All four steps of cellular respiration contribute to ATP production, but the majority of ATP is generated in the third step, the electron transport chain. Here's a breakdown:

1. Glycolysis:

* Yields: 2 ATP (net gain)

* Mechanism: Substrate-level phosphorylation – phosphate groups are directly transferred from molecules to ADP.

2. Pyruvate Oxidation:

* Yields: 0 ATP

* Role: Prepares pyruvate for the citric acid cycle.

3. Citric Acid Cycle (Krebs Cycle):

* Yields: 2 ATP

* Mechanism: Substrate-level phosphorylation.

4. Electron Transport Chain:

* Yields: ~28-34 ATP (depending on the cell and the efficiency of the process)

* Mechanism: Oxidative phosphorylation – electrons move through a chain of protein complexes, generating a proton gradient that powers ATP synthase.

In summary:

* Glycolysis and the citric acid cycle produce a small amount of ATP directly through substrate-level phosphorylation.

* The electron transport chain produces the vast majority of ATP through oxidative phosphorylation.

Let me know if you would like more detail on any of these steps!