Biology · Support and Movement

When muscles need to contract, calcium ions bind with the troponin molecule and cause them to move slightly.
ATP is hydrolysed. This ATP is needed to break the link between the myosin and the actin.

Steps to Muscle Contraction1. A nerve impulse travels to the neuromuscular junction on a muscle cell. The neuromuscular junction is the point where the axons of the nerve meet with the muscle cell.2. Ach is released from the axon to receptors located on the sarcolemma3. The binding Ach causes depolarization of the sarcolemma by opening ion channels and allowing Na+ ions into the muscle cell.4. Na+ ions diffuse into the muscle fibre and depolarization occurs.5. Depolarization creates a wave of action potential (electrical current) across the sarcolemma6. Action potential travels across the sarcolemma and down the T-tubules which triggers the sarcoplasmic reticulum SR to release Ca+7. As Ca+ levels rise, Ca+ ions bind with Troponin which removes the blocking action of Tropomyosin from the Actin binding sites.8. Myosin is now ready to bind with the actin and form cross bridges which begins the contraction process9. In order to contract, ATP binds to the Myosin10. ATP is then hydrolyzed (broken down) to ADP and Pi,which gives the myosin the energy to “cock” its head to the high-energy position.11. Actin and myosin bind together to form a cross bridge12. The myosin heads then pull the actin filaments inward and release the ADP and Pi and return to a low energy position.The myosin is now ready for more ATP to bind and repeat the cycle. This process will continue for as long as there are Ca+ ions and ATP available

Steps to Muscle Contraction1. A nerve impulse travels to the neuromuscular junction on a muscle cell. The neuromuscular junction is the point where the axons of the nerve meet with the muscle cell.2. Ach is released from the axon to receptors located on the sarcolemma3. The binding Ach causes depolarization of the sarcolemma by opening ion channels and allowing Na+ ions into the muscle cell.4. Na+ ions diffuse into the muscle fibre and depolarization occurs.5. Depolarization creates a wave of action potential (electrical current) across the sarcolemma6. Action potential travels across the sarcolemma and down the T-tubules which triggers the sarcoplasmic reticulum SR to release Ca+7. As Ca+ levels rise, Ca+ ions bind with Troponin which removes the blocking action of Tropomyosin from the Actin binding sites.8. Myosin is now ready to bind with the actin and form cross bridges which begins the contraction process9. In order to contract, ATP binds to the Myosin10. ATP is then hydrolyzed (broken down) to ADP and Pi,which gives the myosin the energy to “cock” its head to the high-energy position.11. Actin and myosin bind together to form a cross bridge12. The myosin heads then pull the actin filaments inward and release the ADP and Pi and return to a low energy position.The myosin is now ready for more ATP to bind and repeat the cycle. This process will continue for as long as there are Ca+ ions and ATP available

Steps to Muscle Contraction1. A nerve impulse travels to the neuromuscular junction on a muscle cell. The neuromuscular junction is the point where the axons of the nerve meet with the muscle cell.2. Ach is released from the axon to receptors located on the sarcolemma3. The binding Ach causes depolarization of the sarcolemma by opening ion channels and allowing Na+ ions into the muscle cell.4. Na+ ions diffuse into the muscle fibre and depolarization occurs.5. Depolarization creates a wave of action potential (electrical current) across the sarcolemma6. Action potential travels across the sarcolemma and down the T-tubules which triggers the sarcoplasmic reticulum SR to release Ca+7. As Ca+ levels rise, Ca+ ions bind with Troponin which removes the blocking action of Tropomyosin from the Actin binding sites.8. Myosin is now ready to bind with the actin and form cross bridges which begins the contraction process9. In order to contract, ATP binds to the Myosin10. ATP is then hydrolyzed (broken down) to ADP and Pi,which gives the myosin the energy to “cock” its head to the high-energy position.11. Actin and myosin bind together to form a cross bridge12. The myosin heads then pull the actin filaments inward and release the ADP and Pi and return to a low energy position.The myosin is now ready for more ATP to bind and repeat the cycle. This process will continue for as long as there are Ca+ ions and ATP available

D is correct