Final answer:
Myoglobin is a protein in muscle cells that D. stores oxygen for aerobic respiration, which is essential for sustained energy production during moderate activity.
Step-by-step explanation:
Myoglobin is a protein found in muscle tissues that plays a critical role in muscle metabolism. Unlike hemoglobin, which transports oxygen between the lungs and other tissues, myoglobin's primary function is to store oxygen in muscle cells. This stored oxygen is used for aerobic respiration, a metabolic process that generates energy (in the form of ATP) during sustained, moderate activity. Myoglobin has a higher affinity for oxygen than hemoglobin, enabling muscles to draw oxygen from the blood and maintain aerobic metabolism when sustaining activity levels.
Detailed study on muscle metabolism reveals that, during short, intense activities, muscles derive ATP from sources like creatine phosphate. However, during longer activities that require sustained energy, aerobic pathways are vital. These pathways are fueled by oxygen stored by myoglobin and result in the breakdown of muscle glycogen and other substrates to produce ATP via the Krebs cycle and chemiosmosis.
To sum up the final answer: Myoglobin stores oxygen in muscle cells to support aerobic respiration, a critical process for sustained muscle activity and energy production.