skeletal muscle structure and function

Question 1

give the 4 pieces of evidence for the sliding filament theory during contraction of sarcomere

how do these pieces of evidence support the sliding filament theory

Answer 1

• H zone shortens
• I band shortens
• Z lines move closer together
• A band stays the same length (as the myosin filament)

this provides evidence that the filaments do not contract but do slide over each other

Question 2

why does the A band not change length in the contraction of a sarcomere

Answer 2

the A band is the same width as the myosin filament

Question 3

what 3 proteins are involved in muscle contraction

Answer 3

myosin

actin

tropomyosin

Question 4

how does myosins structure allow it to perform its role in muscle contraction

Answer 4

filaments contain many globular heads attached by a hinge (tail) - this enables them to move back and forth

each globular head has a binding site for actin and one for ATP

Question 5

how many binding sites does each myosin head have and what binds to them

Answer 5

each myosin globular head has a binding site for actin and one for ATP

Question 6

how does actins structure allow it to perform its role in muscle contraction

Answer 6

filaments have binding sites for myosin heads called actin - myosin binding site

Question 7

how does tropomyosins structure allow it to perform its role in muscle contraction

Answer 7

it is a protein found between actin molecules

it blocks the binding sites when muscle isn’t contracting

Question 8

what are the 3 stages to muscle contraction

Answer 8

1. stimulation
2. contraction
3. relaxation

Question 9

explain the process of stimulation of muscle contraction

Answer 9

• action potential reaches neuromuscular junction
• causes Ca2+ ion channels to open
• Ca2+ diffuses into the synaptic knob causing synaptic vesicles to fuse with presynaptic membrane
• release ACh into the synaptic cleft
• ACh diffuses across cleft and binds to receptors on muscle fibre causing depolarisation due to influx of Na+
• action potential spreads through transverse tubules (T) causing Ca2+ ions to be released from sarcoplasmic reticulum into sarcoplasm

Question 10

state 2 functions of Ca2+ in stimulation of muscle contraction

Answer 10

1. causes synaptic vesicles to fuse with presynaptic membrane
2. diffuse out of sarcoplasmic reticulum into sarcoplasm

Question 11

what happens to ACh after it has bound to receptors on sarcolemma
(2 marks)

Answer 11

• hydrolysed by acetylcholinesterase into ethanoic acid (acetic acid) and choline
• reabsorbed into presynaptic membrane and reformed into ACh using ATP

Question 12

state the structure through which an action potential travels within muscle cell fibres

Answer 12

Tranverse (T) tubules

Question 13

name 2 types of transport involved in movement of ACh at neuromuscular junction

Answer 13

diffusion

exocytosis

Question 14

binding of ACh causes influx of which ions causing depolarisation

Answer 14

Na+

Question 15

explain the process of muscle contraction

Answer 15

1. (Ca2+ in sarcoplasmic reticulum diffuse into sarcoplasm)
   Ca2+ bind to tropomyosin causing it to change shape and expose myosin binding site on actin filament
2. the globular heads of myosin molecules bind to actin forming cross bridges (actinomyosin complex) between the 2 filaments
3. myosin heads move together and pull actin filaments toward the middle of the sarcomere causing it to contract - “power stroke”
   this causes ADP to be released
4. another ATP binds to the myosin head and Ca2+ ions activate ATPase to hydrolyse it to ADP and Pi
5. this provides the energy for myosin heads to detach from actin and move back to their original position
6. myosin heads then bind to a new site further along actin, pulling it and causing further shortening of sarcomere
   this process is repeated causing sarcomere to shorten

Question 16

explain the process of muscle relaxation

Answer 16

• when no further action potentials happen, Ca2+ ions are actively transported back into the sarcoplasmic reticulum using ATP
• fewer Ca2+ ions means tropomyosin blocks binding site on actin and myosin heads can no longer bind
• actin filaments slide back to their original position, lengthening the sarcomere

Question 17

state the 3 roles of ATP in myofibril contraction

Answer 17

• provide energy to move myosin head causing actin molecule to slide along
• break down actin - myosin cross bridges, so myosin heads detach from actin filament
• actively transport Ca2+ ions back into sarcoplasmic reticulum after contraction

Question 18

state the 2 roles of Ca2+ ions in myofibril contraction

Answer 18

• binds to tropomyosin causing it to change shape and expose actin - myosin binding site
• activates ATPase to hydrolyse ATP into ADP and Pi

Question 19

state the 2 roles of tropomyosin in myofibril contraction

Answer 19

• moves out of the way when Ca2+ binds
• allows myosin to bind to actin (cross bridge formation)

Question 20

what 3 ways is ATP generate for muscle contraction

Answer 20

• Aerobic respiration
  (regenerated from ATP in respiration of pyruvate in mitochondria)
• Anaerobic respiration
• ATP phosphocreatine system

Question 21

what are the 3 reasons why ATP is needed in muscle contraction

Answer 21

• the movement of the myosin heads (cause actin molecule to slide along)
• the reabsorption of calcium ions into the endoplasmic reticulum by active transport
• break down actin - myosin cross bridges so myosin heads detach from actin filament

Question 22

why muscles contain many mitochondria

Answer 22

they carry out oxidative phosphorylation
this provides lots of ATP
which can then be hydrolysed to provide energy for muscle contraction

(ATP production by aerobic respiration)

Question 23

why do muscles have a large network of blood capillaries

Answer 23

supply glucose and oxygen for respiration

remove waste products like carbon dioxide and creatine

Question 24

what role does phosphocreatine play in muscle contraction

Answer 24

phosphocreatine is stored in cells and acts as a reserve supply of phosphate which combines with ADP to form ATP
which can then be hydrolysed to provide active transport of calcium ions in muscles