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
While cytochromes are real moieties and you will find an abundance of information about them in your text and via the web, it is critical that you realize that the data for solving this assignment is imbedded in the question. This question contains the data you need to answer the question. If you read the book or look at online resources and rely on the order provided there you could be mistaken. The data is in the question, you need to interpret the data, and make inferences based upon that data.
Based on the observations provided, we can determine the order in which the cytochromes occur in the mitochondrial electron transport system as follows:
1. Cytochrome a: When urethane is added to aerobic mitochondria, only cytochrome a begins to absorb light. This suggests that cytochrome a is the first cytochrome in the electron transport chain where electrons are transferred from NADH. Therefore, cytochrome a occurs before cytochromes b and c.
2. Cytochrome b: The question states that when cyanide is absent and oxygen is introduced, the cytochromes stop absorbing light. This indicates that the last cytochrome to give up its electrons to oxygen is the one that is blocked by cyanide. Since cytochrome c continues to absorb light in the presence of oxygen, it must occur after the cytochrome b. Hence, cytochrome b occurs before cytochrome c.
3. Cytochrome c: Purified cytochrome c, even in the presence of oxygen, continues to absorb light. This suggests that cytochrome c is the last cytochrome in the electron transport chain, where electrons are transferred to oxygen. Therefore, cytochrome c occurs last in the mitochondrial electron transport system.
It is important to purify cytochrome c in the last observation because if other cytochromes were present, they might interfere with the absorption of light by cytochrome c. By purifying cytochrome c, any potential contamination from other cytochromes is removed, allowing for a clear and accurate observation of its light absorption properties.
To determine the order in which the cytochromes occur in the mitochondrial electron transport system, let's analyze the observations provided:
1. In the absence of oxygen or in the presence of cyanide, all cytochromes (a, b, and c) absorb light at characteristic wavelengths when they carry extra electrons from NADH.
2. When oxygen is introduced and cyanide is absent, the cytochromes stop absorbing light.
3. When urethane is added to aerobic mitochondria, only cytochrome a absorbs light, while b and c do not.
4. Purified cytochrome c continues to absorb light even in the presence of oxygen.
Based on these observations, we can infer the following:
1. Cytochromes a, b, and c all carry extra electrons from NADH since they absorb light when carrying these electrons.
2. Cytochromes a, b, and c are involved in the transfer of electrons to the next carrier, as they do not absorb light when giving up electrons.
3. Cyanide blocks the transfer of electrons from the last cytochrome to oxygen, indicating that the last cytochrome in the electron transport system is the one affected by cyanide.
4. Urethane specifically affects cytochrome a, as it is the only cytochrome that absorbs light when urethane is present.
Based on these inferences, we can determine the order of cytochromes in the mitochondrial electron transport system:
1. Cytochrome c is the last cytochrome in the electron transport system since it continues to absorb light even in the presence of oxygen.
2. Cytochrome b is the second-to-last cytochrome, as it is not affected by urethane and does not absorb light when giving up electrons.
3. Cytochrome a is the first cytochrome, as it absorbs light when urethane is added and does not absorb light when giving up electrons.
Therefore, the determined order of cytochromes in the mitochondrial electron transport system is a, b, c.
It is important to note that cytochrome a needs to be purified in the last observation because it is the first cytochrome in the electron transport system. Purifying it ensures that any other cytochromes or contaminants present in the mitochondrial sample do not interfere with the observation of cytochrome a's light absorption. This allows for a more accurate determination of the order of cytochromes.