Final answer:
Calcium ions (Ca++) are the molecules that are actively transported in and out of the sarcoplasmic reticulum to facilitate muscle fiber contraction and relaxation in skeletal muscles.
Step-by-step explanation:
Calcium ions (Ca++) are the molecules that pass in and out of the sarcoplasmic reticulum during muscle contraction and relaxation. They play a crucial role in the process of muscle fiber contraction and subsequent relaxation.
In a muscle fiber, as an action potential travels along the sarcolemma and into the T-tubules, it triggers the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. This release of Ca++ initiates the contraction process by binding to the regulatory proteins troponin and tropomyosin, which in turn exposes binding sites on the actin filaments for myosin to attach and form cross-bridges, leading to muscle fiber contraction. For relaxation to occur, these calcium ions are actively pumped back into the sarcoplasmic reticulum using ATP, leading to the detachment of myosin from actin and the muscle fiber returning to its relaxed state. This cyclical process is essential for the control and function of skeletal muscle fibers.