The preparatory reaction, also known as the transition reaction or oxidative decarboxylation of pyruvate, takes place within the mitochondrial matrix of eukaryotic cells.

Here's why:

* Pyruvate is the starting molecule of the preparatory reaction and is produced in the cytoplasm during glycolysis.

* Mitochondria are the powerhouses of the cell and contain the necessary enzymes and coenzymes for the preparatory reaction.

* Mitochondrial matrix is the fluid-filled space within the inner membrane of the mitochondria where the preparatory reaction occurs.

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