Today I will dedicate a post to some general considerations about cellular respiration. This process occurs in the mitochondria More precisely, in the mitochondrial inner membrane. To put it in a simple way, it is an oxidation-reduction process that involves the transport of electrons, from NADH and FADH2 to oxygen. In fact, it is mainly because of this process that we need to breathe oxygen. During the process, the O2 molecules are reduced to H2O, which is the main reason for us to breathe out water vapor. Thus, in fact most of our pulmonary respiration is not more than a consequence of our cellular respiration!
But back to the cellular respiration process... The electrons are received and transported over four complexes, designated I, II, III and IV. These complexes are not more than sets of electron transport proteins, many of which with cofactors specialized in the transport of electrons, such as iron-sulfur centers, heme and flavoproteins. A key element in this process is the presence of transition metals (iron and copper, for example) because, due to the fact that they can oscillate between two oxidation states, they are able to temporarily capture or donate electrons.
For electrons to pass from complex I or II to complex III, there is a lipophilic molecule that will carry them, which is called ubiquinone. For the electrons to move from complex III to IV there is an intermembrane space protein that carries them, designated cytochrome c.
As the electrons move along the complexes, in some of them ther will be a coupled process, which is the proton (H +) pumping from the matrix to the intermembranar space. That means, it will be created a H+ gradient, designated electromotive force , which will accumulate enough energy to drive ATP synthesis via a process called oxidative phosphorylation. The intervening in this process is the mitochondrial ATP synthase.
In future posts I will devote some attention to a detailled explanation of how cellular respiration works...