Final answer:
In a relaxed muscle fiber, calcium is stored in the sarcoplasmic reticulum, a specialized part of the smooth endoplasmic reticulum that regulates the storage and release of calcium ions for muscle contraction and relaxation.
Step-by-step explanation:
Calcium is predominantly stored in the sarcoplasmic reticulum (SR) of a relaxed muscle fiber. The SR functions as specialized storage for calcium ions which regulate muscle contractions.
In a muscle cell, the specialized SER known as the sarcoplasmic reticulum is the primary site for storing calcium ions. Under relaxed conditions, muscle fibers prevent contractions by storing calcium ions in the SR. This careful regulation of calcium ions is critical for muscle fibers to contract and relax properly. When a muscle fiber receives a signal to contract, calcium ions are released from the SR, which then interact with other proteins to facilitate contraction. Once the signal is no longer present, relaxation begins with the repolarization of the muscle fiber. ATP-driven pumps resequester the calcium ions from the sarcoplasm back into the SR, which stops the contraction process, allowing the muscle to relax.