Overview of the mitochondrial respiratory chain and the steps that contribute to formation of ROS. Electrons (e) originate
from NADH and FADH2 that are predominantly generated via the Krebs (citric acid) cycle. The transfer of electrons through
the various complexes of the respiratory chain is coupled with the pumping of protons from the mitochondrial matrix to the
intermembrane space. The resulting proton gradient drives the ATP synthesis at complex V (ATP synthase). Part of the protons
may leak out through uncoupling proteins (UCP) without generating ATP. Normally, electrons transported through the respiratory
chain are transferred at or near complex IV to oxygen, yielding in the end H2O. Electrons present at coenzyme Q (Co Q) in the respiratory chain may be transferred to oxygen yielding ROS. A high proton
gradient may slow down the rate of electron transport through the chain, thereby enhancing ROS production.