Gordon Teasdale

Organic Motion Cycling



The aim of this vignette is to describe the various exercise energy pathways used by our muscle cells during exercise.

Exercise Energy Pathways

The cells in our body require energy in the form of a high energy nucleotide called Adenosine Triphosphate (ATP) to function (Fink, 2008).

All the biochemical reactions in our body require ATP. ATP is the fuel that powers cells including muscle cells.

ATP is produced when glucose is broken down in a process called glycolysis. Our bodies receive glucose by breaking down the carbohydrate in our diet to the smallest single sugar molecule that we are able to absorb, (Fink, 2008)

The process by which the cells in our bodies’ breaks down glucose to produce the energy required is called cellular respiration.

During sport our muscle cells require large amounts of ATP to fuel the energy required.

There are three possible cellular respiration pathways

  1. Alactic anaerobic (without oxygen) or phosphogen system, (Miller, 2009)
  2. Lactic acid anaerobic (without oxygen) system
  3. Aerobic (with oxygen) system

The phosphogen anaerobic energy pathway is used when the cells requirement for energy is immediate, such as sprinting.

When ATP is used to release energy, the substrate Adenosine Diphosphate (ADP) is formed within the cytosol (or Cytoplasm) of the cell.

The phosphogen system energy pathway begins when a molecule of creatine phosphate donates its high energy phosphate group to a molecule of ADP.

The reaction is: ADP + Creatine phosphate àATP + creatine

The enzyme creatine kinase is used during this reaction to bring the substrates ADP and creatine phosphate together and to lower the activation energy of the reaction. (Wilkinson, 2012)

The particular advantages of the Phosphogen system in sport are:

  • The phosphogen system does not require glycolysis to take place. It uses the existing ADP and creating phosphate in the cytosol of the cell. This means that the ATP is formed faster and is able to meet the immediate demands of high energy sport.
  • The phosphogen reactions take place in the cytosol of the cell in close proximity to where ATP is needed, (Wilkinson, 2012) and is therefore extremely beneficial to the immediate high energy requirements of sport.
  • The phosphogen system is the first system to be used during high physical activity.
  • The phosphogen system is a single step reaction which produces ATP quickly and is therefore beneficial to the immediate high energy demands of sport, (Wilkinson, 2012)

The primary disadvantage of the phosphogen system is the fact that there are limited stores of phosphocreatine. When the stores of phosphocreatine are exhausted the cells have to revert to either lactic acid or aerobic respiration.

Typically cells store enough creatine phosphate to produce all out contractions for about 10 seconds, (Miller, 2009)

The anaerobic system is the energy pathway that takes place in the cytoplasm of our cells and does not require oxygen to be present.

An initialization reaction takes place to initialize the glycolysis reaction. Two ATP molecules are required to provide the energy for the initialization reaction.

The Glycolysis reaction produces 4 molecules of ATP. Two molecules of ATP repay the initial energy expenditure of the initialization reaction leaving a net gain of 2 APT molecules available to provide energy to the cell.

The anaerobic respiration equation is:

Glucose à pyruvate àlactic acid + energy

The coenzyme NAD is used in the reaction and acts as an electron accepter.

The anaerobic energy pathway is used when the demand for energy is immediate for example sprinting and when fast muscle contractions are required.

The particular advantages of the anaerobic system in sport are:

  • Anaerobic respiration takes place in the cytosol of the cell close to where the energy is needed.
  • ATP is produced quicker than aerobic respiration with an immediate net gain of 2 ATP molecules after glycolysis. The speed of ATP produced is able to meet the immediate energy demands of sport.
  • The anaerobic respiration system is the second energy system to be used during high physical activity such as sport where ATP is able to be accessed quickly.

The primary disadvantages of anaerobic respiration are:

  • Only two ATP molecules are produced

Can only take place when the enzyme NAD is present. There is a limited supply of NAD in our cells, which is quickly exhausted.

Aerobic respiration is the energy pathway that takes place in the mitochondria of the cell and takes place when free oxygen is available.

Aerobic respiration entails the complete breakdown of the pyruvate sugar molecules that were formed in glycolysis into carbon dioxide (CO2), (Fink, 2008)

When a single glucose molecule is split into two pyruvate molecules during glycolysis the electrons  and hydrogen ions that are released during this reaction are picked up by NAD+ forming NADH

Glucose is oxidized (gains an oxygen) and NAD+ is reduced (loses an oxygen)

Most of the energy released by the breakdown of glucose is carried by the electrons attached the NADH molecule.

The pyruvate molecules are modified as they enter the mitochondria of the cell and release carbon dioxide.

The modified molecules of pyruvate then enter a series of reactions called the Citric Acid cycle or the Krebs cycle.

The Krebs cycle completes the oxidation of glucose and more carbon dioxide is released. 2 ATP’s are formed for every glucose molecule consumed.

Most of the energy released during the oxidation of glucose is carried by electrons attached to NADH and FADH2

The final phase of aerobic respiration is called oxidative phosphorylation or the Electron Transport Chain.

The NADH and FADH2 molecules produced in glycolysis and the Krebs cycle donate their electrons to the electron transport chain.

At the end of the chain oxygen “…exerts a strong pull on the electrons”, (fairfax biology, 2011) and combines with the electrons, hydrogen ions and protons to form water.

The movement of hydrogen ions creates a hydrogen ion gradient. The potential energy from the gradient is used by the ATP synthase complex to synthesize ATP, (ndsuvirtualcell, 2008)

The electron transport chain converts the chemical energy of moving electrons to a form that can be used to drive oxidative phosphorylation which produces 34 ATP molecules for each single glucose molecule.

The particular advantages of the aerobic system in sport are:

  • More ATP is produced per single glucose molecule (38 ATP), which is beneficial to endurance sporting activities such as long distance running and marathon events.
  • The cells in our body are able to aerobically respirate continually as long as there are available stores of glycogen, Proteins or Triglycerides (Fats)
  • Aerobic respiration is the most efficient energy pathway. Aerobic respiration produces the most ATP per single glucose molecule consumed.

No lactic acid is produced during aerobic respiration.


Professor Steven A Fink. (2008). Physiology; Cellular Respiration; Part 1. [Online Video]. 30 December. Available from: http://www.youtube.com/watch?v=WxQeKBHAdn8. [Accessed: 11 December 2013].

Joe Miller. 2009. Anaerobic Respiration and Sprinting. [ONLINE] Available at: http://livehealthy.chron.com/anaerobic-respiration-sprinting-4539.html. [Accessed 11 December 13].

Sarah Wilkinson. (2012). Energy systems in the body: Phosphogen system. [Online Video]. 07 September. Available from: http://www.youtube.com/watch?v=jjSmitf4_w0. [Accessed: 11 December 2013].

Fairfax Biology. (2011). Cellular Respiration Overview. [Online Video]. 05 April. Available from: http://www.youtube.com/watch?v=aXC9jMNIRnE. [Accessed: 12 December 2013].

ndsuvirtualcell. (2008). Cellular Respiration (Electron Transport Chain) . [Online Video]. 03 March. Available from: http://www.youtube.com/watch?v=xbJ0nbzt5Kw. [Accessed: 12 December 2013].