The organelle within cells where energy is released from nutrient molecules is the mitochondria.

Here's why:

* Cellular Respiration: Mitochondria are the powerhouses of the cell. They are responsible for the process of cellular respiration, which is the breakdown of glucose (a sugar) in the presence of oxygen to produce ATP (adenosine triphosphate). ATP is the primary energy currency of cells.

* Krebs Cycle and Electron Transport Chain: Mitochondria house the Krebs cycle (also known as the citric acid cycle) and the electron transport chain, key stages in cellular respiration where the majority of ATP is produced.

While other organelles like the cytoplasm and lysosomes play roles in breaking down nutrients, the mitochondria are the primary site of energy production.