Final answer:
Muscle cells can recharge ADP into ATP using creatine phosphate for quick bursts of energy, lasting about 15 seconds. After that, muscles turn to glycogen for ATP production, lasting about 90 seconds, with lactic acid as a by-product leading to soreness. For sustained energy, aerobic respiration will take over if oxygen is available.
Step-by-step explanation:
During short intense bouts of exercise, muscle cells can recharge ADP (adenosine diphosphate) back into ATP (adenosine triphosphate) primarily using a molecule called creatine phosphate. When exercise begins and energy demands increase, creatine phosphate donates its phosphate group to ADP, creating ATP and creatine. This process, facilitated by creatine kinase, is very quick and serves as the initial energy source for muscular contraction, providing energy for roughly 15 seconds.
After the creatine phosphate stores are depleted, the muscle uses glycogen to produce ATP through a series of about 12 chemical reactions, generating ATP at a slower rate than creatine phosphate. This glycogen-derived ATP can sustain muscle activity for about 90 seconds, all happening anaerobically, meaning without the use of oxygen. A by-product of this anaerobic ATP production is lactic acid, which can lead to muscle fatigue and soreness known as the stitch.
If oxygen is present, the muscles will then switch to aerobic respiration, primarily breaking down glucose to generate a steady supply of ATP for longer-term energy needs. However, this process takes longer and is not the primary source of ATP in short, high-intensity exercise bouts.