3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors

Question 1

What does it mean for muscles to act in antagonistic pairs?

Answer 1

Antagonistic pairs are pairs of muscles that work against each other; as one contracts (agonist), the other relaxes (antagonist). This allows controlled movement.

Question 2

What does it mean for the skeleton to be incompressible?

Answer 2

The skeleton cannot be compressed, meaning it provides a stable framework for muscle action to exert force and create movement.

Question 3

Describe the gross structure of skeletal muscle.

Answer 3

Skeletal muscle consists of bundles of muscle fibres held together by connective tissue. Each fibre is a single multinucleated cell.

Question 4

Describe the microscopic structure of skeletal muscle.

Answer 4

Skeletal muscle is composed of myofibrils arranged in repeating units called sarcomeres, which give the muscle its striated appearance.

Question 5

What is a myofibril?

Answer 5

A myofibril is a long, cylindrical organelle found in muscle fibres, composed of repeating sarcomeres containing actin and myosin filaments.

Question 6

Describe the ultrastructure of a sarcomere.

Answer 6

A sarcomere is the functional unit of a myofibril, bordered by Z-lines. It contains thin actin filaments and thick myosin filaments, with a central M-line and an H-zone.

Question 7

What is the role of actin in muscle contraction?

Answer 7

Actin is a thin filament that interacts with myosin to form actinomyosin bridges. It has binding sites for myosin heads, which are revealed when tropomyosin is displaced by calcium ions.

Question 8

What is the role of myosin in muscle contraction?

Answer 8

Myosin is a thick filament with heads that form cross-bridges with actin. Myosin heads perform a power stroke by hydrolysing ATP, pulling actin filaments towards the M-line.

Question 9

What is the role of calcium ions in muscle contraction?

Answer 9

Calcium ions bind to a protein complex, causing tropomyosin to move away from actin binding sites, allowing actinomyosin bridges to form.

Question 10

What is the role of ATP in muscle contraction?

Answer 10

ATP binds to myosin heads, causing them to detach from actin. Its hydrolysis provides energy for the myosin head to reset and perform another power stroke.

Question 11

What is the role of tropomyosin in muscle contraction?

Answer 11

Tropomyosin is a protein that blocks the binding sites on actin in a relaxed muscle, preventing cross-bridge formation until calcium ions are present.

Question 12

What is the role of phosphocreatine in muscle contraction?

Answer 12

Phosphocreatine provides a reserve of phosphate groups to quickly regenerate ATP from ADP during high-intensity, short-duration exercise.

Question 13

Describe the structure of slow muscle fibres.

Answer 13

Slow muscle fibres have many mitochondria, a high capillary density, and a high concentration of myoglobin, making them red in colour.

Question 14

Describe the structure of fast muscle fibres.

Answer 14

Fast muscle fibres have fewer mitochondria, a lower capillary density, and less myoglobin, making them white in colour.

Question 15

What are the general properties of slow muscle fibres?

Answer 15

Slow fibres contract slowly, are resistant to fatigue, and are adapted for aerobic respiration. They are used in endurance activities.

Question 16

What are the general properties of fast muscle fibres?

Answer 16

Fast fibres contract quickly, fatigue rapidly, and are adapted for anaerobic respiration. They are used in short bursts of activity.

Question 17

Describe the roles of ATP in muscle contraction.

Answer 17

1. To break actinomyosin (bridges);
2. To move/bend the myosin head/arm;
3. (So) actin (filaments) are moved (inwards);
4. For active transport of calcium ions (into the sarcoplasmic/endoplasmic reticulum);

Question 18

Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.

Answer 18

1. (Less/No) tropomyosin moved from binding site
   OR
   Shape of tropomyosin not changed so binding site not exposed/available;
   Ignore troponin.
   Reject active site only once.
2. (Fewer/No) actinomyosin bridges formed;
   Accept actin and myosin do not bind.
   Reject active site only once.
3. Myosin head does not move
   OR
   Myosin does not pull actin (filaments)
   OR
   Less/No) ATP (hydrol)ase (activation);
   Reject ATP synthase.

Question 19

Describe the roles of calcium ions in muscle contraction.

Answer 19

1. Calcium ions diffuse into myofibrils from (sarcoplasmic) reticulum;
2. (Calcium ions) cause movement of tropomyosin (on actin);
3. (This movement causes) exposure of the binding sites on the
   actin;
4. Myosin heads attach to binding sites on actin;

Question 20

What is the role of ATP in myofibril contraction? (2)

Answer 20

1. (Reaction with ATP) breaks/allows binding of myosin to actin/
   actinomyosin bridge;
2. Provides energy to move myosin head;

Question 21

Explain the banding pattern shown in a single sarcomere.

Answer 21

1. Light/I band only actin;
2. H zone/band only myosin;
3. Darkest/overlapping region actin and myosin;