Enzymatic Interconversion of PCr and ATP
Legend: ATP (Adenosine TriPhosphate) is the energy storage molecule of all cells. After donating its energy allotment to the contractile apparatus of muscle, ATP becomes ADP (Adenosine DiPhosphate). During intense physical exertion ATP is rapidly recreated from ADP by the donation of a phosphate group from Phosphocreatine (PCr).
Creatine kinase is the enzyme responsible for swapping the phosphate groups (shown in red) between PCr and ATP. The upward reaction predominates during strenuous exercise when energy (ATP) is needed to fuel explosive movements. The downward reactions primarily takes place during moments of rest and recreates our PCr reserves. The larger our PCr reserves, the longer we can sustain intense muscular activity. This is the basic premise behind creatine supplementation.
Creatine and PCr later spontaneously degrade into creatinine, which is able to escape from the cell.
The production of ATP from PCr has the added advantage that it also neutralizes muscle acidity (H+). This effect should heighten our fatigue threshold during moments of intense physical exertion.
Spontaneous one-way conversion of PCr and Creatine
Legend: The presence of charged groups on creatine and PCr prevent them from transversing the muscle membrane. Creatinine, by contrast, has no charge and is thus able to freely slip through the muscle membrane escaping to the outside. This leakage pathway sets our requirement for new creatine (~2 grams per day, also see Chapter 4 of my creatine guide for greater explanation).
Also note that most of the muscle creatine is in the form of PCr, which also degrades at twice the rate of creatine. Hence, degradation rates should be quite appreciable under resting conditions.