Advanced Placement (AP) Biology Practice Exam

What is substrate-level phosphorylation?

1. The formation of ATP through oxidative phosphorylation

2. The enzyme-catalyzed addition of a phosphate group to glucose

3. The direct formation of ATP from ADP and a phosphate group during catabolism

4. The process of transferring energy between metabolic pathways

The correct answer is: The direct formation of ATP from ADP and a phosphate group during catabolism

Substrate-level phosphorylation is a specific metabolic process that occurs during catabolic reactions, where adenosine triphosphate (ATP) is directly synthesized from adenosine diphosphate (ADP) and an inorganic phosphate (Pi), without the involvement of the electron transport chain or oxidative phosphorylation. This mechanism typically takes place in glycolysis and the Krebs cycle (or citric acid cycle), where specific enzymes facilitate the transfer of a phosphate group from a phosphorylated intermediate substrate directly to ADP. This pathway is distinct from oxidative phosphorylation, which involves a series of electron carrier proteins and the production of ATP through chemiosmosis in mitochondria. The direct nature of substrate-level phosphorylation allows for rapid ATP formation under conditions where oxygen or electron carriers may not be readily available. This is particularly important in anaerobic organisms or during intense exercise in muscle cells. In summary, the key element of substrate-level phosphorylation is the direct coupling of substrate breakdown to the production of ATP, making it a fundamental aspect of metabolism during energetic demands.