Question

Which of the following sequential reactions correctly represent(s) the flow of electrons from NADH to O₂ in plant mitochondrial electron transport chain?

• A. NADH dehydrogenase → Ubiquinone → Succinate dehydrogenase → Cytochrome bc1 → Cytochrome c → Cytochrome c oxidase
• B. NADH dehydrogenase → Succinate dehydrogenase → Ubiquinone → Cytochrome c → Cytochrome bc1 → Cytochrome c oxidase
• C. NADH dehydrogenase → Ubiquinone → Alternative oxidase → Cytochrome c oxidase
• D. NADH dehydrogenase → Alternative oxidase → Ubiquinone

Hint:

In plant mitochondria, the alternative oxidase pathway is an important mechanism that allows the plant to bypass the classical electron transport chain under specific conditions, helping to manage excess energy or stress.

Answer 1

The Correct Option is A, C

In plant mitochondria, the electron transport chain involves several key complexes. The correct electron flow is crucial for understanding the mitochondrial respiration process. Let’s analyze the options: 
- (A): NADH dehydrogenase (Complex I) transfers electrons to Ubiquinone, which then passes them to Succinate dehydrogenase (Complex II). From there, the electrons move to Cytochrome bc1 (Complex III), then to Cytochrome c, and finally to Cytochrome c oxidase (Complex IV), where oxygen is reduced to water. This pathway is the standard electron transport chain and is correct. Hence, option (A) is correct. 
- (B): This option is incorrect because the correct order involves NADH dehydrogenase (Complex I) transferring electrons directly to Ubiquinone, not to Succinate dehydrogenase. Hence, option (B) is incorrect. 
- (C): This pathway is another variation of the electron transport chain where electrons flow from NADH dehydrogenase to Ubiquinone, but instead of the typical flow through Cytochrome bc1, electrons flow through the Alternative oxidase, which bypasses the normal complexes. This pathway is especially prominent under certain conditions, such as when plants experience stress or require an alternative electron flow. Hence, option (C) is correct. 
- (D): This sequence is incorrect because Ubiquinone does not follow Alternative oxidase in the typical electron transport chain. Hence, option (D) is incorrect. 
Thus, the correct answers are (A) and (C).