Phosphagen System

Phosphagens are energy storing compounds that are chiefly found in muscle and nervous tissue in animals

  • They function as an immediate access reserve of high energy phosphates that can be used to make ATP
  • Phosphagens are found in tissues that experience rapidly changing energy needs (muscles and nerves)

A common phosphagen used by animals (including humans) is phosphocreatine (or creatine phosphate – CP)

  • At rest, ATP is hydrolysed to ADP and the phosphate is transferred to creatine to make phosphocreatine
    • This occurs in the mitochondria, where ATP levels are high (thus driving the reaction)
  • During exercise, phosphocreatine is hydrolysed and the phosphate is released to make ATP from ADP
    • This occurs in the muscles, where ADP levels will be high (thus driving the reaction)

ATP–CP Cycle

ATP-CP cycle

Energy Systems

Phosphocreatine synthesises a pool of ATP more rapidly than cell respiration, but reserves are quickly depleted

  • The phosphagen system will be used for the first 10 – 12 seconds of intense exercise (e.g. a sprinter)

Anaerobic respiration provides a more sustained pool of ATP, but produces lactic acid as a by-product

  • Anaerobic respiration (i.e. glycolysis) will be used for the first 1 – 2 minutes of exercise (e.g. 400 m swimmer)

Aerobic respiration requires a constant supply of oxygen, but can produce a large yield of ATP

  • Aerobic respiration will be used for long-distance and less intense exercise activities (e.g. marathons)

Energy System Comparison

energy systems