IB DP Biology: Muscle and motility: sliding filaments, joints and locomotion (HL) (DP Biology) – Revision Notes

These revision notes condense section B3.3 (Muscle and motility) of the IB Biology guide, first assessment 2025, for DP Biology. The whole of B3.3 is additional higher level, so every point here is HL only. For full explanations and worked examples, read the study guide, then test yourself with the practice questions. Also see the IB DP Biology hub, the printable checklist and the free diagnostics.

Key terms

Term	Meaning
Motile	Able to move the whole body from place to place
Sessile	Fixed in one place; parts may still move
Sarcomere	Unit of a myofibril between two Z lines
Actin	Protein of the thin filaments
Myosin	Protein of the thick filaments; heads bind actin
Titin	Elastic protein linking thick filaments to Z lines
Motor unit	One motor neuron plus the fibres it controls
Neuromuscular junction	Synapse between motor neuron and muscle fibre
Antagonistic pair	Two muscles with opposite actions at a joint
Tendon	Joins muscle to bone
Ligament	Joins bone to bone
Range of motion	Angle a joint moves through in one direction

B3.3.1 Movement

• Movement is universal in living organisms.
• Motile example: housefly, moving by wings and legs to feed and find mates.
• Sessile example: oak tree. Leaves turn towards light, stomata open and close, shoots grow towards light.
• Sessile does not mean “does not move”.

B3.3.2 Sliding filament model

Region	Contains	Change in contraction
A band	Whole length of the thick filaments	Unchanged
I band	No overlap, thin filaments alone	Reduced
H zone	Centre of the A band, no thin filaments	Narrows or vanishes
Sarcomere (Z to Z)	Everything	Shorter

Method in steps: the cross-bridge cycle

1. Calcium ions leave the sarcoplasmic reticulum.
2. Calcium binds troponin; tropomyosin moves; binding sites on actin are exposed.
3. Myosin head binds actin (cross-bridge).
4. Power stroke: the head pulls actin towards the centre and sheds ADP and phosphate.
5. New ATP attaches; the head lets go.
6. ATP is hydrolysed; the head resets and binds again.

Worked reminder. Sarcomere 2.8 µm shortens to 2.1 µm. Shortening = 0.7 ÷ 2.8 × 100 = 25%. With an A band of 1.6 µm, the I band changes from 1.2 µm to 0.5 µm, a fall of 58.3%. The filaments slid. They did not shorten.

B3.3.3 Titin and antagonism

• Titin is a giant elastic protein.
• It stores energy when stretched and helps the sarcomere recoil.
• It limits stretch, so it prevents overstretching.
• Force comes only from contraction, so a second muscle must reverse the movement.
• Contraction of one muscle of the pair stretches the other.

Must-know distinction: contraction versus relaxation. Contraction uses ATP and cross-bridges. Relaxation is the end of the calcium signal; the muscle is then lengthened by its antagonist, with titin helping recoil.

B3.3.4 Motor units

• Motor neuron axon branches; each branch ends at one fibre at a neuromuscular junction.
• Impulse arrives, acetylcholine is released, it binds receptors on the sarcolemma, and the fibre depolarises.
• Depolarisation spreads into the fibre and releases calcium.
• All fibres of one unit contract together.
• Small units (few fibres) give fine control; large units give strong force.
• More units recruited means more force.

Worked reminder. 9 units of 60 fibres each, with 0.04 N per fibre: 9 × 60 = 540 fibres; 540 × 0.04 = 21.6 N.

B3.3.5 Skeletons

Feature	Exoskeleton	Endoskeleton
Example	Arthropod such as a crab or insect	Vertebrate such as a human
Position	Outside the body	Inside the body
Muscle attachment	Inner surface of the hard covering	Outer surface of bone, by tendons

• Both give anchorage for muscles and act as levers.
• Moment rule for balance: muscle force × its distance from the pivot = load × its distance from the pivot.

Worked reminder. Load 15 N at 24 cm; muscle inserts 3 cm from the pivot. Force = 15 × 24 ÷ 3 = 120 N.

B3.3.6 Synovial joint (hip)

• Ball-and-socket: head of the femur in the socket of the pelvis.
• Cartilage: reduces friction, absorbs shock.
• Synovial fluid: lubricates, nourishes cartilage, cushions.
• Ligaments: stabilise, bone to bone.
• Tendons: transmit muscle force to bone.
• Muscles: contract in antagonistic pairs to move the joint.

B3.3.7 Range of motion

• Measure with a goniometer, or with computer analysis of images.
• Compare several dimensions of the same joint, such as flexion, extension, abduction, adduction and rotation.
• Range = largest angle minus smallest angle for that movement.
• Use repeat readings and a mean.

Worked reminder. Flexion of 150° and extension of 5° from one starting position give a flexion-extension range of 145°. Another person’s mean of 95° against 70° is a 35.7% increase.

Method in steps: using a goniometer

1. Fix the pivot of the goniometer over the joint centre.
2. Align one arm with each bone.
3. Read the angle at the start and end of the movement.
4. Repeat, then take the mean.

B3.3.8 Intercostal muscles

Muscle layer	Effect on ribs	Breathing role
External intercostals	Raise ribs up and out	Inhalation
Internal intercostals	Pull ribs down and in	Forced exhalation

• Fibres in the two layers run in different directions, so they pull the ribcage opposite ways.
• Contraction of either layer stretches its partner, and the partner’s titin stores potential energy.

B3.3.9 Reasons for locomotion

Reason	Example
Foraging	Wolf pack hunting over a large territory
Escape from danger	Scallop swimming away from a starfish
Searching for a mate	Male moth following pheromones
Migration	Wildebeest following rainfall

B3.3.10 Marine mammals

• Streamlining: reduces drag.
• Flippers: forelimbs modified for steering and balance.
• Fluke: tail moves up and down for thrust.
• Airways: blowhole; a large fraction of lung air is replaced per breath; periodic breathing between dives.

Worked reminder. A breath exchanges 14 L of a 16 L lung: 14 ÷ 16 × 100 = 87.5%.

Must-know distinctions

• Thick versus thin filament: thick is myosin, found only in the A band; thin is actin (with troponin and tropomyosin), attached at the Z line.
• Motile versus sessile: whole-body movement versus fixed position. Both can move internal or growing parts.
• Tendon versus ligament: tendon links muscle to bone; ligament links bone to bone.
• Exoskeleton versus endoskeleton: muscles attach inside a hard outer case, or outside internal bones.
• Flexion versus extension: flexion reduces the angle at a joint; extension increases it. Abduction moves a limb away from the midline and adduction brings it back.
• External versus internal intercostals: one raises the ribs, the other lowers them.
• Fish tail versus whale fluke: side-to-side beat versus up-and-down beat.

Data and graph reminders

• When you are given sarcomere lengths, find the change, then divide by the starting value for a percentage.
• When asked what a constant A band shows, link it to thick filaments keeping their length, so sliding is the explanation.
• When comparing ranges of motion, use means of repeated readings and quote the units (degrees).
• When asked to evaluate a goniometer method, mention error in lining up the arms, differences in starting posture and a small number of repeats.

Quick self-test

1. Name one motile and one sessile species and state one movement of the sessile one.
2. Which band stays the same length during contraction?
3. State what happens to the H zone.
4. Give two roles of titin.
5. Why do muscles work in antagonistic pairs?
6. Define a motor unit.
7. A muscle has 8 motor units of 50 fibres each at 0.04 N per fibre. How much force do all the units give?
8. Name the bones that meet at the hip.
9. Which structure joins bone to bone?
10. State two adaptations of marine mammals for swimming.

Answers

1. Housefly (motile) and oak tree (sessile); the oak turns leaves to the light.
2. The A band.
3. It narrows or disappears.
4. It helps sarcomere recoil after stretch, and it prevents overstretching.
5. Muscle pulls only during contraction, so another muscle must pull the other way.
6. One motor neuron and all the muscle fibres it controls.
7. 8 × 50 = 400 fibres; 400 × 0.04 = 16 N.
8. The femur and the pelvis.
9. A ligament.
10. Any two of: streamlined body, flippers, fluke moving up and down, blowhole with periodic breathing.

Where marks are usually lost

• Writing that actin or myosin filaments “shorten”.
• Forgetting that the A band stays constant.
• Describing the sarcomere without naming the I band, H zone or Z line when asked for observations.
• Mixing up tendons and ligaments.
• Saying muscles “push” or that the biceps “extends” the arm.
• Describing the fluke as moving side to side.
• Not stating the direction in which the ribs move when comparing external and internal intercostals.
• Giving a range of motion without units or without saying which movement was measured.
• Listing a reason for locomotion without a named example.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section B3.3, Muscle and motility, additional higher level.