Explain Your Reasoning.
Some of the mitochondrial electron transport carriers (the cytochromes) absorb light at characteristic wavelengths when they are carrying the extra electrons from NADH but not when they give up the electrons to the next carrier. Thus, the light absorption bands are all present when mitochondria are incubated in the absence of oxygen or when cyanide is present. Cyanide blocks the transfer of electrons from the last cytochrome to oxygen. When cyanide is absent and oxygen is introduced, the cytochromes stop absorbing light. There are three cytochromes in animal mitochondria, called a, b, and c. Another inhibitor of mitochondrial electron transfer is urethane. When urethane is added to aerobic mitochondria, cytochrome a begins to absorb light, but b an c do not. Finally, purified cytochrome c, even in the presence of oxygen, continues to absorb light. Use these observation to decide the order in which the cytochromes occur in the mitochondrial electron transport system. Explain your reasoning for your determined order. Why is it important that the cytochrome a be purified in the last observation