I don't know how specific you need to get for this question. The basic answer would be the mitochondria as it is where the Krebs Cycle, the Electron Transport Chain, and Chemiosmosis (also referred to as oxidative phosphorylation) all occur. Chemiosmosis is where the majority of ATP is produced during cellular respiration, and it primarily occurs in the matrix of the mitochondria as protons move down the gradient through ATP Synthetase channels.