The part of an animal cell whose main job is to break down sugar and release energy that the cell can use is the mitochondrion.

Here's why:

* Mitochondria are the powerhouses of the cell: They are responsible for cellular respiration, the process of converting glucose (sugar) into ATP (adenosine triphosphate). ATP is the primary energy currency of cells.

* Cellular respiration involves three main stages:

* Glycolysis: Glucose is broken down into pyruvate in the cytoplasm.

* Krebs cycle (Citric Acid Cycle): Pyruvate is further broken down, generating electron carriers.

* Electron transport chain: Electrons are passed along a chain of molecules, releasing energy that is used to produce ATP.

While other organelles like the cytoplasm play roles in the initial stages of sugar breakdown, the mitochondrion is the key player in the process that generates the usable energy for the cell.