2.8.2Inquiry Process
Mechanism of Muscle Contraction — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 87 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Structure of a Sarcomere
Contraction happens when thin filaments slide between thick filaments.
Sarcomere
- Sarcomere The repeating unit of a myofibril, lying between two Z lines, which shortens during contraction.
Actin
- Actin Thin filaments attached to the Z lines, which carry binding sites for myosin heads when exposed.
Myosin
- Myosin Thick filaments with projecting heads that can bind to actin and change angle using energy from ATP.
Regulatory proteins
- Regulatory proteins Tropomyosin covers the myosin binding sites on actin at rest, while troponin holds it in place and binds calcium.
Section 2 of 4Cross-Bridge Cycle
1
A nerve impulse reaches the muscle fibre, and calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm.
2
Calcium binds to troponin, which moves tropomyosin aside and exposes the myosin binding sites on actin.
3
Myosin heads, already energised by ATP hydrolysis, attach to actin forming cross-bridges.
4
The heads pivot, pulling the actin filament towards the centre of the sarcomere; this is the power stroke, and ADP and phosphate are released.
5
A new ATP molecule binds the head, breaking the cross-bridge; hydrolysis of ATP re-sets the head so the cycle can repeat.
Section 3 of 4Changes in the Sarcomere
Z lines
- Z lines Move closer together, so the whole sarcomere becomes shorter.
I band
- I band Narrows because the thin filaments slide over the thick filaments.
H zone
- H zone Narrows or disappears as actin filaments approach each other in the centre.
A band
- A band Stays the same length because the length of myosin filaments does not change.
| Region | Relaxed | Contracted |
|---|---|---|
| Sarcomere length | Longer | Shorter |
| I band | Wide | Narrow |
| A band | Same | Same |
Section 4 of 4Exam Checklist & Tips
2.8.2 – Key Points for the Exam
- Sliding filament model The explanation of contraction in which actin and myosin filaments slide past each other without changing length.
- Troponin A protein on actin that binds calcium ions.
- Tropomyosin A protein that blocks the myosin binding sites on actin at rest.
- Cross-bridge The link formed when a myosin head attaches to actin.
- Sarcoplasmic reticulum The membrane network that stores and releases calcium ions in muscle.
Common Mistakes
- Stating that filaments shorten; they slide and keep their length.
- Saying ATP is needed to form the cross-bridge; it is needed to detach the head and to reset it.
- Forgetting that calcium ions bind troponin and not myosin.
- Claiming the A band shortens; the I band and H zone shorten.
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