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Kreb's Cycle

The Kreb's cycle begins with a molecule of acetyl Co-A. In the first phase of the Kreb's Cycle, the Co-A is removed from the compound and the two-carbon molecule of acetyl will join with a four-carbon molecule called oxalacetate to form citrate or citric acid. This is why the Kreb's Cycle can also be called the citric acid cycle. Next, water is removed and than replaced but rearranged to form isocitrate or isocitric acid. In the next two reactions, hydrogens are lost to NAD+ creating NADH+ and releasing carbon dioxide, first creating ketoglutarate than succinyl Co-A. During the reaction to form succinyl Co-A, the coenzyme Co-A, comes back into to cycle to add the Co-A into succinyl. During the reaction to create succinate, the Co-A and energy in the form of ATP and GTP(which will become ATP after another step) will be let off. The next reaction will give off more energy but in the form of hydrogen, not to make NADH+ but to make another energy source, FADH2. The end result of this reaction, is fumerate. After fumerate is made, water or H2O is added and the molecules are rearranged to create malate.  In this last reaction, another hydrogen is given off to make more NADH+, and oxalacetate is made.
As you may notice, oxalacetate, is the four-carbon molecule that acetyl joins with after the Co-A has been removed. This is why the Kreb's Cycle must go through twice, to make sure there is always an oxalacetate present to join with the acetyl.

The end result of the Kreb's cycle is now six molecules of NADH+, two molecules of FADH2, and two molecules of ATP. These will all drop off their electrons at the electron transport chain and than return to the Kreb's Cycle to pick up more electrons. These electrons that are dropped off, will power the electron transport chain!