Which structure within the muscle fiber stores calcium ions necessary for muscle contraction?
T-tubule
Mitochondria
Myofibril
Sarcoplasmic reticulum
The Correct Answer is D
A. T-tubule: T-tubules (transverse tubules) transmit action potentials into the muscle fiber and help trigger calcium release but do not store the bulk of calcium.
B. Mitochondria: Mitochondria produce ATP required for contraction and relaxation but are not the primary reservoir for calcium used in the contraction cycle.
C. Myofibril: Myofibrils are the contractile elements composed of sarcomeres (actin and myosin) and do not serve as the calcium storage organelle.
D. Sarcoplasmic reticulum: The sarcoplasmic reticulum is a specialized endoplasmic reticulum in muscle fibers that stores and releases calcium ions to initiate and terminate contraction.
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Related Questions
Correct Answer is A
Explanation
A. To bind to troponin, allowing cross-bridge formation:Calcium ions bind to troponin, causing a conformational change that exposes myosin-binding sites on actin and permits cross-bridge cycling.
B. To stimulate the release of acetylcholine:Calcium influx into the presynaptic nerve terminal triggers acetylcholine release, but the primary role of calcium in muscle contraction is within the muscle fiber at troponin.
C. To provide energy for muscle contraction:Energy for contraction is supplied by ATP, not by calcium ions.
D. To transport oxygen to muscle cells:Oxygen transport is the role of hemoglobin and myoglobin, not calcium ions.
Correct Answer is B
Explanation
A. Absorbing shock:Shock absorption is mainly performed by articular cartilage and synovial fluid rather than ligaments.
B. Providing joint stability:Ligaments are strong fibrous bands that connect bones to bones, maintaining alignment and stability during movement.
C. Connecting muscles to bones:This function is carried out by tendons, not ligaments.
D. Lubricating the joint:Lubrication is provided by synovial fluid within the joint capsule, not by ligaments.
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