WJEC Biology Practice Exam

ATP is generated during cellular respiration through two main mechanisms: substrate-level phosphorylation and oxidative phosphorylation.

Substrate-level phosphorylation occurs in both glycolysis and the Krebs cycle, where a phosphate group is directly transferred to ADP from a high-energy substrate molecule, resulting in the formation of ATP. This process does not require oxygen and is a relatively quick method of generating ATP.

On the other hand, oxidative phosphorylation takes place in the mitochondria, where it involves the electron transport chain and chemiosmosis. In this process, electrons are transferred through a series of protein complexes, leading to the pumping of protons across the mitochondrial membrane. This creates a proton gradient that drives ATP synthase to convert ADP and inorganic phosphate into ATP. Oxidative phosphorylation is efficient and produces the majority of ATP during cellular respiration, especially in the presence of oxygen.

Thus, both substrate-level phosphorylation and oxidative phosphorylation work together to provide energy in the form of ATP during the cellular respiration process. Other options either focus on processes that are not directly involved in ATP generation during cellular respiration or describe limited aspects of energy production.