ATP and Phosphocreatine

Most ATP is generated via oxidative phosphorylation in the mitochondria.

Only occurs when there is oxygen present. 

ATP is made rapidly by glycolysis.

The end product is glycolysis is pyruvate, which is converted into lactate by lactate fermentation. 

Lactate can quickly build up in the muscles causing fatigue.

Good for short periods of hard exercise.

ATP is made by phosphorylating ADP, adding a phosphate from PCr.

PCr is stored inside cells, ATP is generated quickly.

PCr runs out quickly after short bursts of vigourous exercise. 

ATP-PCr system is anaerobic.