ATP has to be made by adding a phosphate group (Pi) to ADP. In the ATP-phosphocreatine system, the phosphate group comes from phosphocreatine stored inside the muscle fibre cells. An enzyme called creatine kinase breaks down phosphocreatine into Pi and creatine, which releases energy. The energy is used to add Pi to ADP. This system generates ATP very quickly, but only for a few seconds because the phosphocreatine store quickly runs out. It is an anaerobic process and does not produce any lactate, and is used for very short (10 second) bursts of high intensity activity.
Creatine is broken down into creatinine in muscle fibre cells, and creatinine diffuses into the blood for excretion via the kidneys. Therefore it can be found in both blood and urine and can be used as a marker of kidney health.
In anaerobic respiration, ATP is made during glycolysis. This is a good way to rapidly produce ATP in the absence of oxygen, however the build up of lactate means that muscles fatigue after a short time. Therefore anaerobic respiration is good for short bursts of intense exercise.
Aerobic respiration produces large amounts of ATP, mostly during the oxidative phosphorylation stage. However, a ready supply of oxygen is required, so aerobic respiration is best during long periods of low intensity exercise. Of course there is often a combination of all three sources of ATP at work during different activities.
- The ATP-phosphocreatine system uses stored phosphocreatine as a source of phosphate groups to rapidly produce ATP for muscle contraction.
- Anaerobic and aerobic respiration are also sources of ATP for muscle contraction. Aerobic respiration requires oxygen so is good for low intensity activity.