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    So I'm a bit confused about the details of muscle contraction.

    I know that Ca2+ binds to troponin C, causing troponin I to move and expose the actin-myosin binding site, but then I don't understand the next steps, I'm confused about where ATP is relevant and what binds where etc.

    If anyone could clearly explain it that would be really helpful!

    Thanks
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    (Original post by white_o)
    So I'm a bit confused about the details of muscle contraction.

    I know that Ca2+ binds to troponin C, causing troponin I to move and expose the actin-myosin binding site, but then I don't understand the next steps, I'm confused about where ATP is relevant and what binds where etc.

    If anyone could clearly explain it that would be really helpful!

    Thanks
    Okay, I'll take you through this step by step:

    1) Myosin heads bind to actin when the myosin binding sites are exposed by calcium binding to troponin.
    2) An ADP molecule that was bound to the myosin head detaches, causing the myosin head to change its shape (its "conformation") and thus pull the actin with it. This is the "power stroke" and it pulls the Z-lines of the sarcomere together.
    3) Once the myosin head has changed shape, ATP binds to the head and causes it to detach.
    4) ATP hydrolyses and reverts the myosin head back to its original position.
    5) You're left with a myosin head with an ADP attached, so the cycle continues until the muscles are no longer being stimulated.

    So, ATP is actually used to unbind the myosin heads from the actin, that's why when you die, you get rigor mortis, because you don't have any ATP left to stimulate the myosin heads to detach from the actin, so your muscles stay contracted.
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    (Original post by white_o)
    So I'm a bit confused about the details of muscle contraction.

    I know that Ca2+ binds to troponin C, causing troponin I to move and expose the actin-myosin binding site, but then I don't understand the next steps, I'm confused about where ATP is relevant and what binds where etc.

    If anyone could clearly explain it that would be really helpful!

    Thanks
    Hi there,

    I'm also revising this topic and I've written it out into nice little stages to help me remember.

    Step 1: An action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-Tubules to the sarcoplasmic reticulum.

    Step 2: This causes the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm

    Step 3: Calcium ions bind to troponin C, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.

    Step 4: This exposes the binding site, which allows the myosin head to bind.

    Step 5: The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge.

    Step 6: Calcium ions activate the enzyme ATPase, which breaks down ATP (into ADP + Pi ) to provide the energy needed for muscle contraction

    Step 7: The energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action.

    Step 8: ATP also provides the energy to break the action-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved.

    Step 9: The myosin head then reattached to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated.

    Step 10: Many actin-myosin cross bridges form and break very rapdily, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract.

    It's a long process but once you read it over a few times it makes sense
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    (Original post by AortaStudyMore)
    Okay, I'll take you through this step by step:

    1) Myosin heads bind to actin when the myosin binding sites are exposed by calcium binding to troponin.
    2) An ADP molecule that was bound to the myosin head detaches, causing the myosin head to change its shape (its "conformation" and thus pull the actin with it. This is the "power stroke" and it pulls the Z-lines of the sarcomere together.
    3) Once the myosin head has changed shape, ATP binds to the head and causes it to detach.
    4) ATP hydrolyses and reverts the myosin head back to its original position.
    5) You're left with a myosin head with an ADP attached, so the cycle continues until the muscles are no longer being stimulated.

    So, ATP is actually used to unbind the myosin heads from the actin, that's why when you die, you get rigor mortis, because you don't have any ATP left to stimulate the myosin heads to detach from the actin, so your muscles stay contracted.
    Thank you! So the ATP is only used for energy to revert the myosin back to its 'resting' state? and not for the power stroke? Also, what about the inorganic phosphate produced by hydrolysis of ATP?
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    (Original post by AshEntropy)
    Hi there,

    I'm also revising this topic and I've written it out into nice little stages to help me remember.

    Step 1: An action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-Tubules to the sarcoplasmic reticulum.

    Step 2: This causes the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm

    Step 3: Calcium ions bind to troponin C, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.

    Step 4: This exposes the binding site, which allows the myosin head to bind.

    Step 5: The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge.

    Step 6: Calcium ions activate the enzyme ATPase, which breaks down ATP (into ADP + Pi ) to provide the energy needed for muscle contraction

    Step 7: The energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action.

    Step 8: ATP also provides the energy to break the action-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved.

    Step 9: The myosin head then reattached to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated.

    Step 10: Many actin-myosin cross bridges form and break very rapdily, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract.

    It's a long process but once you read it over a few times it makes sense
    Thanks!
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    (Original post by AshEntropy)
    Hi there,

    I'm also revising this topic and I've written it out into nice little stages to help me remember.

    Step 1: An action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-Tubules to the sarcoplasmic reticulum.

    Step 2: This causes the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm

    Step 3: Calcium ions bind to troponin C, causing it to change shape. This pulls the attached tropomyosin out of the actin-myosin binding site on the actin filament.

    Step 4: This exposes the binding site, which allows the myosin head to bind.

    Step 5: The bond formed when a myosin head binds to an actin filament is called an actin-myosin cross bridge.

    Step 6: Calcium ions activate the enzyme ATPase, which breaks down ATP (into ADP + Pi ) to provide the energy needed for muscle contraction

    Step 7: The energy released from ATP moves the myosin head, which pulls the actin filament along in a kind of rowing action.

    Step 8: ATP also provides the energy to break the action-myosin cross bridge, so the myosin head detaches from the actin filament after it's moved.

    Step 9: The myosin head then reattached to a different binding site further along the actin filament. A new actin-myosin cross bridge is formed and the cycle is repeated.

    Step 10: Many actin-myosin cross bridges form and break very rapdily, pulling the actin filament along, which shortens the sarcomere, causing the muscle to contract.

    It's a long process but once you read it over a few times it makes sense
    That was nicely written! Except the ATP doesn't provide energy for the power stroke, it provides energy for the recovery stroke. The power stroke is a passive process, because the myosin head is already in a high energy state, it is simply reverting to a lower energy state by carrying out the power stroke. So in a way, ATP does provide the energy for the power stroke, but not in the way you describe d in steps 7, 8 and 9
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    (Original post by white_o)
    Thank you! So the ATP is only used for energy to revert the myosin back to its 'resting' state? and not for the power stroke? Also, what about the inorganic phosphate produced by hydrolysis of ATP?
    Okay you've got things slightly muddled, which is understandable because I was the same in year 13! The ATP hydrolysis puts the myosin head in an active high energy conformation (shape). This is the shape that it starts off in, it is essentially primed and ready to fire when needed. When the muscle is stimulated, the cross bridges can form and the ADP (and inorganic phosphate) detach, allowing the myosin head to change shape into a lower energy state (which is the energy state that it wants to be in because everything in the universe wants to be in the lowest energy state possible). When ATP binds to the myosin head, the cross bridges break and when ATP is hydrolysed it gives the myosin head energy again to revert to its high energy state
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    (Original post by AortaStudyMore)
    That was nicely written! Except the ATP doesn't provide energy for the power stroke, it provides energy for the recovery stroke. The power stroke is a passive process, because the myosin head is already in a high energy state, it is simply reverting to a lower energy state by carrying out the power stroke. So in a way, ATP does provide the energy for the power stroke, but not in the way you describe d in steps 7, 8 and 9
    Oh dear, my revision book is only teaching me half-the-truth
 
 
 
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