Calcium ions stored in the endoplasmic reticulum play what role in muscle contraction?

• A. DNA replication
• B. Lipid synthesis
• C. Muscle contraction
• D. Hormone secretion

Answer: (C)

Calcium release from the endoplasmic reticulum (the sarcoplasmic reticulum in muscle) triggers contraction by raising the cytosolic Ca2+ concentration. When a muscle fiber is stimulated, Ca2+ is released into the cytosol and binds to troponin C on actin. This binding causes tropomyosin to move away from the myosin-binding sites on actin, allowing myosin heads to attach and perform the cross-bridge cycles powered by ATP, which shortens the sarcomere and produces contraction. After the signal ends, Ca2+ is pumped back into the sarcoplasmic reticulum to stop the contraction and allow relaxation. Other processes like DNA replication or lipid synthesis aren’t driven by this Ca2+ release in this context, and while Ca2+ can play roles in secretion, the direct, muscle-specific effect of SR Ca2+ release is to enable contraction.