Muscle Contraction

Question 1

Describe the structure of a myosin filament.

Answer 1

1. Has hinged globular heads
2. Has a binding site for actin
3. Has a binding site for ATP

Question 2

Describe the structure of an actin filament.

Answer 2

1. Has actin-myosin binding sites

2. Has tropomyosin and troponin nearby

Question 3

What are tropomyosin and troponin?

Answer 3

Proteins

Question 4

What happens when a muscle is relaxed?

Answer 4

1. Tropomyosin blocks the actin-myosin binding site
2. Tropomyosin is held in place by troponin
3. Therefore, the myosin head is prevented from binding to the actin filament so they cannot slide past each other

Question 5

What happens in the first stage of muscle contraction?

Answer 5

1. Action potential from motor neurone stimulates muscle cell
2. Sarcolemma depolarises
3. Sarcoplasmic reticulum releases stored calcium ions into sarcoplasm
4. Calcium ions bind to troponin, causing it to change shape
5. Tropomyosin is pulled out of binding site
6. Binding site is exposed, allowing myosin head to bind
7. Actin myosin cross-bridge forms

Question 6

What happens in the second stage of muscle contraction?

Answer 6

1. Calcium ions activates ATPase
2. ATPase breaks down ATP
3. Energy released moves myosin head, pulling actin along over the top of the myosin filament

Question 7

How does a muscle become relaxed again?

Answer 7

1. Action potential stops
2. ATP provides energy needed to break the actin myosin cross-bridge
3. Myosin head detaches from actin filament
4. Calcium ions leave binding sites on troponin
5. Active transport moves calcium ions back into sarcoplasmic reticulum
6. Troponin returns to original shape
7. Tropomyosin blocks actin-myosin binding site again
8. Myosin heads cannot bind so actin filaments slide back to their relaxed position.

Question 8

Why do slow-twitch muscle fibres appear red in colour but fast-twitch muscle fibres appear white?

Answer 8

Slow-twitch fibres contain lots of myoglobin whereas fast-twitch muscles do not.

Question 9

What is myoglobin?

Answer 9

A red-coloured protein that stores oxygen.

Question 10

Why do slow-twitch muscles need lots of myoglobin but fast-twitch muscles do not?

Answer 10

Slow-twitch fibres can contract slowly for long periods of time using aerobic respiration whereas fast twitch fibres use anaerobic respiration to contract for short periods of time so they would not need myoglobin because they do not respire aerobically.

Question 11

Which muscle fibre contains lots of mitochondria and blood vessels?

Answer 11

Slow-twitch fibres