ATP: The body's energy currency
The only useable source of energy in the body is a compound found in the muscle cells called Adenosine Triphosphate (ATP). All forms of biological work (digestion, production of hormones, transmission of nerve impulses, manufacture and repair of tissue) within the body require energy in the form of ATP.
All sources of energy found in the food that we eat have to be converted into ATP before the potential energy in them can be used. ATP is a high energy compound made up of adenosine (adenine and ribose) and three phosphates.
Image showing the basic structure of ATP:
How does ATP release energy?
Special high-energy bonds exist between the phosphate groups. Energy is released by breaking the bond between the second and third phosphate. ATP is broken down into adenosine diphosphate (ADP) and a free phosphate (Pi) releasing the stored energy. This energy can then used to power the muscle cells.
Image showing the breakdown of ATP to release energy:
The resynthesis of ATP
The quantity of ATP in the muscles is very limited (approximately 2 seconds) so once this breakdown has occurred the ADP needs to be resynthesised (reformed) back into ATP to ensure the constant generation of energy within the body. To do this energy is required to rejoin a phosphate to the ADP to reform ATP. We get this energy from the breakdown of food we have eaten.
Image showing the breakdown of ATP into ADP and the resynthesis of ATP from ADP:
The process by which ATP is resynthesised is called the ATP cycle, and it is the body’s energy systems that are responsible for releasing the energy from food.
The 3 Energy Systems
There are 3 energy systems the body can use to resynthesise ATP:
- PC System
- Lactic Acid System
- Aerobic System
The PC system
The phosphocreatine (PC) system breaksdown a substance called creatine phosphate (found in the muscles). The breakdown of this releases the energy to ‘glue’ or resynthesise the third phosphate molecule back on to ADP to reform ATP.
The PC energy system is capable of reforming ATP very quickly, however, because there is only a very small amount of creatine phosphate stored in the muscles (about 8-10 seconds worth) the energy supply is very limited.
This energy system does not require oxygen to work; it is therefore referred to as an anaerobic energy system. It is also the only system that produces no waste products.
This system can only be used for immediate or very short bursts of activity such as throwing a ball or running for a bus. It is used at the start of exercise for activities which require a brief maximal effort, for example, explosive events such as the long jump, javelin or 100m, and is exhausted after about 10 seconds of maximal activity.
The role of the PC system in team sports is to provide energy for very short but very intense activity, for example when driving hard to the basket in basketball or when charging down an attacker in football.
As energy demands continue after the initial burst of activity the body has to utilise other energy systems, such as the lactic acid system, to continue to make more ATP.
The Lactic Acid System
When the PC stores have run out, energy to resynthesise ATP is provided by the lactic acid system. This system relies on the breakdown of glucose (from carbohydrates) which has been stored in the muscles as glycogen.
The process by which glucose is broken down to release energy is called glycolysis. As the energy is needed quickly, and the body does not have time to deliver oxygen to the muscles, the glucose is broken down without oxygen. It is therefore referred to as anaerobic glycolysis.
It is the breakdown of glucose that provides the energy to resyntheise the third phosphate molecule back on to ADP to reform ATP. Glycolysis is far more complex than PC system since it requires many reactions to occur. Since there is no oxygen present in this process lactic acid is formed.
The lactic acid is a waste product which accumulates in the muscles and blood, causing muscular fatigue which interferes with muscular contractions. When this occurs, exercise intensity has to be reduced to enable the lactic acid to be removed.
This system is used at the start of activities where duration of the exercise is short but very intense for instance from ten seconds to three minutes or when an athlete ‘kicks’ at the start or finish of a race. An example of a sporting activity that uses this system is the 400m. Once the exercise has stopped extra oxygen has to be taken in to remove the lactic acid. This is known as repaying the oxygen debt.
The role of the lactic acid system in team sports is to provide energy when the activity extends beyond 10 seconds but the intensity is still high, for example when there are several phases of play in rugby or during a counter attack in hockey.
A video to show how the anaerobic energy systems work:
The aerobic system
When the intensity of exercise is lower and the duration of exercise longer, energy is provided by the aerobic system. This system differs from the previous two as it uses oxygen to break down glucose or fat to provide the energy to resynthesise ATP. Protein can be used but only tends to be used when glycogen and fat stores are particularly low (i.e. in extreme situations such as starvation).
As oxygen is involved in this process no lactic acid is produced. The only waste products produced are carbon dioxide and water, which do not affect the athletes ability to exercise.
This is a slower but much more efficient system that supplies energy for a longer period of time. It is used when performing sustained activity for longer than three minutes, for example in middle distance swimming, running and cycling, and longer distance events such as rowing, triathalon and road races.
The role of the aerobic system in team sports is to provide for energy recovery after short bursts of activity, for example when getting back into formation after the football has gone out of play or when the football is at the other end of the pitch.
A video to show how the aerobic energy system works:
The interaction of the energy systems
Energy is constantly needed by all of these systems in order for the body to function. For this reason the three energy systems interact with one another.
From very short intense exercise through to very light prolonged activity, all three energy systems make a contribution. However one or two will usually be most dominant.
To get an understanding of how the systems work together we can look at various sports and think about what is happening and why certain energy systems are more or less active.
Try to remember that duration and intensity are the two factors that will determine which system is most active at any given time.
Below is a list of sports and approximate percentages of how much each of the energy systems contributes:
Select one of the team sports listed in the table above and complete the Word document under the Energy Systems section on the LearnZone page (linked below):