Acetyl co-A is too small of a carbon compound to continually breaking off carbons. This is a problem because the goal of the Krebs Cycle is to extract energy in the form of high energy electrons, which are released from the breaking of chemical bonds. To solve this issue, a 4-carbon compound from the last turn of the Krebs Cycle will join with the incoming acetyl co-A 2-carbon compound to form , a 6-carbon compound. As each carbon compound is oxidized by oxygen, high energy electrons are carried by NAD+ and CO2 is released into the atmosphere. During the regeneration of citric acid, so much energy is released that a "back-up" electron carrier, , is also used to gather released electrons. During the entire process, a net gain of 2 ATPs are produced.