schlieren's Bio Q.1: fermentation after TCA inhibition?

[schlieren]

Examkrackers 1001 Biology, Q. 309:
"Fluroacetate, a potent toxin extracted from plants, is converted to fluorocitrate, which is a strong inhibitor of the TCA cycle. Which of the following would be expected in a person exposed to fluoroacetate?
A. an increase in intracellular levels of ATP
B. an increase in intracellular levels of glucose
C. a decrease in levels of ethanol
D. a decrease in the function of the electron transport chain."
Answer is D, and the explanation says "fluoroacetate inhibits the TCA cycle and indirectly the ETC. Thus, pyruvate has no choice but to enter fermentation and produce lactic acid (in animal cells)."

However, Princeton Hyperlearning Workbook, Biology Passage 2, Q. 3:
"If, in an aerobic organism, the Krebs cycle were suddenly arrested while glycolysis proceeded, the cell would most likely experience an increase in:
A. glucose consumption
B. number of cytochromes
C. quantity of lactic acid
D. quantity of ATP"
Answer is A, and the explanation says "In the absence of the Krebs cycle ... ETC should still be functional; there is no reason to assume conditions have become anaerobic ... To compensate for the reduced ATP production, the cell would have to consume more glucose ... Neither would there be an increase in lactic acid; remeber, conditions are not anaerobic."

Since human is aerobic, EK is saying fermentation after TCA inhibition, but TPRH is saying no fermentation. TPRH would give answer B to EK's question, while EK would give answer C to TPRH's question. Who is correct?

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[RogueUnicorn]

these two aren't directly contradictory.. EK says the ETC function is reduced, and TPR is saying it is merely functional with no comment on the activity level. Due to the lower # of substrate for the ETC with TCA cycle inhibition, though, the activity would be lower. That said, neither of these are particularly fantastic Qs..

 

[docelh]

I would caution you against comparing a passage-based question from one source versus a standard multiple choice question in another.

There's probably something in the passage (that we can't see) that is hinting strongly towards a specific answer. In your TPR question for example, the interruption of the Krebs would produce pyruvate which could lead to lactic acid, if not in the cell. Furthermore, there is still a net increase of very small ATP just from glycolysis alone. So there's something in the passage that's probably relevant to the answer.

 

[StIGMA]

TPRH would answer the same thing to EK's question. In the TPRH answer it even says it would increase glucose consumption - ie: decrease the steady-state level of glucose. Ie: there would NOT be an increase in glucose concentration (only consumption).

The answers to each question are the correct answers. You still have oxygen, so NADH will be utilized from glycolysis. However, the TCA cycle is backed up, so the excess pyruvate will also be shunted to lactic acid fermentation. The cell responds by increasing the rate of glycolysis (because ATP is reduced and ADP/AMP increase... stimulating glycolytic enzymes etc.) and even more lactic acid is produced. So glucose consumption is increased AND lactic acid fermentation is increased.

 

[schlieren]

Many thanks to all of you replied! I agree with you that both questions' answers are correct. However, I still have a question over the explanation that fermentation should happen, and need more of your inputs.

As long as there is oxygen, functional ETC can give 2.5 ATP per each NADH->NAD, while fermentation gives no ATP and just waste lactic acid. So, if ETC is viable, fermentation seems to be a waste of energy and NADH. However, if the purpose of fermentation is to clear up pyruvate (such that somehow pyruvate can not be exported out of cell as waste), then there is no NADH if pyruvate is cleared up by fermentation, since for each glucose, fermentation oxidizes all the 2 NADHs that glycolysis produces, then there is no extra NADH to feed to ETC. Any NADH consumed by ETC would mean deficiency of NADH for fermentation, and consequently excess pyruvate. So it still seems to me that fermentation and ETC can not be both functional when Krebs is inhibited?

 

[StIGMA]

Think of how the cell will react. The ETC will strongly suck up the available NADH much more than LDH will (LDH reduces pyruvate to lactate). Once the level of pyruvate rises, their is more substrate for LDH (pyruvate) and available NADH will see LDH and pyruvate more frequently. Eventually there will be a shift from ETC using NADH to increased cellular pyruvate causing increased LDH activity (a shift toward lactate fermentation). Both will be occurring simultaneously at all times, albeit at different rates.