Cell biology and proposed mechanisms
Electron transport chain
The electron transport chain is a series of four protein complexes embedded in the inner membrane of the mitochondrion. It passes electrons stripped from food molecules down a series of carriers to oxygen, and uses the energy released to pump protons across the membrane.
Also called ETC, respiratory chain, complexes I to IV
The sequence
Electrons enter from NADH at complex I or from FADH₂ at complex II, pass via coenzyme Q to complex III, then via cytochrome c to complex IV, and are finally handed to oxygen, which combines with protons to make water. Complexes I, III and IV pump protons out as they work, building the gradient that drives ATP synthase. Regulation and functional role of the electron transport chain supercomplexes, Biochemical Society Transactions (2021), gives a current overview, including the finding that the complexes assemble into larger structures rather than floating independently.
This is why we breathe. Oxygen’s role in respiration is to be the final electron acceptor at complex IV.
The link to red and near-infrared light
Complex IV is cytochrome c oxidase, and it is the molecule at the centre of the photobiomodulation hypothesis. It contains heme and copper centres that absorb red and near-infrared light. The proposal in the IEEE Journal of Selected Topics in Quantum Electronics (2016) review is that absorbed photons displace inhibitory nitric oxide from the enzyme, electron transport speeds up, and ATP output rises.
Why it matters here
Complex IV of this chain is the exact molecule red and near infrared light is proposed to act on, which makes it the physical basis of the whole photobiomodulation story on the Red Light model.