Respiration (both aerobic and anaerobic) begins with glycolysis in the cytoplasm of a cell;
1 glucose molecule (C₆H₁₂O₆) is converted into 2 molecules of pyruvate in a 10-step process, consuming 2 ATPs and yielding 4 ATPs;
Additionally, 2 NAD+ molecules are reduced to NADH molecules;
In aerobic respiration, each pyruvate molecule then goes through the link reaction, getting converted to acetyl co-A in the mitochondrial matrix, producing 1 CO₂ molecule and 1 reduced NADH molecule;
Acetyl co-A then enter the Krebs or tricarboxylic acid cycle (TCA) where 2 CO₂, 1 ATP, 3 NADH (by reduction of NAD+) and 1 FADH₂ molecules are produced;
The NADH and FADH₂ molecules produced are electron carriers required for the electron transport chain (ETC);
The electron transport chain consists of 4 proteins within the inner mitochondrial membrane which are involved in the production of ATP by oxidative phosphorylation;
The first protein (C1) is responsible for oxidising NADH to NAD+, releasing energy used to generate the proton motive force (PMF) by pumping the protons (H+ ions) into the intermembrane space and the electrons released are transferred to a ubiquinone molecule;
The second protein (C2) is responsible for oxidising FADH₂ to FADH, also releasing energy used to generate the proton motive force (PMF) and, similarly to C1, the electrons released are transferred to ubiquinone;
The third protein complex (C3) transfers the electrons from ubiquinone to cytochrome C;
Finally, cytochrome C transfers these electrons to the fourth complex (C4), where they combine with free protons in the matrix and O₂ to to produce H₂0;
The PMF is then harvested via an ATP synthase protein where H+ flood back into the mitochondrial matrix, and the energy released is used to generate ATP from ADP and Pi;
ATP is now ready for use for bodily processes requiring it.
