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Transition Reactions

Transition Reactions take place in the matrix of the mitochondria. Transition Reactions mark the beginning of aerobic respiration but will only take place if  oxygen is available. If there is no oxygen present or not enough, fermentation will begin.

Once one molecule of pyruvate enters the mitochondrial membrane, NAD+ will take two electrons from pyruvate, oxidizing it, releasing carbon dioxide as a waste. A two carbon molecule called acetyl is formed, that will bond with coenzyme A to form acetyl Co-A. Acetyl Co-A, is the main ingredient to begin the Kreb's Cycle. The goal for Transition Reactions is to turn pyruvate into acetyl Co-A, while releasing carbon dioxide as a waste. Now the acetyl Co-A is ready for the Kreb Cycle.
Transition Reactions, as already stated, depends on oxygen so that the carbon has something to bond with to be taken away. If there is no oxygen present, there would be no way to make pyruvate into acetyl Co-A, and since the Kreb's Cycle, can only take on a two-carbon molecule, the Kreb's Cycle will not begin and the pyruate will than carry on into anaerobic respiration and go through fermentation.


The mitochondria, as shown above, has four main parts, the inner and outer membranes, the cristae and the matrix. The mitochondria is said to be very essential in the life cycle for its recognition in the process of apoptosis. Apoptosis is basically a pre-programmed life cycle of a cell.