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In order to initiate glycolysis, 2 ATP are necessary. In the glycolytic process, you generate an additional 4 ATP, which results in a net gain of 2 ATP.

If you don't undergo glycolysis, however, then you have an extra 2 ATP not being used. If you do glycolysis, you still got only 2 ATP. Am I missing something here?

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replace "ATP" with "money" and you have a financial system - they do plenty well :) – shigeta Mar 2 '14 at 13:24
Turning 2 ATP into 4 ATP isn't good? – mgkrebbs Mar 2 '14 at 21:48
you said that glycolysis generates net 2 ATP. That means 2 ATP in excess of what you started with. Plus @Chinmay and Satwik's answers – A. Kennard Mar 3 '14 at 9:48
Hi! You can accept one of the answers (by clicking the checkmark ) if you think it has answered ur question ! – biogirl Mar 3 '14 at 16:44
up vote 3 down vote accepted

Suppose the cellular pool has $x$ATP before starting glycolysis. In the initial phosphorylation steps, we use up two ATP to get the total tally at $(x-2)$ATP. The following steps yields $4$ ATP which brings the final total to $(x+2)$ATP.
Assuming the cell is performing fermentation, the two additional $NADH$ formed will not be contributing to any ATP gain. And hence we earn a total of $2$ ATP in excess of the already existing amount.

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Actually, the NADH contributes to ATP gain in many fermentative systems. – Chinmay Kanchi Mar 2 '14 at 3:19
@ChinmayKanchi You might be correct. But I guess the OP's concern is limited to cases where substrate base phosphorylation is the only source of ATP. – Satwik Pasani Mar 2 '14 at 12:04
@Chinmay Kanchi. Can you give an example of one fermentative system where NADH 'contributes to ATP gain' (and how?). – TomD Dec 31 '15 at 22:11

Remember that glycolysis yields 2 NADH as well as 2 net ATP. This NADH can be used to a terminal electron acceptor to produce an end product with a net gain of ATP. End-products of fermentation can include lactate, acetate, butyrate, propionate and ethanol, all of which generate different amounts of additional ATP.

The exact pathways involved vary according to the organism, but anywhere between 1 (or even 0.5) and 4 additional ATPs can be generated, even without the involvement of oxygen. In the case of aerobic respiration of course, the NADH feeds into the electron transport chain and yields ATP.

EDIT: It's perhaps worth noting that the 2 ATP is gained. See @SatwikPasani's answer for the math.

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But where does the 2 NADH come in? In lactate fermentation, for example, the (one) NADH produced in the GAPdh reaction is reoxidized in the LDH reaction. Glycolysis may be thought of as the splitting of glucose without any net oxidation or reduction, and where the 'purpose' of the LDH reaction is to regenerate the NAD+ (from NADH) to allow the GAPdh reaction to proceed. – TomD Dec 31 '15 at 22:07
In alcohol fermentation, the alcohol dehydrogenase reaction plays a similar role to LDH in lactate fermentation, and there is also no net oxidation or reduction. – TomD Dec 31 '15 at 22:27

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