Final answer:
The anaerobic portion of cellular respiration ends with glycolysis, and the aerobic portion begins when pyruvate is converted to Acetyl CoA before entering the Krebs cycle if oxygen is available.
Step-by-step explanation:
The portion of cellular respiration that is anaerobic concludes with glycolysis, and the aerobic portion begins when pyruvate enters the mitochondria if oxygen is present. If oxygen is available, pyruvate from glycolysis is transported into the mitochondria, where it is converted into Acetyl CoA and enters the Krebs cycle. The Krebs cycle, also known as the TCA (tricarboxylic acid) or Citric acid cycle, is a crucial component of aerobic cellular respiration, in which the derived Acetyl CoA is fully oxidized to carbon dioxide, generating energy carriers NADH and FADH2. These carriers subsequently donate their electrons to the electron transport chain (ETC), which is the definitive stage of aerobic respiration taking place in the inner mitochondrial membrane, and involving oxygen as the final electron acceptor. Therefore, the transition from the anaerobic to aerobic stages of cellular respiration occurs after glycolysis and before the Krebs cycle, thus the correct answer is 'b. After the Krebs cycle'.