7.2

Question 1

Describe the structure of a myofibril

Answer 1

Long tubes of sarcomeres linked together, made of myosin and actin

Question 2

Describe the structure of a sarcomere

Answer 2

One small section of a myofibril between two consecutive Z lines, with a centre m line. Contains parallel myosin and actin filaments.

Question 3

Explain the role of calcium ions in muscle contraction

Answer 3

Calcium ions are released by sarcoplasmic reticulum (Ca2+ channels open) , to increase calcium concentration.
The ions bond to troponin, which then releases tropomyosin from the actin strands, allowing myosin heads to bind to the troponin and cause the overlap of the filaments

Question 4

Describe the sequence of events from the arrival of a nerve impulse at the neuromuscular junction to the contraction of the muscle

Answer 4

• a nerve impulse reaches a neuromuscular junction
• the impulse causes the sarcoplasmic reticulum to release Ca ions into muscle
• Ca ions bind to troponin, unblocking the tropomyosin and exposing actin binding sites
• ATP binds to myosin, which is broken down to ADP and Pi, causing myosin heads to “cock” to high energy position
• myosin binds to actin to form a cross bridge
• myosin heads then move to low energy position, using a power stroke to pull the actin filaments, and then release ADP and Pi

Question 5

Explain what would need to happen in order to get a muscle to relax following contraction

Answer 5

Ca concentration should decrease, to allow tropomyosin to recover the actin binding sites and prevent myosin and actin binding together, allowing muscle to naturally stretch back to relaxed length