Energy systems: Aerobic energy system.

The idea

The aerobic energy system resynthesises ATP when oxygen is available, using carbohydrate and fat as fuels. Glycolysis in the sarcoplasm converts glucose to pyruvate, producing a small ATP yield. Pyruvate then enters the mitochondrion, where the link reaction forms acetylcoenzyme A and the Krebs cycle releases hydrogen. Electrons from hydrogen pass along the electron transport chain, while hydrogen ions are pumped across the membrane and flow through ATP synthase. Oxygen acts as the final electron acceptor to form water. This oxidative phosphorylation yields most of the ATP, with fat oxidation producing more ATP per gram than carbohydrate but at a slower rate. The system dominates during prolonged submaximal exercise, such as a steady 5 km run, because it sustains ATP supply without fatiguing quickly.

What earns the marks

• Aerobic resynthesis of ATP requires oxygen and occurs mainly in the mitochondria.
• Glycolysis breaks glucose into pyruvate in the sarcoplasm and yields a small amount of ATP.
• The link reaction converts pyruvate into acetylcoenzyme A, which enters the Krebs cycle.
• Hydrogen removed in the Krebs cycle provides electrons that pass along the electron transport chain.
• Hydrogen ions are pumped across the membrane and flow through ATP synthase to produce ATP.
• Oxygen is the final electron acceptor, combining with hydrogen to form water.
• Oxidative phosphorylation produces the largest ATP yield of the three energy systems.
• Fat oxidation yields more ATP per gram than carbohydrate but at a slower rate.
• The aerobic system predominates during prolonged submaximal exercise.

Common mistakes and how to put them right

• Thinking the aerobic system only uses fat.

  Put it right: It also uses carbohydrate, and carbohydrate is the main fuel at higher submaximal intensities.

• Believing glycolysis happens in the mitochondria.

  Put it right: Glycolysis occurs in the sarcoplasm, while the Krebs cycle and electron transport chain occur in the mitochondria.

• Stating that hydrogen ions pass along the electron transport chain.

  Put it right: Electrons pass along the chain, while hydrogen ions are pumped across the membrane.

• Confusing the aerobic system with the ATP-PC system.

  Put it right: The ATP-PC system is anaerobic and lasts only a few seconds, whereas the aerobic system sustains prolonged exercise.

Exam tips

• Link each stage to its location: sarcoplasm for glycolysis, mitochondrial matrix for the link reaction and Krebs cycle, inner mitochondrial membrane for the electron transport chain.
• Use the phrase final electron acceptor when explaining oxygen's role, and state that water is formed.
• Compare fuel use by intensity: fat dominates at low intensity, carbohydrate contributes more as intensity rises within the aerobic range.
• When a question asks why the aerobic system suits prolonged exercise, refer to its large ATP yield and lack of fatiguing by-products.