Cellular respiration question (from Examkrackers)

[KungFuPanda123]

The question is:

"Assuming that 2 ATP are consumed in moving NADH from the cytosol to the mitochondrial matrix, and all other conditions are optimal, how many ATP are produced per glucose molecule?"
A) 18
B) 32
C) 36
D) 38

Here's my thought process:
"All other conditions are optimal" is telling me that aerobic respiration will happen (i.e. glycolysis AND the krebs cycle/electron transport chain will happen). Going to assume 3 ATP per NADH and 2 ATP per FADH2 for the sake of this question, since EK said to assume so in the review.
Glycolysis = 2 ATP + 2 NADH = 8 ATP equivalents
Pre-Krebs = 2 NADH = 6 ATP equivalents
Krebs = 2 GTP + 6 NADH + 2 FADH2 = 24 ATP equivalents
Total produced = 38 ATP
Usually, 1 ATP is necessary to transport the NADH produced in Glycolysis into the matrix for the electron transport chain, but according to this question, 2 ATP are needed for each NADH. Thus, I'm going to say that there is a net total of 38 - 2x2 = 34 ATP. This is not one of the answer choices.
I originally wanted to simply say (D), because 38 ATP IS PRODUCED. And then, you use up some ATP to get a lower NET total (normally 36, but in this case, 34).

Here's the explanation EK has:
(B) is correct. The net product of glycolysis for one glucose molecule is 2 ATP and 2 NADH. The additional steps of aerobic respiration produce 34 ATP. This is a net of 36 ATP without accounting for the NADH. Since 2 ATP are used to move each NADH in, this results in the loss of 4 ATP and net product of 32 ATP.

---- I think EK's explanation is wrong. How do the "additional steps of aerobic respiration produce 34 ATP?" Seems to me like the additional steps produce 6+24=30. Also, why do they not factor in the glycolysis NADH into the final calculation of ATP?

Hope someone can give me some clarifications if I'm wrong or agree with me if I'm right. Thanks!

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[amy_k]

We have the following going into ETC:

4 ATP (2 from glycolysis and 2 from krebs cycle)
10 NADH (2 from acetyl coA conversion and 6 from krebs cycle)
2 FADH2 (From krebs cycle)

We get 32 ATP (optimally) from etc as follows:

3ATP per NADH: 3x10 = 30, but we lose 2ATP transporting NADH which gives 28
2ATP per FADH2: 2x2 = 4

4 ATP + 32 ATP = 36 ATP.

The 32 ATP from ETC + 2ATP from Krebs cycle = 34 ATP from "additional steps of aerobic respiration," giving a total of 36 ATP when you count the 2ATP from glycolysis.

Hope that helps!

 

[amy_k]

Also, the initial question states "2 ATP are consumed in moving NADH from the cytosol to the mitochondrial matrix", i.e. 1 per NADH, not 2 per NADH, it is saying 2 total, this might have been tripping you up!

 

[sazerac]

The answer key in the OP says 2 ATP are used to move each NADH in. Not 2 total.

 

[amy_k]

You are right, it does say that in the post - can we check OP is it saying 1 ATP per NADH or 2? It should be 2 total (1 ATP per NADH transported)

 

[KungFuPanda123]

I know that it usually takes 1 ATP per NADH to transport it from the cytosol to the mitochondrial matrix, but I think the purpose of this question was to test a situation where it might actually take 2 ATP per NADH. (I'm making this assumption based off of the answer key, where it states "2 ATPs are used to move each NADH in")

 

[amy_k]

It would be 30atp then surely? Theres got to be a mistake in there somewhere then whether its the answer, solution, or initial info

 

[KungFuPanda123]

It would be 30atp then surely? Theres got to be a mistake in there somewhere then whether its the answer, solution, or initial info

That's what I'm thinking... I quoted the question and answer exactly as it's written in the book. I don't get how it can make sense - I'm hoping that someone out there can definitively say that it is wrong so that I can put this to rest and move on, knowing that there is no correct answer for this question.

 

[sazerac]

When you find mistakes in your review materials, it means you are nearly ready for the exam.

 

[Labrat07]

I learn something new here. Thanks. I have one question regarding this though. Some source say NADH = 2.5 and FADH =1.5 . Why is that? I assume we only need to know 3 and 2?

 

[KungFuPanda123]

I learn something new here. Thanks. I have one question regarding this though. Some source say NADH = 2.5 and FADH =1.5 . Why is that? I assume we only need to know 3 and 2?

I learned 2.5 and 1.5 in AP Bio in high school. It seems like that number is actually more accurate - not only because 2 NADH's actually require 1 ATP each to get transported across but also because apparently ATP Synthase needs 10 protons per 3 ATP's synthesized. NADH sends a total of 10 protons across the membrane in the Electron Transport Chain for an eventual total of 10/4 = 2.5 ATP's. FADH2 sends a total of 6 protons across the membrane in the Electron Transport Chain for an eventual total of 6/4= 1.5 ATP's. (Those numbers are from here: http://www.life.illinois.edu/crofts/bioph354/ATP_2e_ratios.html)

I feel like cellular respiration is such a toss up when it comes to exact numbers, due to the fact that there are a lot of different generalizations/assumptions made that vary the net total ATP's produced to an extent.

 

[amy_k]

If we go off the following information:

The net product of glycolysis for one glucose molecule is 2 ATP and 2 NADH. The additional steps of aerobic respiration produce 34 ATP. This is a net of 36 ATP without accounting for the NADH. Since 2 ATP are used to move each NADH in, this results in the loss of 4 ATP and net product of 32 ATP.

We have the following calculation:

2 ATP from glycolysis
2 ATP from Krebs cycle
10 NADH (2 from acetyl coA conversion and 6 from krebs cycle)
2 FADH2 (From krebs cycle)

Going into ETC we have 4 ATP + 10 NADH + 2 FADH2

We then get from ETC cycle:
3 ATP per NADH = 30 ATP
2 ATP per FADH2 = 4 ATP

That is 34 ATP from ETC but we also have 4 from glycolysis and krebs cycle which gives 38 net (as OP stated in their original calculation), since they say

The additional steps of aerobic respiration produce 34 ATP

I think this should be 36 ATP and they have missed out the ATP in Krebs cycle. The additional steps of respiration produce 36 ATP. The other possibility is they meant to say 1 ATP per NADH giving 2 total. Either way there is a mistake somewhere as the information they give doesn't add up to any of the answers.

Then we lose 2 ATP per NADH = 38 - 4 = 34 ATP should be the answer

 

[amy_k]

I learn something new here. Thanks. I have one question regarding this though. Some source say NADH = 2.5 and FADH =1.5 . Why is that? I assume we only need to know 3 and 2?

The maximum possible yield of ATP is 38

We lose 2 ATP transporting NADH (1 per NADH) into the mitochondria, so the yield would be 36, if we got 3 ATP per NADH and 2 ATP per FADH2. But 2.5 for NADH and 1.5 per FADH2 is a more accurate approximation due to the leaky membrane and other inefficiencies, which would give us 30 ATP.

To be safe, be aware of both 3/2 and 2.5/1.5 for the exam, as at the moment it is not simply one or the other. The most up to date and accurate is 2.5/1.5, and the older values will likely be phased out in time. The question writers can test you on either of these ratios but not both in the same question, so it will become obvious from the answer choices which are being used.

 

[sazerac]

Back in 2010 I was taking a physiology class from the nursing department and a biology class from the science department at the same time. The premed biology class could make 32 ATP per glucose, while in the nurse physiology class we could make all 38.

I concluded that nurses were just more efficient.