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6.4 Aerobic Metabolism: Cellular Respiration

The Aerobic System

So far, we have discussed the two most immediate anaerobic energy systems: the ATP–CP system and glycolysis. Although these systems are able to provide ATP very quickly, their major limitation is their relatively short duration of action. In the Aerobic System, carbohydrate, protein, and fat can be used to generate ATP. As demonstrated by the diagrams below, this system is significantly more complex than anaerobic metabolism.

See caption
Collage of athletes participating in endurance-based sports that rely on the aerobic energy system. Includes long-distance runners on a trail, competitive cyclists in a race, and soccer players in a game. These activities require sustained ATP production using oxygen over extended periods. Image (top left) “Two men running” by RUN 4 FFWPU, Image (bottom left) “Cycling” by Pixabay, Image (right)” Three men playing soccer” by Pixabay, Pexels License

The Krebs Cycle & The Electron Transport Chain (ETC)

Cellular respiration, which includes the Krebs cycle and the Electron Transport Chain (ETC), collectively referred to as aerobic metabolism, can provide ATP on a virtually limitless basis as long as fuel (carbohydrate, fat, or protein) and oxygen are available. While this text will not dive deep into specific pathways of cellular respiration, it will examine key differences between its method of ATP production in comparison to Anaerobic metabolism.

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Click on the icons below to learn more about each step in the Krebs cycle and the ETC.

Image Description

Citric Acid Cycle:

  • 4 Carbon Dioxide: Each Acetyl CoA enters the cycle, releasing 2 CO₂. Four carbon dioxide diffuse out of the cell.
  • Two ATP: Two ATP ( 1 ATP per pyruvate, two pyruvates total) are released into the cell for energy use.
  • Oxaloacetate: Acetyl CoA drops the acetic acid off onto oxaloacetate. These 2 carbons get released as 2 CO₂ and oxaloacetate remains in the cycle to be reused. This repeats for the 2nd acetyl coA, producing a total of 4 CO₂).
  • 6 NADH and 2 FADH₂: Produces 6 NADH, 2 FADH₂ (3 NADH and 1 FADH₂ per pyruvate, two pyruvates total), that bring high-energy electrons to the ETC.
Image Description

Electron Transport Chain (ETC)

  • NADH and FADH₂: NADH and FADH₂ from previous stages drop high-energy electrons off at ETC, becoming NAD+ and FAD for reuse.
  • ATP:  Each NADH yields 3 ATP (10 × 3 = 30 ATP), each FADH₂ yields 2 ATP (2 × 2 = 4 ATP), total of 34 ATP produced through redox reactions
  • H₂O: O₂ serves as the final electron acceptor in the ETC, facilitating ATP production and generating water as a waste product.

Collectively, cellular respiration produces enough ATP for several hours of moderately intense physical activity by relying on a supply of glucose (carbohydrate), protein, fat, and, of course, oxygen. The amount of ATP produced greatly exceeds that of anaerobic metabolic pathways, but becomes impaired when oxygen is no longer present or is not present in sufficient quantities. In this case, cellular respiration would cease, and glycolysis would begin.

“8.4 Glycolysis” from Nutrition and Physical Fitness by Angela Harter Alger is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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The Foundations of Human Movement and Physical Fitness Copyright © 2025 by A.J. Stephen; Sarah Fraser; and Connor Dalton is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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