When a cell is carrying out aerobic respiration it obtains 38 ATPs from the oxidation of one molecule of glucose. Here is where they come from:

In glycolysis:

2 ATPs put in and 4 produced by SLP = a net of ...............................2

2 NADH2 go to ETS where 3 ATPs produced/NADH2 .......................6

In the Krebs Cycle:

via SLP .............................................................................2

8 NADH2 (-> ETS where 3 ATPs produced/NADH2...................... 24

2 FADH2 (-> ETS where 2 ATPs produced/FADH2) .......................4

Total ...............................................................................38

Should you memorize the above?

NO! What you should know is that 38 ATPs are produced and you should understand that in respiration most of the energy obtained is gotten from reduced coenzymes going to the ETS where oxidative phosphorylation occurs.

What about when a cell is carrying out fermentation?

When a cell carries out fermentation only glycolysis occurs. Under these conditions the only ATP that the cell obtains is from SLPAs noted above, 2 ATP are put in and 4 are produced resulting in the net production of 2 ATP.

What about the reduced NAD (NADH2) that is produced in fermentation?

It becomes reoxidized by donating its 2 hydrogens to an organic molecule - as an example it can donate its hydrogens to pyruvic acid and convert it to lactic acid. The pyruvic acidhas become reduced (to lactic acid) and the NADH2 become oxidized back to NAD.