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I have been through the process of aerobic respiration a few times in different text books and almost every book quotes a different value for the number of ATP molecules produced. The consensus seems to be 30–32, but why is there disagreement and why aren’t the numbers exact?

The possible reasons I can think of are:

  1. Phosphorylation of ADP is not directly coupled to redox reactions.

  2. The number of ATP molecules produced depends on the shuttle used to transport electrons from the NADH in the cytosol to the mitochondria, i.e. whether it’s FADH2 which enters the ETC or NADH. (I have a silly question on that, but I need confirmation, does the electron carrier chosen depend on availability?)

  3. The proton-motive force can be used to drive other cellular processes other than the production of ADP? (I'm guessing.)

Are these suggestions correct, or is there some other explanation?

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    $\begingroup$ Does this answer your question? Energy released during the production of ATP? $\endgroup$ – David Dec 15 '19 at 15:05
  • $\begingroup$ I don't think so,no, none of the answers explain why the numbers are inexact. $\endgroup$ – Jaca Dec 16 '19 at 18:53
  • $\begingroup$ You are right there, but at least they give numbers, unlike the current answers here. I could tell you why, but will wait until the current answers have been given a chance to address that question. In the meantime you might consult Berg on NCBI Bookshelf. $\endgroup$ – David Dec 16 '19 at 19:27
  • $\begingroup$ what notable reason is ATP synthase is not perfect and sometimes fail to have all available sites completely filled by hydrogen ions while rotating which can cause fluctuations in efficiency, basically it can occasionally slip a gear. The ratio may also vary with temprature, sciencedirect.com/science/article/pii/S0005272896001375 $\endgroup$ – John Jan 14 at 6:39
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You are totally spot on with #1. ADP phosphorylation pump is driven by the proton gradient and ADP availability, while the the electron transport chain slows when it becomes too hard to pump protons. Furthermore, other processes (mostly channels) may use up a proton from the gradient.

The different electron carriers have store different amounts of energy (NADH is more energetic than FADH) and differ in the uncatalysed energy required to release the electron pair, hence why there is not a one size fits all. NADPH vs. NADH is a special case as NADPH is anabolic (builds), while NADH is catabolic (the electron pair is used for energy generation) and the enzymes in the cell keep it so that NAD+ levels are higher than NADH, and NADPH higher than NADP+. The passing of a pair from NADH to NADPH (a membrane bound transhydrogenase) actually utilises a proton off the gradient to drive it.

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It depends on the mode of transport used to transfer reduction equivalent (NADH or FADH2) across the inner mitochondrial membrane.

If glycerol phosphate shuttle transfers hydrogen atoms across the membrane, it uses FADH2 and 3 ATPs are produced. And in case of malate-aspartate shuttle NADH is used for transport and 5ATPs are synthesized. Each FADH2 produces only 1.5 ATPs, whereas NADH produce 2.5 ATPs.

So, this results in the difference in overall ATP production during Aerobic respiration.

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There is a fixed number of NADH and FADH2 produced per glucose oxidizes (assuming we are starting from the beginning of glycolysis) so reason 2 is unimportant for variability. The third reason you are right that the cell uses the proton gradient for additional purposes such as pumping ADP and Pi into the mitochondria by secondary active transport but we know that this requires 1 proton overall per ATP synthesised so this doesn't contribute to the variability in the number. The proton gradient may be used for other transport processes across the inner mitochondrial membrane and this might contribute to variability in the number as we aren't sure how much of the proton gradient is used by these other processes or if these processes are always active. The other reason (probably the most important reason) is that protons can leak back across the membrane without producing ATP and the extent of leakage measured depends on the quality of membrane preparation and the experiment performed (and the equipment used) so there is variation in the amount of leakage and hence the number of protons required per ATP synthesised, resulting in variation in the overall number of ATP synthesised in one round of aerobic respiration.

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