Question

which organelle of the skeletal muscle cell is responsible for releasing stored calcium ions upon electrical stimulation?
a.) sarcoplasm
b.) sarcolemma
c.) myofibril
d.) sarcoplasmic reticulum (sr)

Explanation:

The sarcoplasmic reticulum (SR) in skeletal muscle cells stores calcium ions. When an electrical stimulus (action potential) reaches the muscle cell, the SR releases these stored calcium ions. This release is crucial for muscle contraction as calcium binds to troponin, causing a conformational change that allows myosin heads to interact with actin filaments.

• Sarcoplasm is the cytoplasm of the muscle cell, not specifically for calcium release.
• Sarcolemma is the plasma membrane of the muscle cell, involved in conducting action potentials but not calcium storage/release.
• Myofibrils are the contractile units (containing actin and myosin) within the muscle cell, not the calcium - releasing organelle.

Answer:

d. Sarcoplasmic Reticulum (SR)