3.6.3 - Skeletal Muscles

Question 1

Describe the roles of calcium ions and ATP in the contraction of a myofibril.

Answer 1

• calcium ions diffuse into myofibrils from
  (sarcoplasmic) reticulum
• (calcium ions) cause movement of tropomyosin (on actin)
• (this movement causes) exposure of the
  binding sites on the actin
• myosin heads attach to binding sites on actin
• hydrolysis of ATP (on myosin heads) causes myosin heads to bend
• (bending) pulling actin molecules
• attachment of a new ATP molecule to each myosin head causes myosin heads to detach (from actin sites)

Question 2

actin

Answer 2

thin filament

Question 3

myosin

Answer 3

thick filament

Question 4

I-band

Answer 4

thin filament (actin) only
light band

Question 5

H-zone

Answer 5

thick filament (myosin) only

Question 6

slow-twitch fibres e.g. human calf muscle

Answer 6

• contract more slowly
• provide less powerful contractions over a longer period
• adapted to endurance work
• adapted for aerobic respiration to avoid lactic acid build-up
• have a large store of myoglobin (red molecule that stores oxygen)
• have a rich supply of blood vessels to deliver glucose and oxygen
• have numerous mitochondria to produce ATP

Question 7

fast-twitch fibres e.g. human biceps

Answer 7

• contract more rapidly
• produce powerful contractions for a short period
• adapted to intense exercise
• have thicker, more numerous myosin filaments
• have a high concentration of glycogen
• have a high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly
• have a store of phosphocreatine which can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction

Question 8

muscle contraction

Answer 8

• usual cholinergic synapse stuff
• muscle is depolarised
• action potential travels down T-tubules → sarcoplasmic reticulum
• action potential opens calcium ion protein channels on sarcoplasmic reticulum and calcium ions diffuse into sarcoplasm
• calcium ions bind to protein on tropomyosin which changes shape
• tropomyosin pulled away from binding site on actin
• myosin head binds to actin and form crossbridge
• myosin head changes angle and moves/pulls actin filament along
• molecule of ADP released
• ATP binds to myosin head, causing it to detach from actin filament
• calcium ions activate ATP hydrolase
• hydrolysis of ATP to ADP by ATPase provides energy for myosin head to return to original position
• recocking of myosin head

Question 9

muscle relaxation

Answer 9

• calcium ions actively transported back into the sarcoplasmic reticulum
• uses energy from hydrolysis of ATP
• tropomyosin blocks actin filament again
• myosin heads unable to bind
• contraction stops, muscle relaxes

Question 10

phosphocreatine

Answer 10

• stored in muscle
• acts as a reserve supply of phosphate
• available immediately to combine with ADP and so re-form ATP
• replenished using phosphate from ATP when the muscle is relaxed

Question 11

evidence for sliding filament mechanism

Answer 11

when a muscle contracts:
- the Z-lines move closer together (the sarcomere shortens)
- the I-band becomes narrower
- the H-zone becomes narrower
- the A-band remains the same width

Question 12

A-band

Answer 12

thick filament (myosin) AND overlap of thin and thick filaments (actin and myosin)