Citric acid cycle/ capacitor questions

[km1865]

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Hi,

When doing the net totals for a single pyruvate molecule undergoing the citric acid cycle, 1ATP, 3NADH, 1FADH2 and 3CO2 are produced through a single turn. But this doesn't include the NADH produced from the conversion of pyruvate to acetate does it? Because the net CO2 seems to include the one CO2 molecule from the conversion of pyruvate to acetate (since 2 CO2 molecules are produced from the citric acid cycle itself, discounting the conversion of pyruvate to acetate). Following similar logic then, shouldn't the NADH from the conversion of pyruvate to acetate also be included in the net total, making it 4NADH then? Correct me if i'm wrong please.

Also, the reason why prokaryotes net 38ATP per molecule of glucose rather than the 36 in eukaryotes is because they dont have to expend energy shuttling NADH from glycolysis through the inner mitochondrial membrane, correct?

I was really confused about the reasoning behind why the charge decreases as a function of time for both the charging and the discharging of a capacitor? Ive been thinking that during the charging the current decreases because the voltage across the resistor decreases. That makes sense according to Ohm's law, V=IR. But when discharging a capacitor why is it that we shift from looking at current from a resistor perspective to a capacitor perpspective? Basically, why is the reasoning that as charge decreases from the capacitor, the voltage of a capacitor decrease causing the voltage across the resistor to increase resulting in an increase in the current across the resistor wrong?

Sorry about the length, but any help would be appreciated! Thanks

 

[Arctangent]

I can answer a bit of the citric acid/respiration question.

For a single pyruvate molecule, I believe I learned the same numbers of products as you, except 1 GTP instead of 1 ATP, though they can be seen as equivalents to some extent. It does not include the 1 NADH from the conversion of pyruvate to acetyl CoA, or the 1 NADH per pyruvate produced in glycolysis, so the net NADH production is 5 per pyruvate through all of cellular respiration.

As for the 2 ATP discrepancy, I've heard two different explanations. One, from my MCAT course instructor, was the same as yours; shuttling through the membranes of the mitochondria takes energy. The course textbook for TPR, however, states that actually the 2 NADH from glycolysis release electrons into the electron transport chain at the same protein as FADH2, so you get 1 less ATP than for other ATP.

 

[Axon hillock]

I can answer a bit of the citric acid/respiration question.

For a single pyruvate molecule, I believe I learned the same numbers of products as you, except 1 GTP instead of 1 ATP, though they can be seen as equivalents to some extent. It does not include the 1 NADH from the conversion of pyruvate to acetyl CoA, or the 1 NADH per pyruvate produced in glycolysis, so the net NADH production is 5 per pyruvate through all of cellular respiration.

As for the 2 ATP discrepancy, I've heard two different explanations. One, from my MCAT course instructor, was the same as yours; shuttling through the membranes of the mitochondria takes energy. The course textbook for TPR, however, states that actually the 2 NADH from glycolysis release electrons into the electron transport chain at the same protein as FADH2, so you get 1 less ATP than for other ATP.

This is actually the same reason. This is called the glycerophosphate shuttle and it transfers NADH from cystol to the mitochondrial matrix which yields only 1.5 molecules of ATP.

 

[Axon hillock]

Hi,


I was really confused about the reasoning behind why the charge decreases as a function of time for both the charging and the discharging of a capacitor? Ive been thinking that during the charging the current decreases because the voltage across the resistor decreases. That makes sense according to Ohm's law, V=IR. But when discharging a capacitor why is it that we shift from looking at current from a resistor perspective to a capacitor perpspective? Basically, why is the reasoning that as charge decreases from the capacitor, the voltage of a capacitor decrease causing the voltage across the resistor to increase resulting in an increase in the current across the resistor wrong?

Sorry about the length, but any help would be appreciated! Thanks

Are you sure the charge decreases as you charge a capacitor? I'm pretty sure the charge, Q, increases as you charge the capacitor because your storing the electrical charge.

 

[Arctangent]

This is actually the same reason. This is called the glycerophosphate shuttle and it transfers NADH from cystol to the mitochondrial matrix which yields only 1.5 molecules of ATP.

My impression was that the textbook's reason for the deficiency was that the NADH were basically equivalent to FADH2 with respect to where the electrons started in the electron transport chain. So I was imagining that if the -1ATP deficiency was due to where the electrons start in the ETC, then there wasn't any noticeable energy loss due to the transport through the membrane. I would think though, given that the inner membrane is fairly impermeable, that transport would likely take energy, so a transport based explanation for the loss of net ATP would make sense. Do you mean that the shuttle's energy loss due to crossing the membrane and the opportunity cost from reducing an electron carrier further down the chain sums up to 1ATP per NADH?

 

[Axon hillock]

My impression was that the textbook's reason for the deficiency was that the NADH were basically equivalent to FADH2 with respect to where the electrons started in the electron transport chain. So I was imagining that if the -1ATP deficiency was due to where the electrons start in the ETC, then there wasn't any noticeable energy loss due to the transport through the membrane. I would think though, given that the inner membrane is fairly impermeable, that transport would likely take energy, so a transport based explanation for the loss of net ATP would make sense. Do you mean that the shuttle's energy loss due to crossing the membrane and the opportunity cost from reducing an electron carrier further down the chain sums up to 1ATP per NADH?

That's actually not true but it's far outside the scope of the MCAT.

Anyway what I meant is that the so called "energy loss" in crossing the membrane is due to the glycerophosphate shuttle which is responsible for the shuttling of the cystolic NADH b/c they yield 1.5 molecules of ATP. In reality, there is another shuttle called the Malate Aspartate shuttle which gives 2.5 ATP per NADH. This is why the amount of ATP formed in Eurkaryotic cell is usually written 30-32 ATP depending on the shuttle taken. This is all outside the scope of the MCAT though so don't give it too much thought. Just know Prokaryotes generate more ATP. Hope its a little clearer, if not let me know.

 

[Arctangent]

That's actually not true but it's far outside the scope of the MCAT.

Anyway what I meant is that the so called "energy loss" in crossing the membrane is due to the glycerophosphate shuttle which is responsible for the shuttling of the cystolic NADH b/c they yield 1.5 molecules of ATP. In reality, there is another shuttle called the Malate Aspartate shuttle which gives 2.5 ATP per NADH. This is why the amount of ATP formed in Eurkaryotic cell is usually written 30-32 ATP depending on the shuttle taken. This is all outside the scope of the MCAT though so don't give it too much thought. Just know Prokaryotes generate more ATP. Hope its a little clearer, if not let me know.

That cleared it up quite a bit; I was wondering about that for a while since I had heard seemingly contradictory explanations. Thanks!