The Sliding Filament Model

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What can you physically see happen to the different bands and zones when a muscle contracts?
Light band becomes narrower, H-zone becomes narrower and the Z-lines move closer together (sarcomere is shorter)
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What is the structure of myosin?
A myosin filament is lots of myosin molecule tails aligned together with the globular heads protruding out
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What is the structure of actin?
2 filaments of actin twisted around each other, there are myosin binding sites but these are blocked by 2 proteins (tropomyosin and troponin), when muscles relaxed the binding sites are blocked
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What happens when an action potential reaches a neuromuscular junction?
It stimulates the calcium ion channels to open, so Ca2+ ions move into the presynaptic knob causing the vesicles containing acetylcholine be released into synaptic cleft by exocytosis, the ATH binds to receptors on the sarcolemma which depolarises it
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What happens after the sarcolemma is depolarised to cause the myosin to bind to the actin
Na+ ions depolarise sarcolemma, stimulates sarcoplasmic reticulum which floods sarcoplasm with Ca+ ions which bind to the tropomyosin, this moves tropomyosin away so myosin heads can bind to actin sites (actin-myosin bridges formed)
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Once the myosin heads are bound to the actin, how does it cause the muscle to contract?
The myosin heads flex, pulling the actin filament along, molecule of ADP on myosin released so ATP can attach to it, which causes the head to detach, then the heads reattach onto the actin further down and the process starts again
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Why are the calcium ions also essential for the binding and unbinding of the myosin heads?
The calcium ions activate ATPase activity in myosin which hydrolyses the ATP into ATP and the myosin head uses the energy released to return to is original position
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What are the 3 main ways ATP is generated in the muscles
Aerobic respiration, anaerobic respiration and creatine phosphate
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Aerobic respiration
There are lots of mitochondria in the sarcoplasm and most of the ATP used by the muscle is regenerated during oxidative phosphorylation, this only occurs in presence of O2 so long periods of low-intensity exercise
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Anaerobic respiration
When O2 is used up faster than the blood can replace it, ATP is generated anaerobically in glycolysis, producing pyruvate which is converted into lactate (which builds up and causes muscle fatigue), short periods of high-intensity exercise
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Creatine phosphate
CP is a chemical stored in the muscles, it acts as a reserve supply of phosphate so ADP can be phosphorylated into ATP, this store is used up quickly so is used in bursts of vigorous exercise and CP is replenished when muscles are relaxed
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Card 2

Front

What is the structure of myosin?

Back

A myosin filament is lots of myosin molecule tails aligned together with the globular heads protruding out

Card 3

Front

What is the structure of actin?

Back

Preview of the front of card 3

Card 4

Front

What happens when an action potential reaches a neuromuscular junction?

Back

Preview of the front of card 4

Card 5

Front

What happens after the sarcolemma is depolarised to cause the myosin to bind to the actin

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