USA Biology Olympiad (USABO) Practice Exam

Which metabolic process primarily occurs in the mitochondria after glycolysis?

1. Oxidative phosphorylation

2. Krebs cycle

3. Replication

4. Transcription

The correct answer is: Krebs cycle

The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle, primarily occurs in the mitochondria after glycolysis. Following glycolysis, which takes place in the cytoplasm and breaks down glucose into pyruvate, the pyruvate molecules are transported into the mitochondria. Once inside the mitochondria, each pyruvate is converted into acetyl-CoA, which is the substrate that enters the Krebs cycle. This cycle is crucial for cellular respiration as it generates high-energy electron carriers, namely NADH and FADH2, alongside ATP and carbon dioxide as byproducts. The high-energy electrons produced during the Krebs cycle are then utilized in the process of oxidative phosphorylation, which also occurs in the mitochondria, but the Krebs cycle is the fundamental metabolic pathway that follows glycolysis in the mitochondrial matrix. In contrast, replication and transcription occur in the nucleus and are not part of the cellular respiration process. Replication refers to DNA synthesis, and transcription involves the synthesis of RNA from a DNA template, neither of which is related to the metabolism of glucose and energy production. Oxidative phosphorylation, while also occurring in the mitochondria, follows the Krebs cycle and relies on the electron