Which organelle is ultimately responsible for breaking down carbohydrates into usable energy at the molecular level?

Mitochondria

Explanation:

They are membrane bound organelles found in large numbers in most cells.

They are responsible for breaking down carbohydrates into usable energy such as ATP at the molecular level.

They are the sites for the biochemical activities of respiration and energy generation which is why they are regarded as the power house of the cell.