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Discrepancies in different prep books, which to believe?

Discussion in 'MCAT Discussions' started by adillon, Mar 1, 2007.

  1. adillon

    adillon CuriousDoc 2+ Year Member

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    Jul 8, 2006
    We know that 1 molecule of glucose produce 36 ATP from glycosis and TCA cycle in eukaryotes. However, in Princeton Review book, they counted 30, and explained it by saying that "the number reflect the most recent understanding of ATP synthesis, and as such, may not appear in some textbooks that still cling to the previously established counts of 36 ATP per glucose in eukaryotes..."

    Which one do we use in the EXAM???:confused:
     
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  3. gujuDoc

    gujuDoc 10+ Year Member

    13,872
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    Feb 21, 2004
    30
     
  4. gujuDoc

    gujuDoc 10+ Year Member

    13,872
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    Feb 21, 2004
    Now i'm going to give you the very long lesson in biochemistry. hope I don't confuse you.

    First let's do a little recall and recall that

    Glycolysis: 2 NADH, 2 Net ATP
    Pyruvate Dehydrogenase Cycle: 2 NADH
    Citric Acid Cycle: 1 FADH2, 1 GTP, and 3 NADH

    In olden times it was believed that the matter of ATP/Oxygen (P:O ratio) was determined by a simple chemical equation and purely a chemical reaction. This led to values of 3 ATP per NADH and 2 ATP per FADH2.


    So if 1 GTP conerts to ATP and you have 4 NADH and 1 FADH2, that would give you 4*3 = 12. That accounts for th first 2 phases. You also have 2 net ATP from glycolysis. In TCA, you have 2 acetyl coa which will get you two rounds of the TCA cyce. So if you have 3 NADH and 1 FADH2 and 1 GTP per turn of the cyle that wasa believed to give you (2(3*3)+2(1*2)+2) = 24

    24 +12 = 36

    In the past several years however, a scientist by the name of Mitchell proved that the processes of oxidative phosphorylation and electron transport are coupled together by not only a chemical gradient but also an electrical gradient which was driven by a proton motive force.

    When taken into account, the proton motive force has proven that for every ATP produced 4 protons are driven back into the matrix.

    This change the P:O ratios so that NADH actually yields 2.5 ATP/Oxygen molecule and FADH2 yields 1.5 ATP per Oxygen molecule.

    So now redoing the caculations above we see that if there are 4 NADH from glycolysis and PDC, we get 4(2.5)+2net ATP + [2(3 NADH (2.5) + (2(2FADH2 (1.5))] + 2GTP which are converted to ATPs = 32.

    So the last piece of the puzzle is why 30 instead of 32. I'm going to create another post for that one.
     
  5. gujuDoc

    gujuDoc 10+ Year Member

    13,872
    31
    Feb 21, 2004
    Ok so I hope my above explanation wasn't too confusing.

    So now I'm going to get to the last piece of the puzzle. Why 30 and not 32. In bacteria it is 32 in eukaryotes it is 30.

    why? Recall that glycolysis takes place in the CYTOSOL.

    On the other hand, PDC, TCA, and the coupled processes of electron transport take and oxidative phosphorylation take place in the matrix of the mitochondria. Also recall the mitochondria is like a gram neg. bacteria in the sense that it has a double membrane system in which things cannot freely cross the inner membrane.

    As a result the 2 NADH from glycolysis will not be able to freely cross the membrane and must go through specialized transport systems to get access into the matrix. There are 2 main transport systems used:

    Malate-aspartate shuttle and glycerol 3 phosphate shuttle.

    In the G3P shuttle, the 2 NADH pass their electrons to intermediates that then transfer the electrons to 2 FADH2. So by the time the electrons reach the inside of the matrix they are now carried by FADH2 leading to a 1.5 ATP to Oxygen ration decreasing the number of total ATP by 2.

    Cells using the malate aspartate shuttle don't decrease the overall amount because they reoxidize NADH back to NAD+ while passing the electrons to oxaloacetate to create malate. Then inside the matrix, they again reduce the NAD+ back to NADH and reoxidize malate back to oxaloacetate. oxaloacetate is then converted to Aspartate and transported back out of the mitochondria.

    Since NADH remains the carrier throughout in the Mal. Asp. shuttle, those cells will produce 32 ATP and not 30 ATP in total.

    Cells that usually use G3P shuttle include cells of the brain and skeletal muscle, while the heart, liver, and other cells use the Mal-Asp. shuttle.

    But for the purposes of MCAT you only need to understand the G3P shuttle concepts.
     
  6. gujuDoc

    gujuDoc 10+ Year Member

    13,872
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    Feb 21, 2004
    Sorry not trying to post continually but again, to reiterate:

    What you need to know for the MCAT is the most current number, 30 ATP total bc 2.5 ATP/NADH and 1.5 ATP/FADH2. The previous understanding of 36 is outdated and not used anymore except in some literature. In the MCAT they'd more then likely use the newly accepted values. Please try to understand the concepts I presented earlier and understanding why it is 30 and not 36 will be easier for you. Don't just memorize. Try to ingrain the actual concept down and your life will be easier.
     
  7. meds2008

    meds2008 Junior Member 5+ Year Member

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    Dec 17, 2004
    I'm not being sarcastic: this is an interesting academic discussion and well/clearly presented.

    But in terms of the MCAT, most biology questions in the biological sciences are related to the passage/table/diagram that is presented. The questions relying strictly on memorization are in the minority. A question that would straight up ask for the number of ATP produced per molecule glucose under aerobic conditions in a mammal or prokaryote . . . don't bet on it. That's why AAMC practice tests are important, etc . . . just to keep it real. Just to make sure you don't fill your head with info for nothing.

    Now they could ask what a kinase or phosphorylase may do or something that underlines the importance of acetyl Co A, but for the AAMC (not kap or tpr, etc) to ask the number of ATP and have possible answers like 30, 32, and 36 where the data cannot be determined by the passage, I would humbly suggest that it's not going to happen or that question is one of the 1-2 questions that they admit to putting in exams that won't count.

    Just my 2 cents.
     
  8. gujuDoc

    gujuDoc 10+ Year Member

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    Feb 21, 2004
    Yeah I agree with you. I was just explaining it for the OP so they understood the concept. But I agree that the MCAT is very highly focused on being able to interpret graphs, data tables, and other given information in the passage. However, to do that you need to understand some essentials first. That's why the key to any section, not just bio, is to understand the key concepts and then practice a lot and get used to the style of questioning.
     
  9. killinsound

    killinsound Physician 10+ Year Member

    907
    3
    Aug 30, 2006
    i think if they were to present the newer version of ATP synthesis, they would do so in a passage describing how he figured it out.
     

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