The cell organelle responsible for the release of energy as ATP is the mitochondrion.

Here's why:

* Mitochondria are often called the "powerhouses of the cell." They are responsible for cellular respiration, a process that breaks down glucose (sugar) in the presence of oxygen to produce ATP.

* ATP (adenosine triphosphate) is the primary energy currency of cells. It is used for various cellular processes, including muscle contraction, protein synthesis, and active transport.

While other organelles like the chloroplast (in plant cells) also generate energy, their primary function is photosynthesis, not ATP production for the cell's overall energy needs.