3.6.3 Skeletal muscles

Question 1

Antagonistic pairs

Answer 1

Agonist - contracting muscle

Antagonist - relaxing muscle

Question 2

Sarcolemma

Answer 2

Cell membrane of muscle fibres

Question 3

Sarcoplasm

Answer 3

Cytoplasm of muscle fibres containing many mitochondria

Question 4

Sarcoplasmic reticulum

Answer 4

network of internal membranes throughout the sarcoplasm to store and release Ca2+ needed for contraction

Question 5

Myofibril

Answer 5

Contains 2 types of protein filaments arranged in overlapping units - sarcomeres
Myosin (thick) and actin (thin)

Question 6

Features of muscle fibres (cell)

Answer 6

• specialised elongated fused cells
• have many nuclei so can make proteins easily at any point along their length
• have many mitochondria to provide ATP for muscle contraction

Question 7

Myofibril structure

Answer 7

• Myosin - thick protein filament
• Actin - thin protein filament
• I band actin only
• H zone myosin only
• A band - actin and myosin

Question 8

How does the sarcomere change during muscle contraction?

Answer 8

Filaments slide over each other

• A band never changes length
• Sarcomere shortens (z lines closer)
• H zone shortens
• I band shortens

Question 9

Uses of ATP in muscle contraction

Answer 9

1. required for movement of myosin heads (that causes actin filaments to slide)
2. Breaks actin-myosin cross bridge
3. For active transport of Ca2+ back into reticulum (if no ATP muscle wouldn’t relax)
4. Mitochondria generate ATP by respiration

Question 10

What causes muscle fatigue?

Answer 10

lactic acid build up

Question 11

Sliding filament theory

Answer 11

1. Sarcolemma is depolarised, causing Ca2+ channels to open
2. Ca2+ diffuse from sarcoplasmic reticulum into sarcoplasm into myofibril
3. Ca2+ binds to troponin causing it to change shape
4. This causes tropomyosin to move and expose the myosin binding site on actin
5. Myosin heads bind, forming actin myosin cross bridges
6. Ca2+ activates ATP hydrolase in the head - ATP hydrolysed into ADP and Pi releasing energy
7. Energy causes myosin head to bend, pulling the actin with it
8. ATP binds to head, breaking cross bridge, myosin head attaches to actin further along

Question 12

How is energy provided for contraction by

1. Aerobic respiration
2. Anaerobic respiration
3. ATP-Phosphocreatine system

Answer 12

1. Most ATP generated by oxidative phosphorylation in mitochondria (requires O2)
2. ATP made rapidly by glycolysis
3. ATP made by phosphorylating ADP with phosphate from phosphocreatine

ADP + Phosphocreatine (Pi) -> ATP + Creatine

PCr system in anaerobic and alactic

PCr stores inside cells, generates ATP rapidly but runs out after a few seconds

Question 13

Slow twitch muscle fibres

Answer 13

• slow less powerful contractions over long time period
• aerobic (uses oxygen as final e- acceptor) so less fatigue as no lactic acid
• for endurance work pr maintaining posture
• Large store of oxygen in myoglobin, lots of mitochondria, lots of blood vessels (short diffusion distance)

Question 14

Fast twitch muscle fibres

Answer 14

• fast powerful contractions
• sprint work
• anaerobic respiration producing lactic acid so muscles become fatigued quickly
• less myoglobin, mitochondria and blood vessels (no O2 needed)
• more phosphocreatine to supply ATP
• larger glycogen stores - for glycolysis

Question 15

Function of myoglobin

Answer 15

• binds to oxygen in muscles
• very high affinity for oxygen
• unloads oxygen when partial pressure is low
• acting as a store for oxygen