As stated earlier, the most common acceptor in respiration reactions is NAD. Thus, stet is transferred to Co-Q via-FMN (Flavin mononucleotide).

At one step in Kreb’s cycle, hydrogen is accepted by FAD. From FAD, it is transferred to Co-Q.

At Co-Q, the hydrogen undergoes ionization, i.e. it splits into electrons and protons.

      2H ® 2e^- + 2H⁺

Therefore, from Co-Q onwards, only the electrons are transferred through the ETS along the chain of the electron carrying molecules. The protons (H+) are transferred across the inner membrane into the intermembrane space.

The first electron acceptor in the chain is cytochrome. b. It accepts the electrons from Co-Q and transfers them first to cytochrome C1, and then to cytochrome C1. From cytochrom C1, electrons are accepted by cytochrome oxidase. In the final step, cytochrome oxidase donates electrons to the free molecular oxygen. In other words, oxygen acts as the last electron acceptor in the respiratory chain (aerobic respiration).

Oxygen receives 2 electrons through the ETS and 2 protons directly from the aqueous medium to form one molecule of water.

  1/2O₂ + 2 e- + 2H+ ®H₂O (Terminal oxidation)

Utilization of oxygen at the end of the ETS is the final step in aerobic respiration and is called terminal oxidation.

There is a progressive decrease in the level of electron energy as they flow along the Chain. This is given out as free energy. There are three sites in the ETS at which enough free energy is released. At each site, this energy is utilized to form ATP from ADP and iP (i.e. inorganic phosphate = H₃PO₄).

Formation of ATP using the energy released during terminal oxidation is called oxidative phosphorylation.

As can be seen in Figure 5.4 for the respiratory chain, each molecule of reduced co-enzyme NADH ₂ produces 3#ATP through the ETS, while each FADH2 yields 2ATP through the ETS.