Energy system

Energy Systems and Energy Production

Characteristics of the Three Energy Systems

• There are three (3) main energy systems:

  • Alactacid OR ATP/PC system

  • Lactic acid system or anaerobic glycolysis system

  • Aerobic system

• The body requires energy in the form of Adenosine Tri-Phosphate (ATP) to convert it from chemical energy to mechanical (movement) energy.

• Energy systems provide the energy required by muscles for movement.

ATP-PC Energy System

• This system produces energy by breaking down phosphocreatine (PC) to resynthesise ATP through reactions that do not require oxygen (anaerobic).

• All activities carried out above 100% VO2 max depend on anaerobic energy supply.

  • VO2 max – Maximum rate of oxygen consumption, usually expressed as (L of O2/min).

• If PC has not had time to replenish, this will be powered by the anaerobic glycolysis system.

Characteristics of ATP-PC System

• As the ATP is broken down into ADP and P, the ADP reacts with the PC (without the presence of oxygen) in the myocyte to produce another ATP and C (1 ATP per PC).

• Source of Fuel

  • The alactacid energy system or ATP/PC (Adenosine Tri-Phosphate/Phospho-Creatine) energy system uses the ATP that is immediately available within the muscle cell (myocyte).

• The PC runs out quickly, resulting in this system no longer being available until it has begun to recover.

• Efficiency of ATP Production

  • The alactacid energy system (ATP/PC) has a very fast rate of ATP production but has a very limited store of fuel.

• Duration that the System Can Operate

  • The alactacid energy system (ATP/PC) does not last very long due to the limited fuel source and fast ATP production.

  • The amount of ATP store supplies energy for a max of 2-3 seconds.

  • The alactacid system will deplete its fuel in 8 seconds when used at maximal intensity but can take as long as 12 seconds if used at a lower intensity.

• Cause of Fatigue

  • Fatigue in the alactacid energy system (ATP/PC) is caused by the depletion of fuel.

  • Once the immediate stores of ATP and PC run out, the system needs to recover before it can be used again.

• By-products of energy production

  • The alactacid energy system (ATP/PC) has no by-products other than heat, which is a by-product of every energy system.

• Process and rate of recovery

  • The alactacid energy system (ATP/PC) recovers as the creatine in the cell connects to the free phosphates again, storing them as PC to be used when they are needed again.

  • This process takes up to 2 minutes for complete recovery, but can be half restored at around the 30-second mark.

• What sort of activities will use the ATP-PC system the most?

  • Due to the speed of ATP production and the short duration of the fuel source, the alactacid energy system (ATP/PC) is the dominant system in activities such as a 100m sprint, discus, javelin, high jump, and other sports of very short duration.

  • The alactacid energy system (ATP/PC) is also used to provide brief periods of high intensity within many other sports.

  • Examples include kicking a ball during soccer or rugby league, or a short sprint where maximal effort is needed but lasts only 5-10 seconds.

Summary of ATP-PC System

• ATP-PC system is anaerobic; does not depend on oxygen to release energy.

• Provides the most rapidly available source of ATP, due to the simplicity of chemical reactions and ready availability of PC in the muscles (PC broken down to C and P).

• ATP-PC system limited by the amount of PC stores in muscles (about 4x the amount of ATP) – lasts about 10 seconds at maximal intensity.

• Once PC has been depleted, can only be replenished when there is sufficient energy in the body – usually occurs through aerobic pathway or during recovery.

• ATP must be resynthesised from another substance, typically glycogen from the muscles and liver using the anaerobic glycolysis system.

• The anaerobic glycolysis system takes over at approximately 40-50% depletion of PC stores.

Anaerobic Glycolysis (Lactic Acid) System

• Anaerobic glycolysis refers to energy created by the incomplete breakdown of glucose when oxygen isn’t available.

• More ATP is produced when lactic acid is broken down to lactate and hydrogen.

• Hydrogen combines with pyruvate to form lactate -> converted to glycogen.

• ATP must be re-formed from ADP and Pi (a free phosphate molecule) but again this requires energy.

• This energy is obtained from the breakdown of glycogen.

• Glycolysis - is the process of breaking down glycogen.

• The LA system sees this occurring without oxygen; hence, it is referred to as anaerobic glycolysis.

• In the absence of oxygen, pyruvic acid (pyruvate and hydrogen ions – H+) is produced as the main by-product.

• As an increase in H+ occurs, this causes a decrease in muscle pH (more acidic), which in turn decreases the activity of glycolytic enzymes and hence the rate of ATP resynthesis, contributing to fatigue.

• Recovery can take between 20 minutes to 2 hours, depending on intensity and duration of exercise.

• The lactic acid system produces ATP very rapidly.

• For each molecule of muscle glucose and glycogen, two molecules of ATP are produced (1:2).

• Note: Two molecules of lactic acid are also produced.

• The Lactate Inflection Point (LIP)

  • Refers to the point where the body can prevent the accumulation of hydrogen ions in muscles.

  • Lactate production rates match removal.

  • Beyond this point, lactic acid is produced faster than it can be oxidised or broken down, and lactate accumulates in the muscle and moves into the bloodstream.

  • The point at which lactate levels begin to rise rapidly is known as the Onset of Blood Lactate (OBLA).

Characteristics of Anaerobic Glycolysis System

• The lactic acid energy system uses carbohydrates (CHO) as its only source of fuel and relies on anaerobic glycolysis for its production of ATP.

• Glycolysis is the breakdown of glucose to produce ATP. In anaerobic glycolysis the glucose (sourced from glycogen in the muscle or glucose in the blood) is turned into lactic acid as it produces ATP.

• Source of Fuel

• This system produces ATP at a fast rate and can produce a lot of ATP.

• The lactic acid system produces 2 ATP for each glucose molecule it breaks down; however, it also produces lactic acid in the process.

• Efficiency of ATP Production

• The lactic acid system lasts between 30 seconds and 3 minutes depending on the intensity.

• The less intense the activity the longer it will last because it will be producing lactic acid at a slower rate at the lower intensity levels.

• Duration that the system can operate

• The cause of fatigue in the lactic acid system is the build up of pyruvic acid in the muscle.

• Pyruvic acid is made up of two molecules; pyruvate and a hydrogen ion (H+).

• Without oxygen the body converts the pyruvate and two H+ to lactate.

• This helps to reduce the acidity of the muscle and allows anaerobic glycolysis to last longer, as the lactate is removed from the muscle and taken to the liver where it is converted to a useful fuel source such as glucose.

• However, in continued high intensity activity the lactate cannot be removed fast enough, which results in a build up of pyruvic acid.

• It is specifically the build up of the H+ within the muscle that causes fatigue.

• It does this by increasing the acidity of the muscle and causing the enzymes needed for anaerobic glycolysis to slow down.

• Cause of fatigue

Aerobic Energy System

• Aerobic glycolysis refers to energy provided by the complete breakdown of glucose with oxygen.

• This causes aerobic glycolysis to take more time, but can continue to supply energy for much longer, as long as there is fuel.

• When oxygen supply meets demand, the body is said to have reached steady state.

• After approximately 75 seconds of maximal exercise, energy is being used almost equally between aerobic and anaerobic systems.

• After only 30-60 seconds, oxygen uptake can be as high as 90% of an athlete's max (VO2 max).

Characteristics of Aerobic Energy System

• Source of Fuel

  • The aerobic system can use CHO, fats, and protein as its source of fuel, though protein is used sparingly.

  • The aerobic system uses aerobic glycolysis, the Krebs cycle, and the electron transport chain in its production of ATP.

  • It is the presence of oxygen, which allows this energy system to use these various fuel sources.

• Efficiency of ATP Production

  • The aerobic system is very efficient in producing ATP.

  • It produces 38 ATP molecules per glucose, but the rate of production is medium and cannot cope with the higher intensity levels.

  • For each fatty acid molecule metabolised, 130 molecules of ATP are produced. $(1:130)$

• Duration that the system can operate

  • This energy system can produce ATP continuously for well over an hour.

  • In fact, it may not have a limit as long as fuel sources can be found (you will die if this energy system cannot be used).

  • However, your muscle glycogen will deplete after about an hour of exercise, which will result in an increased need for oxygen as fats become the dominant fuel source and uses more oxygen per ATP produced than CHO.

• Cause of Fatigue

  • Though this system does not need to stop, a reduction in intensity will occur when CHO stores deplete.

  • Since fats require more oxygen to produce ATP than CHO, an athlete will normally decrease their intensity when their main fuel source switches from CHO to fats.

  • This is often called