What stores calcium in muscle cells?

• A. Sarcoplasmic reticulum (SR)
• B. Sarcolemma
• C. Sarcoplasm
• D. Mitochondria

Answer: (A)

Calcium is stored primarily in the sarcoplasmic reticulum, a specialized network of tubules surrounding each muscle fiber’s myofibrils. It keeps calcium ions concentrated inside the SR using active pumps, so the cytoplasm stays low in Ca2+ when the muscle is relaxed. When the muscle is stimulated, Ca2+ channels in the SR release calcium into the cytoplasm, raising its level in the sarcoplasm and enabling the interaction of actin and myosin that drives contraction. After contraction, Ca2+ is pumped back into the SR to reset the cell and end the signal. The other structures serve different roles: the sarcolemma is the cell membrane, the sarcoplasm is the cytoplasm where calcium acts, and mitochondria can take up calcium but are not the primary storage site.