Which of the following describes substrate-level phosphorylation?

Substrate-level phosphorylation is the process of synthesizing ATP directly from ADP and a phosphorylated intermediate during specific metabolic reactions. This occurs without the involvement of an electron transport chain or oxidative phosphorylation. The key characteristic of substrate-level phosphorylation is that it involves a direct transfer of a phosphate group from one molecule—often a metabolic intermediate—to ADP, forming ATP.

In glycolysis, for instance, certain reactions involve the transfer of a phosphate group to ADP, resulting in the production of ATP. This stands in contrast to other methods of ATP generation, such as those occurring in the mitochondria during oxidative phosphorylation, where ATP synthesis is driven by the proton gradient established by electron transport.

The other options describe different biochemical processes that do not involve the specific mechanism of substrate-level phosphorylation, making the choice that accurately defines this concept the most appropriate answer.