Electron Transport Chain.

[solo-king]

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For a mutation that caused pores to open in the inner membrane of the mitochondria, which of the following effects would be seen in the organism?
A
Decreased production of ATP
B
Reduced ability to perform the Krebs cycle
C
Reduced affinity of pyruvate for Coenzyme A
D
Decreased efficiency of the electron transport chain

The answer is A.

I was able to eliminate B and C immediately but choose D over A . I can understand why A is correct but i don't se how D is wrong either. Anyone?

 

[StarFall]

D refers to the ETC specifically. The pores of the membrane do not affect any part of the ETC. It affects the H+ gradient. The ETC can still be efficiently pouring out H's, but with the pores you'll have no gradient. A is the better answer.
@solo-king

 

[EParker37]

@StarFall is spot on, the ETC only refers to the movement of electrons through the various proteins from NADH and FADH2 eventually ending up at Oxygen. It will be pumping tons of H out of the matrix into the inner membrane, which will then proceed to re-enter the matrix and bypass ATP-synthase.

 

[Altius Premier Tutor]

A great way to understand this conceptually is to consider the "uncoupling" of oxidative phosphorylation from ATP production. Your "holes in the membrane" scenario is an example of uncoupling. Many chemicals/drugs are called simply "uncouplers"--somebody chose that name because their impact is literally an un-linking of two things that themselves keep on running, but just aren't connected anymore. It's like driving down the freeway and losing the connection between your engine and your drive train. The engine keeps running (Kreb's, ETC), but because it isn't "coupled" to the drive train/wheels (ATP synthase) anymore, you lose your forward force.

Thinking of it that way helps on problems like this and many others. Functions in answers B, C and D aren't decreased because you only "uncoupled" from the engine, you didn't shut-off or destroy the engine. The MCAT could ask a ton of questions about processes up or down the chain that you could now answer knowing it is "uncoupling" not "destroying."

For that very reason, many students mistakenly think that ATP synthase is suddenly FROZEN by such drugs. If there are a few protons in the vicinity they can still move down their gradient and for a time you'll get a little ATP (think of your car still coasting forward for a while after your engine is disconnected)--you just aren't connected to that oxidative phosphorylation engine anymore, so pretty quickly the proton gradient will be gone. That is why Answer A in your question was "decreased" ATP production and not something like "no ATP production." Lastly, remember thermogenin from brown fat. The body uses it for generating heat by purposeful decoupling....that heat energy from protons passing directly into the matrix wouldn't be possible if that oxidative phosph/ETC engine was shut down. It is the continued churning of the engine that drives the ETC/gradient and provides the energy, going in this case to heat instead of ATP.