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

This study guide covers section B3.3 (Muscle and motility) of the IB Biology guide, first assessment 2025, for DP Biology. The guide states that B3.3 has no SL content, so everything here is additional higher level and is HL only. It applies to the examination sessions that follow the first assessment in 2025. Use it with the revision notes and the practice questions, and see the IB DP Biology hub, the printable checklist and the free diagnostics.

What this unit covers

Section	What you must be able to do	Tier
B3.3.1	Explore movement across a range of organisms, taking in at least one that is motile and one that is sessile	HL only
B3.3.2	Explain sarcomere shortening in terms of actin and myosin filaments sliding past each other	HL only
B3.3.3	Explain titin’s roles and why antagonistic muscles are needed	HL only
B3.3.4	Describe a motor unit: motor neuron, muscle fibres and neuromuscular junctions	HL only
B3.3.5	Explain skeletons as anchorage and levers; exoskeleton and endoskeleton	HL only
B3.3.6	Describe movement at a synovial joint, using the human hip	HL only
B3.3.7	Compare range of motion in several dimensions; measure angles with a goniometer or image analysis	HL only
B3.3.8	Explain intercostal muscles as an antagonistic pair	HL only
B3.3.9	Give reasons for locomotion with an example of each	HL only
B3.3.10	Explain swimming adaptations in marine mammals	HL only

B3.3.1 Movement as a universal feature

All living organisms show movement of some kind. A motile organism moves its whole body from place to place. A sessile organism stays fixed in one place but still moves parts of itself.

• Motile example: a housefly moves by beating wings and walking on legs. It moves to find food, mates and egg-laying sites.
• Sessile example: an oak tree is fixed by its roots. It still moves: leaves orient towards light, stomata open and close, and growing shoots bend (a tropism). Cytoplasm streams within its cells.

Movement needs ATP and a mechanism that turns its energy into force. In animals that mechanism is mostly muscle.

B3.3.2 Sliding filament model

Skeletal muscle is built from muscle fibres. Each fibre contains bundles called myofibrils. A myofibril is a chain of sarcomeres joined end to end. One sarcomere is the stretch between neighbouring Z lines.

Two protein filaments overlap inside a sarcomere:

• Thick filaments are made of myosin and sit in the dark A band.
• Thin filaments are made mainly of actin. They attach to the Z lines and extend towards the centre.

The lighter I band contains thin filaments only. The H zone, at the middle of the A band, contains thick filaments only.

The model: during contraction, myosin heads pull the thin filaments towards the centre of the sarcomere. The filaments do not shorten. They slide past each other, so the overlap increases.

Observations that support this:

• The A band stays the same length.
• The I band becomes shorter.
• The H zone becomes narrower or disappears.
• The distance between Z lines falls.

The cross-bridge cycle

1. A nerve impulse leads to calcium ions being released from the sarcoplasmic reticulum into the cytoplasm of the fibre.
2. Calcium ions bind to troponin on the thin filament. Tropomyosin moves aside and exposes binding sites on actin.
3. A myosin head carrying ADP and phosphate binds to actin, forming a cross-bridge.
4. The head changes shape (the power stroke) and pulls the thin filament towards the centre, letting go of its ADP and phosphate.
5. Fresh ATP attaches to the head, and the head lets go of actin.
6. ATP is hydrolysed, which resets the head so it can bind again further along.

Without ATP the heads stay attached, which explains stiffness after death.

Worked example 1: sarcomere lengths

A relaxed sarcomere is 2.4 µm long and its A band is 1.6 µm. During contraction it shortens to 1.8 µm. The A band stays at 1.6 µm. (Simplified data supplied for the example.)

• Shortening: 2.4 − 1.8 = 0.6 µm. As a percentage: 0.6 ÷ 2.4 × 100 = 25%.
• I band length (both halves): relaxed 2.4 − 1.6 = 0.8 µm. Contracted 1.8 − 1.6 = 0.2 µm.
• Percentage change in I band: (0.2 − 0.8) ÷ 0.8 × 100 = −75%.
• Interpretation: the A band is constant because the thick filaments keep their length. The I band shrinks because thin filaments slide into the A band. This supports sliding rather than filament shortening.

A 30 mm fibre has 30 000 ÷ 2.4 = 12 500 sarcomeres in series. If each shortens by 0.6 µm, the fibre shortens by 7.5 mm.

B3.3.3 Titin and antagonistic muscles

Titin is an immense, elastic protein. One end is anchored at the Z line and the other is linked to the thick filament near the middle of the sarcomere. It does two jobs:

• It helps a stretched sarcomere to recoil to its resting length, because it stores elastic potential energy when stretched.
• It prevents overstretching, because it resists being extended beyond a limit and keeps the thick filaments centred.

Muscle tissue exerts a pulling force only while contracting. It cannot push. Muscles therefore work in antagonistic pairs: when one contracts, it stretches the other. The stretched muscle is then ready to contract and reverse the movement. Example: at the elbow, the biceps flexes the joint and the triceps extends it.

B3.3.4 Motor units

A motor unit is one motor neuron plus all the muscle fibres it controls. The motor neuron’s axon branches, and each branch ends on one fibre at a neuromuscular junction.

At the junction:

1. An action potential arrives at the axon terminal.
2. Acetylcholine is released by exocytosis into the synaptic cleft.
3. Acetylcholine binds receptors on the muscle fibre’s membrane (the sarcolemma), which depolarises.
4. The depolarisation spreads along the membrane and into the fibre, triggering calcium release.

All fibres in one motor unit contract together. Muscles that need fine control, such as those moving the eye, have small motor units with few fibres each. Muscles that need strong, coarse force have large units. The nervous system adjusts force by recruiting more motor units.

Worked example 2: recruiting motor units

A muscle has 40 motor units averaging 150 fibres each. Assume each active fibre exerts 0.05 N (supplied data).

• Total fibres: 40 × 150 = 6000.
• Recruiting 12 units activates 12 × 150 = 1800 fibres.
• Force: 1800 × 0.05 = 90 N.
• Recruiting all 40 units gives 6000 × 0.05 = 300 N.

B3.3.5 Skeletons: anchorage and levers

A skeleton does two jobs in movement. It gives muscles something firm to anchor to, and its bones or plates act as levers, pivoting about a joint.

• Arthropods have an exoskeleton, a hard external covering. Muscles attach to its inner surface.
• Vertebrates have an endoskeleton inside the body. Muscles attach to the outside of the bones by tendons.

A lever changes the size or speed of a force. In the forearm the elbow is the pivot, the biceps tendon attaches close to the pivot, and the load is carried in the hand.

Worked example 3: the forearm lever

A 20 N load is held 30 cm from the elbow. The biceps inserts 4 cm from the elbow. For balance, moments are equal (values supplied).

• Muscle force × 4 = 20 × 30.
• Muscle force = 600 ÷ 4 = 150 N.
• The hand moves 30 ÷ 4 = 7.5 times as far as the insertion point for the same contraction. The arrangement loses force but gains range and speed.

B3.3.6 The synovial joint

A synovial joint lets bones move against each other with low friction. Use the human hip as your model: the rounded head of the femur fits into the cup-shaped socket of the pelvis, which makes a ball-and-socket joint.

Part	Role
Bones	Act as levers and provide attachment points
Cartilage	Smooth covering on bone ends; reduces friction and absorbs some shock
Synovial fluid	Lubricates the joint, supplies nutrients to cartilage and cushions
Joint capsule	Encloses the joint and holds the fluid
Ligaments	Join bone to bone; stabilise the joint and limit excess movement
Muscles	Contract to produce movement; work in antagonistic pairs
Tendons	Join muscle to bone and transmit the pulling force

You need not name hip muscles or ligaments, but you should name the femur and the pelvis.

B3.3.7 Range of motion

Range of motion is the angle through which a joint can move in a given direction. A ball-and-socket joint can move in several dimensions: flexion and extension, abduction and adduction, and rotation.

You can measure it with a goniometer, a protractor with two arms. Place the pivot over the joint, align one arm with each bone, and read the angle at each end of the movement. You can also use computer analysis of images, marking points on the joint and measuring the angle in software.

Worked example 4: comparing ranges

A student measures hip flexion five times (degrees): 118, 122, 120, 115, 125. The mean is 600 ÷ 5 = 120°. Knee flexion data give a mean of 140°. The knee therefore flexes further than the hip in this person. After warming up, the hip range rises from 105° to 118°: (118 − 105) ÷ 105 × 100 = 12.4% increase.

To compare fairly, measure the same movement in the same posture, repeat readings, and keep one person measuring.

B3.3.8 Intercostal muscles

Intercostal muscles lie between the ribs. They are an antagonistic pair acting on the ribcage.

• External intercostals have fibres running in one direction. Contraction raises the ribs up and out, increasing thorax volume for inhalation.
• Internal intercostals have fibres running in the opposite direction. Contraction pulls the ribs down and in, which is useful in forced exhalation.

The different fibre orientations mean each layer pulls the ribcage in the opposite direction. When one layer contracts, it stretches the other, and the stretch stores potential energy in the sarcomere protein titin. The stored energy helps the stretched layer to recoil.

B3.3.9 Reasons for locomotion

Reason	Example
Foraging for food	A wolf pack covers long distances to hunt
Escaping from danger	A scallop swims by clapping its shell valves when touched by a starfish
Searching for a mate	Male moths follow a female’s pheromone plume
Migration	Arctic terns travel between polar regions; wildebeest follow rainfall

Worked example 5: migration speed

A bird migrates 9500 km in 8 days (supplied data). Mean distance per day = 9500 ÷ 8 = 1190 km (3 s.f.). If it flies for all 24 hours, that is 1190 ÷ 24 = about 49.5 km per hour. Real birds rest, so flying speed is higher.

B3.3.10 Swimming in marine mammals

Whales, dolphins and seals show adaptations for life in water:

• Streamlining: a smooth, tapered body, with few external projections, reduces drag.
• Flippers: the forelimbs are modified into flippers, used mainly for steering and balance.
• Fluke: the tail forms a horizontal fluke. Powerful muscles move it up and down, which drives the animal forward. (Fish tails move side to side.)
• Airways: the nostrils form a blowhole on top of the head. A large share of the air in the lungs is replaced in each breath, so the animal can breathe periodically between dives.

Worked example 6: swimming

A dolphin swims at 5.0 m per second with a tail beat frequency of 2.0 Hz (supplied). Distance per beat = 5.0 ÷ 2.0 = 2.5 m. If its body is 2.5 m long, that is one body length per beat. If a breath exchanges 9 L of a 12 L lung volume, the exchange is 9 ÷ 12 × 100 = 75%.

Common errors

• Saying filaments “shorten” or “contract”. They slide.
• Saying the A band shortens. It stays constant.
• Saying muscles push.
• Mixing up tendons (muscle to bone) and ligaments (bone to bone).
• Writing “fluke moves side to side”.

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.