3.6.3 Muscle contraction

Question 1

Describe one example of how muscles act in antagonistic pairs.

Answer 1

E.g.

• The biceps and triceps work together to move an arm.
• As one muscle contracts, the other relaxes.

Question 2

What are transverse (T) tubules?

Answer 2

Inward foldings of sarcolemma (cell membrane) across the muscle fibres, which stick into the sarcoplasm.

Question 3

Name the two proteins that make up myofibrils.

Answer 3

Myosin and Actin

Question 4

Describe how the lengths of the different bands in a myofibril change during muscle contraction.
[2 marks]

Answer 4

• The A-bands stay the same length during contraction.

- The I-bands get shorter.

Question 5

Describe one way that ATP can be generated in contracting muscles.

Answer 5

One of three:

• Aerobic respiration: via oxidative phosphorylation in the cell’s mitochondria.
• Anaerobic respiration: made rapidly by glycolysis.
• ATP-Phosphocreatine (PCr) System: made by phosphorylating ADP.

Question 6

State three differences between slow and fast-twitch skeletal muscle fibres.

Answer 6

Slow-twitch muscle fibres:

• contract slowly.
• can work for a long time without getting tired.
• energy’s released slowly through aerobic respiration.

Fast-twitch muscle fibres:

• contract very quickly
• get tired very quickly
• energy’s released quickly through anaerobic respiration using glycogen.

Question 7

Rigor mortis is the stiffening of muscles in the body after death. It happens when ATP reserves are exhausted.
Explain why a lack of ATP leads to muscles being unable to relax.
[3 marks]

Answer 7

• Muscles need ATP to relax because ATP provides the energy to break the actin-myosin cross bridges.
• If the cross bridges can’t be broken, the myosin heads will remain attached to the actin filaments,…
• …so the actin filaments can’t slide back to their relaxed position so the muscle stays contracted.

Question 8

Bepridil is a drug that blocks calcium ion channels.
Describe and explain the effect this drug will have on muscle contraction.
[3 marks]

Answer 8

• The muscles won’t contract…
• …because calcium ions won’t be released into the sarcoplasm, so tropomyosin will continue to block the actin-myosin binding sites.
• This means no actin-myosin cross bridges can be formed.

Question 9

What is meant by an antagonistic pair of muscles?

Answer 9

• A pair of muscles that work together to move a bone.

- One muscle in the pair relaxes as the other contracts.

Question 10

Describe the structure and function of T-tubules.

Answer 10

• The T-tubules are parts of the sarcolemma that fold inwards across the muscle fibre and stick to the sarcoplasm.
• They help to spread electrical impulses across throughout the sarcoplasm so they reach all parts of the muscle fibre.

Question 11

Why do muscle fibres contain lots of mitochondria?

Answer 11

To provide the ATP that’s needed for muscle contraction.

Question 12

Describe the structure of an A-band in a myofibril and describe its appearance under an electron microscope.

Answer 12

• An A-band contains myosin filaments and some overlapping actin filaments.
• Under an electron microscope it appears as a dark band.

Question 13

What is the sliding filament theory of muscle contraction?

Answer 13

Myosin and actin filaments slide over one another to make the sarcomeres contract.

(the myofilaments themselves don’t contract)