Poison in cellular respiration.

[alexwaylo2008]

When a poison such as cyanide blocks the electron transport chain during cellular respiration, glycolysis and the krebs cycle soon grind to halt as well. Which of the following is the best explanation for this?

a) A high level of NADH is present in the cell.
b) the uptake of oxygen stops because electrobn transport was inhibited.
c) NAD+ and FAD are not available for glycolysis and the krebs cycle does not continue.
d) electrons are no longer available from the electron transport chain to power glycolysis and the Krebs cycle.
e) they run out of ATP

I think it would be C, but I'm not very sure. Could someone confirm me my answer please?

 

[Kalirren]

Both a) and c) are correct, though I think c) is more likely to be the right answer. If the electron transport chain is blocked, then the electron carriers NADH and FADH2 cannot unload their electrons into it, which means NAD+ and FAD are not regenerated. Since glycolysis and the Krebs cycle both require NAD+ (and the Krebs cycle requires FAD), both of these processes stop running.

 

[t-money]

I think it is B. Cyanide is a compettive inhibitor to oxygen. It binds to Hemoglobin far more effeciently than Oxygen molecules do.

 

[Kalirren]

Statement b) is true too (though the toxicity of cyanide results from its binding to cytochrome oxidase in the electron transport chain as stated in the problem, not to haemoglobin), but it's not the most direct cause of stopped glycolysis/Krebs cycle. A cell that could eliminate its buildup of NADH/FADH2 could still run glycolysis even under oxygen starvation - this is what your muscle cells do during anaerobic exercise.

 

[t-money]

How does cyanide get to the cells so effectively then?

 

[Kalirren]

Depends on how it's introduced. If it's inhaled, then it enters the bloodstream via the lungs, proceeds directly to the heart, and kills quickly. Oral toxicity is more gradual, but also passes through the bloodstream. In either case, if too much cyanide gets into the heart or CNS tissue, it binds irreversibly to cytochrome oxidase and kills the cell.

 

[t-money]

Depends on how it's introduced. If it's inhaled, then it enters the bloodstream via the lungs, proceeds directly to the heart, and kills quickly. Oral toxicity is more gradual, but also passes through the bloodstream. In either case, if too much cyanide gets into the heart or CNS tissue, it binds irreversibly to cytochrome oxidase and kills the cell.

Let me rephrase my question. How does cyanide dissolve so well in the blood stream. I realize what it does metabolically, what I was trying to point out though is a characteristic of poisons.

 

[Spirochete]

Cyanide competes with oxygen for binding at respiratory complex 4, Cytochrome C Oxidase. NADH is still able to feed electrons into the chain, they just never reach their final destination. That should help answer your question.

 

[Spirochete]

How does cyanide get to the cells so effectively then?

Small, uncharged polar molecules such as water diffuse reasonably well through cell membranes. Cyanide (HCN) may be slightly larger than water but it is also less polar. Remember the Hydrogen is bonded to carbon which is a mostly non polar bond.