In mitochondria, it sees the first donated electron reduce Cytochrome C while the second passes through cytochromes b566 ("b 566") and b562 ("b 562").  
In photosystem II, this process involves electron transfer to and within the cytochrome b/f complex.  In mitochondria, it sees the first donated electron reduce Cytochrome C while the second passes through cytochromes b566 ("b 566") and b562 ("b 562").  Assimilation of matrix protons into the radical molecule replenishes the reducing agent, ubiquinol.  Each ubiquinol oxidation is accompanied by the release of four protons into the intermembrane space, producing the pH gradient for chemiosmotic ATP synthesis.  FTP, identify this proton-motive cycle sharing its single-letter designation with a key coenzyme.
In photosystem II, this process involves electron transfer to and within the cytochrome b/f complex.  
Q cycle