Muscle Contraction

Question 1

Describe the structure of Myosin

Answer 1

These are fibrous protein molecules with a globular head
The fibrous part of the myosin molecule anchors the molecule into the thick filament

Question 2

Describe the structure of Actin

Answer 2

These are globular protein molecules
Many actin molecules link together to form a chain
Two actin chains twist together to form one thin filament
A fibrous protein known as tropomyosin is twisted around the two actin chains
Another protein known as troponin is attached to the actin chains at regular intervals

Question 3

During muscle contraction what happens to the length of sarcomeres?

Answer 3

sarcomeres within myofibrils shorten as the Z discs are pulled closer together

Question 4

What is the name of the model which explains muscle contraction ?

Answer 4

the sliding filament model of muscle contraction

Question 5

Explain how the sliding filament process of muscle contraction occurs.

Answer 5

An action potential arrives at the neuromuscular junction
Calcium ions are released from the sarcoplasmic reticulum (SR)
Calcium ions bind to troponin molecules, stimulating them to change shape
This causes troponin and tropomyosin proteins to change position on the actin (thin) filaments
Myosin binding sites are exposed on the actin molecules
The globular heads of the myosin molecules bind with these sites, forming an actinomysoin crossbridge
Energy is released from ATP causing myosin heads to bend pulling the actin filaments towards the centre of the sarcomere and causing the muscle to contract.
ATP binds to the myosin heads.
The enzyme ATP hydrolase hydrolyses ATP into ADP and inorganic phosphate which causes the myosin heads to release from actin and move back to their original position.

Question 6

What is ATP used for at the end of muscle contraction ?

Answer 6

Used to break the Actinomyosin crossbridges

without ATP, myosin stays locked onto the actin. (this happens after death)

Question 7

Where are calcium ions stored in the muscle fibre ?

Answer 7

In the Sarcoplasmic Reticulum

Question 8

Describe the sequence of events following a nerve impulse arriving at a neuromuscular junction ?

Answer 8

Action potential arrives at the neuromuscular junction
acetylcholine is released into the synaptic cleft
An action potential is produced in the sarcolemma
The action potential is transmitted down the transverse tubules to the centre of the muscle fibre
Calcium ions are released from the sarcoplasmic reticulum
(Sliding filament model can continue)

Question 9

Describe the sequence of Events when the action potential stops

Answer 9

Acetylcholine is destroyed at the neuromuscular junction by acetylcholineesterase
Stops stream of action potentials along sarcolemma
Sarcoplasmic reticulum stops releasing calcium ions
calcium ions pumped back into sarcoplasmic reticulum-uses ATP
Fall in the level of calcium ions cause tropomyosin to move back to its original position
Actin binding sites are now blocked so myosin heads cannot bind with actin
Muscle contraction stops

Question 10

What is energy from ATP used for in muscle contraction ?

Answer 10

The movement of myosin heads
The reabsorption of calcium ions into the sarcoplasmic reticulum by active transport

Question 11

What do muscle fibres contain a store of so that ATP can be produced if oxygen cannot be supplied quickly enough for aerobic respiration ?

Answer 11

PHOSPHOCREATINE

Question 12

How can PHOSPHOCREATINE be used to synthesise ATP ?

Answer 12

by rapidly transferring a phosphate to ADP to make ATP to replace the ATP that has been broken down.