IB Biology · Theme B: Form and function · HL only
B3.3 Muscle and motility
A one-page summary of B3.3 Muscle and motility, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do muscles contract and cause movement?
What are the benefits to animals of having muscle tissue?
What B3.3 covers
The sliding filament mechanism
- B3.3.1 · HLMovement is a spectrum, not a switch
- B3.3.1 · HLEven "rooted" organisms move
- B3.3.2 · HLMuscle is built in a strict hierarchy
- B3.3.2 · HLGetting Ca²⁺ to exactly the right place
- B3.3.2 · HLThe sliding filament model, step by step
- B3.3.3 · HLTitin resets the sarcomere for free
- B3.3.3 · HLOne muscle can only pull
- B3.3.4 · HLOne neuron, every fibre it commands
- B3.3.4 · HLMotor unit size trades precision for power
Skeletons, levers and joints
- B3.3.5 · HLArthropods anchor muscle to a rigid shell
- B3.3.5 · HLVertebrates anchor muscle from within
- B3.3.6 · HLThe hip: a joint built to move without wearing out
- B3.3.6 · HLLigaments hold it together, tendons make it move
- B3.3.7 · HLNot every joint moves the same way
- B3.3.8 · HLThe ribcage moves by the same logic as a limb
Why animals move at all
- B3.3.9 · HLMovement always serves a purpose
- B3.3.10 · HLA body built to move through water, not air
- B3.3.10 · HLBreathing has to fit around swimming
B3.3 Muscle and motility: summary
Sarcomere & sliding filament
- Muscle → fascicle → fibre → myofibril → sarcomere.
- Actin + myosin slide; filaments don't shorten.
Excitation-contraction
- SR stores Ca²⁺; T-tubules carry the impulse in.
- ATP breaks the cross-bridge, not forms it.
Titin & antagonistic pairs
- Titin recoils the sarcomere passively.
- Muscle only pulls — needs a partner to reset.
Motor units
- Small = precision (eye); large = power (quads).
- Neuron + all fibres it innervates.
Skeletons & joints
- Exo- vs endoskeleton: anchorage + lever.
- Synovial joint; ball-socket vs hinge ROM.
Why & how animals move
- Forage, flee, mate, migrate.
- Marine mammals: streamlining, flippers, blowhole.
Key terms
- Sarcomere
- The repeating contractile unit of a myofibril, bounded by Z discs.
- Sliding filament model
- Actin and myosin filaments sliding past each other, without shortening themselves, to contract a sarcomere.
- Cross-bridge cycle
- The repeating sequence of myosin binding actin, ATP releasing it, and the power stroke pulling the filaments.
- Titin
- An elastic protein linking myosin to the Z disc, providing passive elastic recoil.
- Antagonistic muscles
- A pair of muscles that act in opposition, one stretching the other as it contracts.
- Motor unit
- A motor neuron and every muscle fibre it innervates.
- Range of motion (ROM)
- The angular distance and directions through which a joint can move.
- Synovial fluid
- Lubricating fluid within a joint capsule that reduces friction and absorbs shock.
- Porpoising
- Leaping clear of the water while swimming, reducing drag at speed.
Sample exam questions
Three of the 47 multiple-choice questions for B3.3. Try each one before opening the answer.
Question 1. The functional unit of a skeletal muscle fibre, the sarcomere, is defined as the region between:
- One Z-line and one M-line
- Two adjacent Z-lines (Z-discs)
- The A-band and the I-band
- Two adjacent M-lines
Show the answer
Answer: B. A sarcomere extends from one Z-line to the next. It is the smallest contractile unit. During contraction, Z-lines move closer together as the sarcomere shortens.
Question 2. What is the role of Ca²⁺ in muscle contraction?
- Ca²⁺ depolarises the sarcolemma, initiating the action potential
- Ca²⁺ directly hydrolyses ATP for the power stroke
- Ca²⁺ binds to troponin, moving tropomyosin away from myosin-binding sites on actin, allowing cross-bridge formation
- Ca²⁺ binds to myosin heads, triggering detachment from actin
Show the answer
Answer: C. Ca²⁺ released from SR binds to troponin C → tropomyosin shifts → myosin-binding sites exposed on actin → cross-bridges form. Ca²⁺ is the regulatory signal; it doesn't provide energy.
Question 3. The internal and external intercostal muscles have different fibre orientations. What functional consequence does this have when one layer contracts?
- The contracting layer stretches the other layer, storing elastic potential energy in titin within the stretched muscle's sarcomeres
- Both layers always contract simultaneously, cancelling each other out
- The contracting layer permanently shortens the other layer
- The layers act completely independently with no mechanical interaction
Show the answer
Answer: A. Because the internal and external intercostal muscles have different fibre orientations, when one layer contracts it stretches the other; this stretching stores elastic potential energy in the titin of the stretched muscle's sarcomeres, linking to the passive elastic recoil role of titin.
Linking questions
Questions that connect B3.3 to other parts of the course, the kind that come up in Paper 2.
- The sarcoplasmic reticulum is a specialised form of smooth ER (B2.2). How does its Ca²⁺ storage and release function relate to the general role of SER in calcium homeostasis? (see B2.2)
- ATP for muscle contraction is produced by aerobic respiration in mitochondria (C1.2). Explain why Type I fibres have more mitochondria than Type IIb, relating this to their different metabolic strategies. (see C1.2)
- Ca²⁺ acts as a second messenger in both muscle contraction and cell signalling (C2.1). Compare the role of Ca²⁺ in triggering contraction with its role in neurotransmitter release at synapses. (see C2.1)
- During intense exercise, muscles switch to anaerobic glycolysis producing lactate. How does this relate to the different ATP capacities of the three fibre types, and why does lactate accumulation limit performance? (see C1.2)
Practise B3.3
Study notes, every question and full markschemes for B3.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.