CourseCourseQ BankQuestion BankDocsDocuments

B3.3.2 (HL)—Sliding filament model

Sliding filament contraction occurs when calcium exposes actin binding sites and ATP-powered myosin heads pull actin inward to shorten sarcomeres.

Syllabus
First assessment 2025
Objective
B3.3.2
Level
HL

Exam analysis

Chance of appearing26%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–3

Common command terms

  • Explain
  • Draw
  • Identify
  • Describe
  • Analyse
  • State
  • Outline
  • Label
  • Deduce

Scoring notes

Common mistake
Saying actin or myosin filaments shorten instead of sliding past each other.

Recent exam appearances

November 2025Paper2 ["HL"] · TZ13(b)[ 3 ]B3.3.2 (HL)—Sliding filament model
May 2025Paper1A ["HL"] · TZ225[ 1 ]B3.3.2 (HL)—Sliding filament model
November 2024Paper2 ["HL"] · TZ07(c)[ 7 ]B3.3.2 (HL)—Sliding filament model
May 2024Paper1 ["HL"] · TZ137[ 1 ]B3.3.2 (HL)—Sliding filament model
May 2024Paper3 ["HL"] · TZ23(b)(ii)[ 2 ]B3.3.2 (HL)—Sliding filament model
Practice this objective

Coverage 2012–2025 · Updated 15 Jul 2026

Sliding Filaments Shorten a Sarcomere

HL only

A sarcomere extends between two Z lines. Thin actin filaments anchored at the Z lines slide past central thick myosin filaments, increasing overlap and shortening the sarcomere without shortening either filament.

Calcium released from the sarcoplasmic reticulum binds troponin, changing its shape and moving tropomyosin away from myosin-binding sites on actin. Energized myosin heads can then form cross-bridges.

Cross-bridge cycle: myosin-ADP-Pi binds actin → Pi and ADP release drives the power stroke → ATP binding detaches myosin → ATP hydrolysis re-cocks the head. Cycling continues while calcium and ATP are available.

During contraction the Z lines move closer, the I band and H zone narrow, and the A band stays the same length because thick myosin filament length is unchanged.

ATP does not directly pull actin: it permits detachment and re-cocking, while the myosin power stroke generates force. Actin and myosin slide; they do not shrink.

Sequence Contraction

HL only
Ordered contraction states.

Skeletal muscle contraction occurs when myosin heads repeatedly pull actin filaments towards the centre of each sarcomere. The filaments slide past one another; neither filament becomes shorter.

  1. Expose the binding sites: calcium ions bind to troponin, shifting tropomyosin away from binding sites on actin.
  2. Pull: a myosin head attaches to an exposed actin site. Release of ADP and phosphate during the power stroke changes the head angle and pulls actin towards the sarcomere centre.
  3. Detach: ATP binds to myosin, causing the head to detach from actin.
  4. Reset: ATP is hydrolysed to ADP and phosphate, re-cocking the detached head so it can attach again.
  5. While calcium remains available and ATP is supplied, repeated cycles increase actin and myosin overlap, bring the Z-lines closer and shorten the sarcomere.

ATP does not pull actin directly: it enables myosin detachment and re-cocking, while the power stroke of an attached myosin head produces the pull. Sliding shortens the sarcomere, not the actin or myosin filaments themselves.

Sliding filament model

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Draw / Identify.

Command terms

Explain / Draw / Identify / Describe / Analyse / State / Outline / Label / Deduce

What earns marks

Build the answer around this relationship: Sarcomeres are repeating contractile units between Z lines.

Watch for

Saying actin or myosin filaments shorten instead of sliding past each other.

Representative question

Question 1

[Maximum number: 8]

Actin and myosin are two proteins found in muscles. Explain how skeletal muscle contracts, including the interaction of these proteins.

Muscle And Motility

HL only

A strong answer links movement benefit, muscle contraction mechanism, force transfer, joint range, and locomotion adaptations. For contraction, use calcium-troponin-tropomyosin and ATP-driven myosin cross-bridge cycling. For movement, use antagonistic muscles, tendons, ligaments, skeletons as levers, and joint type. For locomotion, link body form to survival or swimming advantage. Locomotion can improve survival and reproductive success.

  • Contraction answers need calcium, actin binding sites, myosin cross-bridges, ATP, and sarcomere shortening.
  • Movement answers need antagonistic pairs because muscles contract but do not actively extend.
  • Joint and skeleton answers need tendon versus ligament, lever action, synovial fluid, and range of motion.
  • Locomotion answers should link examples to food, escape, mate finding, or migration: blackbirds feeding, hares escaping predators, orangutans finding mates, and whales migrating.

Concept essentials

  • Sarcomeres are repeating contractile units between Z lines.
  • Calcium binds to troponin and causes tropomyosin to expose actin binding sites.
  • Myosin heads form cross-bridges with actin and pull actin inward.
  • ATP binding detaches myosin heads, while ATP hydrolysis re-cocks them.
  • Contraction shortens sarcomeres and light bands, not the actin or myosin filaments.