The part of cellular respiration directly responsible for the bulk production of ATP is oxidative phosphorylation, which occurs in the electron transport chain located in the inner mitochondrial membrane.

Here's why:

* Glycolysis and the Krebs cycle (also called the citric acid cycle) generate a small amount of ATP through substrate-level phosphorylation. This process directly transfers a phosphate group from a substrate molecule to ADP, producing ATP.

* Oxidative phosphorylation, however, utilizes the high-energy electrons generated during glycolysis and the Krebs cycle. These electrons are passed along a chain of protein complexes embedded in the inner mitochondrial membrane.

* The energy released from electron movement is used to pump protons (H+) across the membrane, creating a proton gradient.

* This gradient represents potential energy, which is then harnessed by ATP synthase, a protein complex that uses the proton flow to generate ATP from ADP and inorganic phosphate.

In essence, oxidative phosphorylation is the primary ATP-producing pathway in cellular respiration, generating the vast majority of the cell's energy currency.