IB Biology · Theme B: Form and function · HL only
B3.3 Muscle and motilityHL only
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
Movement and muscle contraction
- B3.3.1 · HLA motile species: the sloth
- B3.3.1 · HLA sessile species: coral
- B3.3.1 · HLMovement in a plant
- B3.3.2 · HLInside a muscle
- B3.3.2 · Data booklet · HLThe sarcomere
- B3.3.2 · HLThe sliding filament model
- B3.3.2 · HLHow myosin pulls actin
- B3.3.3 · HLTitin: a molecular spring
- B3.3.3 · HLAntagonistic muscles
- B3.3.4 · HLA motor unit
- B3.3.4 · HLThe neuromuscular junction
Skeletons, joints and the ribcage
- B3.3.5 · HLAn exoskeleton
- B3.3.5 · HLAn endoskeleton
- B3.3.6 · HLThe hip: a synovial joint
- B3.3.6 · HLMuscles and tendons move the hip
- B3.3.7 · HLRange of motion
- B3.3.7 · Skill · HLMeasuring a joint angle
- B3.3.8 · HLThe intercostal muscles
Why and how animals move
- B3.3.9 · HLMoving to find food
- B3.3.9 · HLMoving to escape danger
- B3.3.9 · HLMoving to find a mate
- B3.3.9 · HLMigration
- B3.3.10 · HLBuilt for swimming
- B3.3.10 · HLLimbs became flippers
- B3.3.10 · HLBreathing between dives
B3.3 Muscle and motility: summary
Movement
- All organisms move; motile (sloth) vs sessile (coral).
- Plants move too: the Venus flytrap.
Sarcomere
- Z disc to Z disc; actin thin, myosin thick.
- Contraction: I band and H zone narrow, A band constant.
Contraction
- Calcium ions expose sites; myosin heads pull actin.
- ATP detaches and re-cocks the heads.
Relaxation and motor units
- Titin recoils; antagonistic pairs stretch each other.
- Motor unit: one neuron, its fibres, neuromuscular junctions.
Skeletons and joints
- Exo vs endoskeleton: anchorage and levers.
- Hip: cartilage, synovial fluid, ligaments, tendons; range of motion.
Locomotion
- Food, danger, mate, migration: one example each.
- Marine mammals: streamlined, flippers, fluke up and down, blowholes.
Key terms
- Motile / sessile
- able to move from place to place / fixed in one place.
- Sarcomere
- the unit of a myofibril, from one Z disc to the next.
- Sliding filament model
- actin slides past myosin, so the sarcomere shortens.
- Cross-bridge
- a myosin head bound to actin.
- Titin
- an elastic protein that helps sarcomeres recoil.
- Antagonistic muscles
- a pair in which each stretches the other.
- Motor unit
- one motor neuron and the fibres it stimulates.
- Synovial joint
- a joint with cartilage and synovial fluid, e.g. the hip.
- Range of motion
- how far a joint can move in each direction.
Sample exam questions
Three of the 47 multiple-choice questions for B3.3. Try each one before opening the answer.
Question 1. What is the sarcomere?
- A bundle of muscle fibres and nerves
- The repeating unit of a myofibril
- A single actin filament
- The plasma membrane around a fibre
Show the answer
Answer: B. A sarcomere runs from one Z disc to the next; myofibrils are chains of sarcomeres.
Question 2. Which is the correct order of events in a cross-bridge cycle?
- ATP binds; head attaches; head detaches; power stroke
- Power stroke; head attaches; ATP binds; head detaches
- Head attaches; power stroke; ATP binds; head detaches
- Head detaches; actin shortens; ATP binds; head attaches
Show the answer
Answer: C. The head binds actin, pulls it towards the centre, then ATP binds and the head detaches and is re-cocked.
Question 3. Why can the internal and external intercostal muscles move the ribcage in opposite directions?
- Their fibres run in different directions.
- They are attached to different bones.
- One is cardiac muscle.
- They contract at the same time.
Show the answer
Answer: A. The different orientations of the fibres move the ribs in opposite directions.
Linking questions
Questions that connect B3.3 to other parts of the course, the kind that come up in Paper 2.
- What are the advantages and disadvantages of dispersal of offspring from their parents? (see B4.2)
- In what ways does locomotion contribute to evolution within living organisms?
- The external and internal intercostal muscles move the ribcage during ventilation (B3.1). How does their antagonistic action change the volume of the thorax? (see B3.1)
- Acetylcholine is released at the neuromuscular junction (C2.2). How is transmission at this junction similar to transmission at a synapse between two neurons? (see C2.2)
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.