Which statement about calcium ions in muscle contraction is correct?

• A. Calcium release from the sarcoplasmic reticulum triggers cross-bridge cycling
• B. Calcium binds to troponin to cause immediate shortening
• C. Calcium is stored in mitochondria to provide energy for contraction
• D. Calcium has no role in contraction

Answer: (A)

Calcium release from the sarcoplasmic reticulum is the trigger that starts contraction by allowing cross-bridge cycling to proceed. When an action potential stimulates a muscle fiber, calcium is released into the cytosol. This calcium binds to troponin C on the actin filament, causing the tropomyosin-troponin complex to move and expose the myosin-binding sites on actin. With those sites open, myosin heads can attach, perform their power strokes, and cycle using ATP to shorten the sarcomere.

The other statements miss key details: calcium isn’t stored in mitochondria to provide energy—ATP supplies the energy for contraction; calcium’s role isn’t to cause immediate shortening by itself without the regulatory steps, and calcium absolutely has a crucial role in contraction, not none.