Sugar molecules are broken down to release energy in the mitochondria of an animal cell.

Here's a breakdown:

* Glycolysis: The first stage of sugar breakdown happens in the cytoplasm of the cell. Here, glucose (a sugar) is broken down into pyruvate. This process generates a small amount of ATP (energy).

* Krebs Cycle (Citric Acid Cycle): Pyruvate enters the mitochondria and is further broken down in a series of reactions called the Krebs cycle. This process generates more ATP and electron carriers (NADH and FADH2).

* Electron Transport Chain: The electron carriers from the Krebs cycle deliver electrons to the electron transport chain, located within the inner membrane of the mitochondria. This chain of reactions drives the production of the majority of ATP, the cell's primary energy currency.

So, while the initial steps of sugar breakdown happen in the cytoplasm, the majority of energy release from sugar occurs within the mitochondria.