Which energy system uses glycolysis, Krebs cycle, and electron transport chain to produce ATP and can utilize glucose, fats, or proteins?

• A. Lactic acid system
• B. Aerobic energy system
• C. ATP-PCr system
• D. Glycolysis only

Answer: (B)

The aerobic energy system relies on oxidative metabolism that uses glycolysis, the Krebs cycle, and the electron transport chain, with oxygen as the final electron acceptor. It can use glucose, fats, or proteins as fuel. In practice, glycolysis in the cytosol breaks down glucose to pyruvate; with oxygen, pyruvate enters the mitochondria and is converted to acetyl-CoA to feed the Krebs cycle, which generates NADH and FADH2 that drive the electron transport chain to produce the bulk of ATP. Fats contribute via beta-oxidation to form acetyl-CoA, while proteins can supply substrates that enter either glycolysis or the Krebs cycle. This combination allows high ATP yield over longer periods, making it the system that supports sustained activity and can utilize multiple fuel sources. By contrast, systems that rely on glycolysis alone (producing lactate) or ATP-PCr provide much quicker, short-lived energy and do not involve all three pathways.