7.1.3 - Muscle Contraction

Question 1

Describe Thick Myosin Filaments
(3 Points)

Answer 1

~ Fibrous protein molecules with a globular head.

~ Are hinged, so they can’t move back and fourth.

~ Each myosin head has a binding site for actin and another for ATP.

Question 2

Describe Thin Actin Filaments
(5 Points)

Answer 2

~ Globular protein molecules.

~ 2 actin chains, twist together to form a chain.

~ Have binding sites for myosin heads, called actin-myosin binding sites.

~ Fibrous protein tropomyosin, twists around the 2 actin chains.

~ Troponin, is attached to the actin chains at regular intervals.

Question 3

Describe The Binding Sites, When Resting
(2 Points)

Answer 3

~ Actin-myosin binding sites is blocked by tropomyosin, held in place by troponin.

~ So the myofilaments can’t slide past each other, as the myosin heads can’t bind to the actin binding site.

Question 4

Describe The 1st Part Of The Sliding Filament Theory
(3 Points)

Answer 4

1) Action potential arrives at the presynaptic membrane, causing voltage gated calcium channels to open.

2) Calcium ion influx, causes vesicles to fuse with the pre-synaptic membrane and release acetyl choline (Neurotransmitter) via exocytosis.

3) Acetyl choline diffuses across the synapse and binds to ACH-gated calcium channels, causing depolarisation.

Question 5

Describe The 2nd Part Of The Sliding Filament Theory
(7 Points)

Answer 5

4) Calcium is released from the store in the SR, and binds to troponin.

5) Causes a conformational change in tropomyosin, exposing the actin-myosin binding site.

6) Globular head of myosin, which has a ADP molecule bound, binds to the binding site on actin, forming cross-bridges.

7) The formation of cross-bridges, causes the myosin heads to bend, causing the ADP molecule to release. (Power stroke).

8) ATP binds to the myosin head, causing a conformational change, meaning it is no longer complementary to the actin-myosin binding site.

9) ATPase hydrolyses ATP into ADP + Pi, causing myosin heads to move back to the resting position. (Recovery stroke).

• Process is repeated, until the muscle is fully contracted.

Question 6

Describe What Happens When Muscle Stimulation Stops
(5 Points)

Answer 6

~ Calcium ions leave their binding sites on troponin molecules.

~ They are transported back to the SR, via active transport.

~ Troponin molecules return to their original shape, pulling the tropomyosin molecules to block the actin-myosin binding sites.

~ No cross-bridges are formed, so no muscle contraction can occur.

~ Sarcomere lengthens again, as actin filaments side back to their relaxed positions.