The carriers of the mitochondrial electron transport chain act to transfer the electrons derived from oxidation of substrates down an electrochemical gradient until finally oxygen is reduced to water. At three stages in the re-oxidation of NADH there is phosphorylation of ADP to yield ATP; in the re-oxidation of reduced flavins there are only two steps at which ADP is phosphorylated to ATP.
A more detailed view of the arrangement of the carriers in the electron transport chain is shown below - each carrier is reduced by the one above it, and in turn is reoxidised by reducing the carrier below, until finally oxygen is reduced to water. This diagram shows the three complexes of carriers involved in the re-oxidation of NADH; each complex is associated with phosphorylation of ADP to ATP. Complex II is not shown here - it is the reaction of reduced flavins with ubiquinone, which is not associated with phosphorylation of ADP to ATP.
