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15.1.1214-15 · Homeostasis and Control

Sliding Filament Model of Muscular Contraction — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 27 of 31 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

15.1.12 Sliding Filament Model of Muscular Contraction — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the thick and thin filaments and the cycle of cross-bridge formation in contraction.Myosin molecules — fibrous protein with a globular head.Fibrous part —…

  • Arrangement
  • Tropomyosin
  • Fibrous part — anchors the molecule in the thick filament
  • Actin — globular protein molecules linked in a chain
  • Structure — two actin chains twist together to form one thin filament
  • Troponin — attached to the actin chains at regular intervals

Section 1 of 8Thick filaments

Thick filaments
Myosin molecules
  • Myosin molecules fibrous protein with a globular head.
Fibrous part
  • Fibrous part anchors the molecule in the thick filament.
Arrangement
  • Arrangement many molecules lie side by side, with the heads pointing away from the M line.

Section 2 of 8Thin filaments

Thin filaments
Actin
  • Actin globular protein molecules linked in a chain.
Structure
  • Structure two actin chains twist together to form one thin filament.
Tropomyosin
  • Tropomyosin a fibrous protein twisted around the two chains.
Troponin
  • Troponin attached to the actin chains at regular intervals.

Section 3 of 8The sliding filament model

1Contraction
  • Contraction sarcomeres in myofibrils shorten as the Z discs are pulled closer.
2Process
  • Process actin filaments slide past myosin filaments.

Section 4 of 8Steps of contraction

1
An action potential arrives at the neuromuscular junction.
2
Ca2+ are released from the sarcoplasmic reticulum.
3
Ca2+ bind to troponin, which changes shape.
4
Troponin and tropomyosin change position on the actin.
5
Myosin-binding sites are exposed.
6
Myosin heads bind the sites, forming cross-bridges.
7
The heads move and pull the actin towards the centre of the sarcomere.

Section 5 of 8Repeating the cycle

1
ATP is hydrolysed, allowing the myosin heads to release the actin.
2
The heads move back to their original positions.
3
They bind to new sites closer to the Z disc.
4
The heads move again, pulling the actin further and shortening the sarcomere.
5
The heads hydrolyse ATP once more to detach.
6
The cycle repeats while the binding sites are unblocked and ATP is available.

Section 6 of 8Requirements

Calcium
  • Ca2+ must be present to move troponin and tropomyosin.
ATP
  • ATP needed for detachment of the myosin heads.

Section 7 of 8Key terms

TermMeaning
Sliding filament modelContraction by actin filaments sliding past myosin.
Cross-bridgeLink formed when a myosin head binds actin.
MyosinThick filament protein with a globular head.
ActinGlobular protein forming the thin filament.
TropomyosinProtein twisted around actin that blocks binding sites.
TroponinProtein on actin that binds Ca2+.
Z discAttachment point of actin filaments.

Section 8 of 8Exam technique

1
Saying the filaments shorten — they slide, and the sarcomere shortens.
2
Saying ATP is only for the power stroke — ATP is needed for detaching the heads.
3
Saying Ca2+ bind to myosin — they bind to troponin.
4
Saying heads bind before the sites are exposed — tropomyosin must move first.

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