The body’s three energy systems:
Your body has three energy systems. They are the phosphagen, anaerobic, and aerobic systems. The phosphagen system is used for activities less than 10 seconds. The anaerobic system is for activities 2 minutes or less. The aerobic system is for any activity lasting longer than two minutes. All of your energy systems are active at the same time but one is always more dominant. It is simply the relative amount of energy each system is supplying that varies with the type of activity being performed. Adenosine triphosphate (ATP) is the usable form of energy for the body and stored to some degree in most cells. The food we eat must first be broken down and converted to ATP before use by the body. The building blocks of ATP are also its by-products during breakdown. These by-products/building blocks are adenosine diphosphate (ADP) and inorganic phosphate (Pi).
The rest of this article is an in-depth breakdown of how the actual processes occur.
The phosphagen system is used only in short durations for intense activities lasting less than 10 seconds. You may see this system referred to as ATP/CT (Creatine Transport), ATP/CP (Creatine Phosphate), or some variation but they are all the same thing. The terms creatine phosphate and phosphocreatine are the same molecule.
It doesn’t consume oxygen or produce lactate during its use. No carbohydrates or fats are used during this process either. It uses a combination of adenosine triphosphate (ATP) and phosphocreatine (PCr) stored in the muscles. This phosphocreatine donates its phosphate group during rephosphorylation (addition of a phosphate group) to resynthesize an ADP molecule back to ATP using the enzyme creatine kinase. This newly formed ATP molecule can be used immediately to produce more useable energy. The phosphagen system is the quickest way to resynthesize ATP.
There is a limited store of ATP and PCr in the muscles and so rapid energy demands quickly deplete the available stores. This rapid depletion leads our bodies to use the next energy system under sustained activity. The main purpose of the phosphagen system is the rapid availability rather than quantity of energy.
One of the mechanisms used to increase the available amount of phosphocreatine for use is via creatine supplementation.
The anaerobic system is used for intense activities lasting less than 2 minutes. The anaerobic system doesn’t use oxygen and comes from a Greek word which translates as living without air. Glycolysis refers to the breakdown of either blood glucose or muscle glycogen (the stored from of glucose). Glycogen is first broken down into glucose through a process called glycogenolysis. This process produces pyruvic acid (pyruvate is the anion). If enough oxygen is present, pyruvate enters the mitochondria for oxidation. If oxygen is not plentiful, it is converted into lactate.
Two molecules of ATP are available for use for every glucose molecule involved in glycolysis. Four ATP molecules are produced as the end product but two are used during the preparatory phase and so a net gain of two ATP is produced. When demands are placed on the body, but available oxygen is deficient; the buildup of hydrogen ions causes acidosis (that familiar muscle burning feeling).
Any activity that is longer than 2 minutes is dominated by the aerobic system. The aerobic system is responsible for most of the cellular energy in the body, but it is also the slowest way to resynthesize ATP. Aerobic respiration consists of glycolysis, the Krebs cycle (aka citric acid cycle) and the electron transport chain.
During the breakdown of glycogen, and glucose; they are first metabolized through glycolysis. The resulting pyruvate is used to form acetyl-CoA which then enters the Krebs cycle. Acetyl-CoA is broken down to carbon dioxide and hydrogen with a net gain of two more ATP molecules produced.
If the hydrogen ions aren’t dealt with, acidosis will occur. So the hydrogen ions enter the electron transport chain and are combined with oxygen to produce water. This part of the electron transport chain is called oxidative phosphorylation. Oxidative phosphorylation is the last stage of the aerobic system and produces the largest ATP yield. The name “oxidative phosphorylation” in this case can be simplified to; oxidation (a loss of electrons) and phosphorylation (addition of a phosphate group).
NADH+ and FADH+ molecules provide the electrons, hydrogen ions, and energy needed for ADP phosphorylation. Electrons and hydrogen ions are accepted by oxygen to produce H2O, and a phosphate group is added to ADP to produce ATP. The total theoretical ATP yield is 34 from oxidative phosphorylation.
Fat is the largest store of energy in the body and as such aerobic metabolism has an almost unlimited supply of energy. The body breaks down triglycerides into free fatty acids and glycerol via lipolysis.
The free fatty acids are then transported to mitochondria where it is used to produce acetyl-CoA through beta-oxidation. Fat metabolism is identical to carbohydrate metabolism after acetyl-CoA formation.
Fatty acids take more time to breakdown compared to glucose because of the increased need of oxygen.
Free fatty acid oxidation produces exponentially more ATP molecules than oxidation of glucose or glycogen. The often cited example is oxidation of the fatty acid palmitate which produces 129 molecules of ATP. However, that is the theoretical maximum yield. Depending on how many ATP molecules are calculated for transport and other reactions the number is closer to 106 ATP per complete oxidation cycle for palmitate.