centre), and cytochrome c proteins. A prosthetic heme group is found in cytochrome proteins. Complex III moves protons through the membrane and transfers electrons to cytochrome c for transmission to the fourth protein and enzyme complex. Cytochrome c accepts electrons from Q; however, unlike Q, cytochrome c can only accept one electron at a time.
4. Complex-IV
The cytochrome proteins c, a, and ${a_3}$ make up the fourth complex. Two heme groups (one in each of the cytochromes a and ${a_3}$) and three copper ions make up this complex (a pair of $C{u_A}$ and one $C{u_B}$ in cytochrome ${a_3}$). Until the oxygen is entirely reduced, the cytochromes hold one oxygen molecule extremely firmly between the iron and copper ions.

Note:
Any electrons taken from one molecule are always transferred to another in biological processes, thus anytime one molecule is oxidised, another is reduced. The propensity of such oxidation-reduction processes, or redox reactions, to occur spontaneously, like any other chemical reaction, is determined by the free-energy change ($\vartriangle G$ ) for electron transfer, which is determined by the respective affinities of the two molecules for electrons.