The sarcoplasmic reticulum stores _______ when a muscle is relaxed and releases it for binding to _______ during contraction

Question

BlackTom AI · Accepted Answer

Begin by restating the question in my own words to set the context: The sarcoplasmic reticulum (SR) stores a certain ion when the muscle is relaxed and releases it to interact with another molecule during contraction.
Option 1: 'Sodium/troponin' — This pairing is incorrect for the SR's storage role. Sodium ions are not stored in the SR for initiating muscle contraction, and troponin is a calcium-binding protein, not directly related to sodium.
Option 2: 'Sodium/tropomyosin' — Again, sodium is not the key ion stored by the SR for contraction, and tropomyosin’s role is to block or expose myosin-binding sites along actin, not to bind a signaling ion released from the SR. This choice confuses the ionic file with the regulatory proteins involved.
Option 3: 'Calcium/troponin' — This option aligns with established physiology: the SR stores calcium ions (Ca2+) when the muscle is relaxed; during contraction, Ca2+ is released from the SR and binds to troponin, which triggers a conformational change that moves tropomyosin away from myosin-binding sites on actin, enabling cross-bridge cycling. This is the core mechanism of excitation-contraction coupling.
Option 4: 'Calcium/tropomyosin' — Although calcium is indeed the stored ion, it does not bind to tropomyosin directly to initiate contraction. Calcium binds to troponin, not to tropomyosin, which means this option misstates the binding partner and would not correctly describe the molecular sequence leading to contraction.
Summary of the contrasts:
- The first and second options falsely pair ions with troponin or tropomyosin and misidentify the storage or binding partners.
- The fourth option correctly identifies calcium as the ion but incorrectly assigns tropomyosin as the binding target, which would not produce the regulatory change needed for contraction.
- The third option precisely matches the physiological sequence: Ca2+ is released and binds to troponin, enabling the exposure of myosin-binding sites on actin for contraction to proceed.
Therefore, selecting the calcium/troponin combination accurately reflects the established mechanism of calcium release from the SR and its interaction with troponin to regulate muscle contraction.

ANSC20001_2025_SM1 Week 3 - Muscle (Example MCQs)

The sarcoplasmic reticulum stores _ when a muscle is relaxed and releases it for binding to _ during contraction

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ANSC20001_2025_SM1 Week 3 - Muscle (Example MCQs)

The sarcoplasmic reticulum stores _______ when a muscle is relaxed and releases it for binding to _______ during contraction

View Explanation

Log in for full answers

Similar Questions

Question at position 24 In the sliding filament model of muscle contraction, the myofilaments slide over each other, resulting in the overlapping of actin and __________. myosin actin tropomyosin troponin

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More Practical Tools for Students Powered by AI Study Helper

Making Your Study Simpler

The sarcoplasmic reticulum stores _ when a muscle is relaxed and releases it for binding to _ during contraction

Application-Level Multiple-Choice Items Magnesium is an important mineral that works with calcium in the body to:

The antagonist muscle(s) passively lengthen during the lifting phase and passively shorten during the lowering phase. To lift (or push or pull) a resistance the agonists are _concentrically contracting, to control the resistance back down the agonists are eccentrically __ contracting.