Which cellular structure is responsible for energy production via ATP synthesis?

The mitochondria are the cellular structures primarily responsible for energy production through ATP synthesis. They are often referred to as the "powerhouses" of the cell due to their critical role in the cellular respiration process, where glucose and other nutrients are converted into ATP (adenosine triphosphate), the main energy currency of the cell.

Mitochondria have a double-membrane system that creates compartments within the organelle, which is essential for the processes of oxidative phosphorylation and the electron transport chain. In these processes, energy from electrons is used to pump protons across the inner mitochondrial membrane, creating a proton gradient. ATP synthase then utilizes this gradient to synthesize ATP from ADP and inorganic phosphate.

The nucleus, while pivotal in storing and replicating genetic information, does not participate in ATP production. Ribosomes are responsible for protein synthesis and do not synthesize ATP. The endoplasmic reticulum plays a role in protein and lipid synthesis and processing but is not involved directly in ATP production. Thus, the mitochondria are unique among the choices for their dedicated function in energy production through ATP synthesis.