Question

Cytochrome c can be reduced directly by small molecules such as ascorbate (vitamin C). If ascorbate is added to the following preparations of mitochondria (which include succinate and oxygen), what would you predict would be the effect on oxygen consumption in each case, and why?

a. mitochondria that have been treated with oligomycin

b. mitochondria that have been treated with cyanide

c. mitochondria that have been treated with rotenone

d. mitochondria that have been treated with antimycin A

Answer 1

a. Oxygen consumption would stop because oligomycin inhibits ATP synthase hence ATP formation, which is coupled to the activity of the electron-transport chain.

b. Oxygen consumption would stop because Complex IV is inhibited and the entire chain backs up.

c. Oxygen consumption would fall at a lower rate because rotenone inhibits Complex I, but the presence of succinate will enable electrons transfer to Complex II.

d. Oxygen consumption would stop because antimycin A inhibits the transfer of electrons from complex iii to complex iv.