How many molecules of ATP are actually produced in aerobic respiration?

Question

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?

Answer 1

The discrepancy between 30 and 32 molecules of ATP quoted in the question may well arise from the two possible ways in which the reducing equivalents of the two molecules of NADH generated in glycolysis may be shuttled across the inner mitochondrial membrane into the mitochondrion (suggestion 2).

If one assumes that the malate-aspartate shuttle is used, then NADH is transported into the mitochondrion as such, and the yield of ATP is the same as for those molecules of NADH generated in the mitochondrion — 2.5 ATP per NADH from the most generally accepted measurement.

However, if one assumes the glycerol phosphate shuttle is used then the reducing equivalents are transferred from NADH to FADH₂, from which only 1.5 molecules ATP is produced per molecule.

In summary, using the malate–aspartate shuttle a total of 5 molecules of ATP (2 x 2.5) will be produced from the two molecules of glycolytic NADH, whereas using the glycerol phosphate shuttle there will be only 3 molecules of ATP (2 x 2.5). This gives a difference of 2 molecules of ATP, which would cover the range in the question.

As the relative proportions of the two shuttles varies between tissues, a range of values is actually possible.

Answer 2

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.

Answer 3

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.