Final answer:
Eukaryotic cells and aerobic prokaryotic cells synthesize the majority of their ATP through substrate-level phosphorylation and oxidative phosphorylation, with the latter being the primary mode in the electron transport system.
Step-by-step explanation:
The two mechanisms that eukaryotic cells and aerobic prokaryotic cells combine to synthesize the majority of their ATP are substrate-level phosphorylation and oxidative phosphorylation. During glycolysis and the Krebs cycle, some ATP is produced via substrate-level phosphorylation, but most ATP is generated through oxidative phosphorylation, a separate process that takes place in the cytoplasmic membrane of prokaryotes and the inner mitochondrial membrane of eukaryotes. This process involves the electron transport system (ETS), where electrons from NADH and FADH2 are passed down a series of carriers to create an electrochemical gradient across the membrane, which then drives the production of ATP.