Question

calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. where are calcium ions stored within the fiber?
calcium ions are stored in the transverse tubules.
calcium ions are stored in the nuclei.
calcium ions are stored in the sarcoplasmic reticulum.
calcium ions are stored in the mitochondria.

Explanation:

In skeletal muscle fibers, calcium ions are stored in the sarcoplasmic reticulum. The sarcoplasmic reticulum is a specialized endoplasmic - reticulum like structure in muscle cells that releases calcium ions during muscle excitation - contraction coupling, which is essential for muscle contraction. Transverse tubules are involved in transmitting the electrical signal, nuclei contain genetic material, and mitochondria are the power - houses of the cell for energy production, not calcium storage sites.

Answer:

Calcium ions are stored in the sarcoplasmic reticulum.