Creatine phosphate shuttles high-energy phosphate from mitochondria to sites of muscle contraction. Careful observation of ATP levels in red striated muscle has shown that the provision of energy is more complicated than it might appear at first. The amount of ATP needed for a single contraction may be greater than all the ATP immediately available to a sarcomere. Yet even after relatively long exercise, ATP levels in the sarcomeres remain essentially constant. Only after extreme exhaustion do ATP levels begin to fall. This finding suggests that ATP is an intermediary, and not the ultimate, energy storage compound in these muscles. Indeed, it has been known for many years that the high-energy compound steadily depleted during muscular activity is creatine phosphate. An energy-rich muscle has lots of creatine phosphate, whereas a fatigued muscle has little creatine phosphate, and also has decreased ATP and increased ADP and AMP levels.

As its high phosphate transfer potential suggests (see Figure 3.7), this compound is capable of phosphorylating ADP very efficiently. The reaction is catalyzed by the enzyme creatine kinase as follows:

Creatine + ATP <=> Creatine Phosphate + ADP

The reaction is strongly endergonic as written. However, the level of ATP is very high in mitochondria, so the reaction proceeds to the right. Creatine phosphate then diffuses from mitochondria to the myofibrils, where it provides the energy for muscle contraction. High levels of ADP formed in the myofibrils during contraction favor the reverse reaction namely, resynthesis of ATP - at the expense of creatine phosphate cleavage to creatine. This example shows that one must consider not only the standard free energy change but also the actual concentrations of all reactants and products when predicting the direction of a reaction in vivo.

Whether energy is stored as ATP or as a compound like creatine phosphate, that energy must eventually be made available as chemical energy, if it is to drive the synthesis of other high-energy compounds. It also can be transduced to other forms of energy, including mechanical energy or electrical energy. Transduction to mechanical energy occurs in muscle contraction or ciliary motion, whereas transduction to electrical energy occurs in membrane depolarization or in pumping ions across a membrane.