Final answer:
Creatine kinase catalyzes the transfer of a phosphate group from creatine phosphate to ADP, rapidly regenerating ATP for initial muscle contraction efforts. The energy from creatine phosphate lasts only briefly, after which muscles turn to glycolysis for ATP production.
Step-by-step explanation:
Muscle contraction requires the fast regeneration of ATP, which is achieved by creatine kinase using creatine phosphate. When muscle cells are at rest, ATP converts creatine into ADP and creatine phosphate, which acts as a store of energy. ATP is formed when creatine phosphate returns its phosphate group to ADP during muscle contraction. This process happens quickly and is appropriate for the first energy bursts needed for muscle contraction, like when lifting weights or running. But this energy source is finite—it lasts for only 8–10 seconds—and the muscle must then turn to glycolysis to keep making ATP.
Anaerobic respiration, or longer-term glycolysis, is a method of generating ATP without the need for oxygen. It is capable of extending the time that creatine phosphate is available for muscle contraction. Muscles resort to breaking down glucose and glycogen through a process called glycolysis when the ATP generated by creatine phosphate runs low.