DP Biology HL B3.3 Muscle and Motility

DP Biology HL B3.3 Muscle and Motility - Full notes

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DP Biology HL | B3.3 Muscle and Motility

DP BIOLOGY · THEME B: FORM AND FUNCTION

B3.3

Muscle and Motility

Full notes for HL · IB-style

Contents

1	Movement and motility
2	Skeletal muscle structure
3	The sarcomere and the sliding filament model
4	The control of contraction: calcium and ATP
5	Muscles, bones and joints
6	Antagonistic muscles and the elbow
7	Movement in other organisms
8	Sessile and motile organisms
9	Structure of striated muscle
10	The sarcomere
11	The sliding filament model
12	Measuring sarcomere length
13	Titin and muscle elasticity
14	Antagonistic muscles
15	Joints and the skeleton
16	Range of motion
17	Intercostal muscles and breathing
18	Adaptations for swimming
19	Muscle fatigue and fibre types
20	Mixed review: worked problems
21	Additional exam-style practice
22	Motility in single cells
23	Common misconceptions and links
24	Exam-style question with markscheme
25	Check your understanding
26	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
distinguish movement and motility with examples
describe the structure of skeletal muscle from fibre to myofibril
draw and label a sarcomere and explain the sliding filament model
explain the roles of calcium, troponin, tropomyosin, ATP and titin
explain how muscles, bones and joints produce movement
explain antagonistic muscle action at the elbow
explain reasons for locomotion and adaptations for swimming

B3.3 is an Additional Higher Level (AHL) subtopic. It is studied by HL students only.

Guiding questions

How do muscles contract and cause movement? What are the benefits of motility?

How this topic is examined

B3.3 is an HL-only subtopic. It is examined through sarcomere diagrams and electron micrographs (identify bands, measure lengths), the sliding filament mechanism, and joint structure.

Be precise about which band changes length during contraction: the sarcomere and I band shorten, the A band does not.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Movement and motility

Many organisms move; motile organisms move from place to place.

Think about it

A sunflower turns to follow the Sun and a cheetah sprints after a gazelle. Are both of these 'motility'?

Movement is any change in position of an organism or part of it. Sessile organisms such as plants and corals move parts of their bodies but stay fixed in one place. Motility is the ability of an organism to move from place to place by its own effort: locomotion.

Organisms move for many reasons: to find food, escape predators, find mates, migrate to better conditions, and disperse to new habitats. Animal locomotion uses muscles acting on a skeleton or on fluid; single cells use cilia and flagella.

Quick check

1. Define motility.

2. Give three reasons for locomotion.

Answers: 1. the ability to move from place to place 2. food, escape, mates, migration, dispersal

2 Skeletal muscle structure

Muscles are bundles of fibres, which are bundles of myofibrils.

A skeletal muscle consists of bundles of muscle fibres. Each fibre is a single very long cell, formed by the fusion of many cells, so it is multinucleate. It is surrounded by a plasma membrane (sarcolemma) and contains many mitochondria to supply ATP, a specialized endoplasmic reticulum called the sarcoplasmic reticulum that stores calcium ions, and many myofibrils.

Myofibrils run the length of the fibre. They are made of repeating units called sarcomeres joined end to end, which give skeletal muscle its striated (striped) appearance under the microscope.

Figure 1. Levels of organization in skeletal muscle.

Quick check

1. Why are muscle fibres multinucleate?

2. What does the sarcoplasmic reticulum store?

Answers: 1. formed by fusion of many cells 2. calcium ions

3 The sarcomere and the sliding filament model

Myosin pulls actin towards the centre.

A sarcomere is the unit between two Z lines. It contains two types of protein filament. Thin filaments of actin are attached to the Z lines and extend towards the centre. Thick filaments of myosin lie in the centre, overlapping with the actin. Myosin filaments have heads that can bind to actin.

Bands seen under the microscope: the A band (dark) is the length of the myosin filaments, including overlap with actin; the I band (light) contains only actin; the H zone is the central part of the A band with only myosin; the M line is the centre of the sarcomere.

During contraction, the myosin heads pull the actin filaments towards the M line. The filaments themselves do not shorten; they slide past each other, increasing the overlap. This is the sliding filament model. As a result, the sarcomere shortens, the I band and H zone shorten, but the A band stays the same length.

A large elastic protein, titin, runs from the Z line to the M line. It keeps myosin centred, prevents overstretching, and helps the sarcomere recoil after contraction.

Figure 2. A relaxed sarcomere: actin attached to Z lines, myosin in the centre.

Region	Contains	During contraction
Sarcomere	Z line to Z line	shortens
A band	myosin (with overlapping actin)	stays the same
I band	actin only	shortens
H zone	myosin only	shortens (may disappear)

WORKED EXAMPLE 1 Measuring sarcomere length

In an electron micrograph at ×20 000, the distance between two Z lines is 46 mm. Calculate the actual sarcomere length.

Solution

46 mm = 46 000 µm in the imageM1

Actual length = 46 000 ÷ 20 000 = 2.3 µmA1

Common mistake

Saying the actin and myosin filaments shorten. They slide past each other; only the sarcomere, I band and H zone shorten.

Quick check

1. Which bands shorten during contraction?

2. What is titin?

Answers: 1. sarcomere, I band and H zone (A band stays the same) 2. an elastic protein that prevents overstretching and aids recoil

4 The control of contraction: calcium and ATP

Calcium unlocks the binding sites; ATP powers the stroke.

When a motor neuron stimulates a muscle fibre, an impulse spreads along the sarcolemma and into the fibre. This causes the sarcoplasmic reticulum to release calcium ions into the cytoplasm around the myofibrils.

In a resting muscle, the binding sites on actin are covered by a protein called tropomyosin, held in place by troponin. Calcium ions bind to troponin, which changes shape and moves tropomyosin aside, exposing the binding sites on actin.

The cross-bridge cycle then repeats: (1) a myosin head, already 'cocked' by energy from hydrolysis of ATP, binds to actin, forming a cross-bridge; (2) releasing ADP and phosphate, the head pivots (the power stroke), pulling actin towards the M line; (3) a new ATP binds to the myosin head, causing it to detach from actin; (4) ATP is hydrolysed, and the energy re-cocks the head ready for the next cycle. Many heads working asynchronously produce smooth contraction.

When stimulation stops, calcium is actively pumped back into the sarcoplasmic reticulum (using ATP), tropomyosin covers the binding sites again, and the muscle relaxes. After death, ATP runs out, so myosin heads cannot detach from actin: this causes rigor mortis.

Figure 3. The cross-bridge cycle.

Quick check

1. What does calcium bind to?

2. What is the role of ATP in the cross-bridge cycle?

3. Why does rigor mortis occur?

Answers: 1. troponin 2. detaches myosin from actin and re-cocks the head 3. no ATP, so myosin cannot detach from actin

5 Muscles, bones and joints

Muscles pull on bones across joints.

Muscles can only pull (contract); they cannot push. To move a body part, muscles pull on bones, which act as levers, rotating about joints. Muscles are attached to bones by tendons (tough, inelastic collagen), and bones are attached to each other at joints by ligaments.

The exoskeleton of arthropods (made of chitin) serves the same role from the outside: muscles attach to its inner surface.

A synovial joint, such as the elbow, knee or hip, allows free movement. Its structure: cartilage covering the ends of the bones, reducing friction and absorbing shock; a synovial membrane that secretes synovial fluid, which lubricates the joint; a joint capsule that encloses the joint; and ligaments that hold the bones together and limit movement.

Structure	Function
Cartilage	smooth surface; reduces friction; absorbs shock
Synovial fluid	lubricates; nourishes cartilage
Synovial membrane	secretes synovial fluid
Joint capsule	encloses and protects the joint
Ligaments	connect bone to bone; stabilize
Tendons	connect muscle to bone

Quick check

1. Distinguish tendons and ligaments.

2. What is the role of synovial fluid?

Answers: 1. tendons: muscle to bone; ligaments: bone to bone 2. lubricates the joint, reducing friction

6 Antagonistic muscles and the elbow

Pairs of muscles pull in opposite directions.

Because muscles only pull, they work in antagonistic pairs: when one contracts, the other relaxes, and they cause opposite movements. At the elbow, the biceps contracts to flex (bend) the arm, while the triceps relaxes; the triceps contracts to extend (straighten) the arm, while the biceps relaxes. The biceps is attached to the radius, the triceps to the ulna; the humerus is the upper arm bone.

Antagonistic muscles also work in insects: in the leg of a grasshopper, flexor and extensor muscles inside the exoskeleton bend and straighten the leg for jumping. Joints vary in their range of motion: the elbow is a hinge joint (movement in one plane); the hip is a ball and socket joint (movement in many planes).

Quick check

1. Why do muscles work in antagonistic pairs?

2. Which muscle flexes the elbow?

Answers: 1. muscles can only pull; one muscle reverses the movement of the other 2. biceps

7 Movement in other organisms

Adaptations for swimming.

Many organisms are adapted for locomotion in water. Swimming marine mammals such as dolphins show: a streamlined body shape that reduces drag; flippers (modified forelimbs) for steering; a powerful tail fluke moved up and down by muscles; a blowhole for breathing at the surface; and blubber for buoyancy and insulation.

Fish have streamlined bodies, fins and a swim bladder; their muscles are arranged in segments (myotomes) that contract in sequence. Single cells such as sperm move with flagella; Paramecium uses cilia.

8 Sessile and motile organisms

Why move?

Motile organisms move their whole body (most animals); sessile organisms are fixed in place (plants, corals, barnacles) but may move parts. Reasons for locomotion: finding food, escaping predators, finding mates, migration and dispersal.

WORKED EXAMPLE 2 Examples (basic)

Give two reasons why animals need locomotion.

Solution

Foraging for foodA1

Escaping predators; finding mates; migrationA1

9 Structure of striated muscle

From muscle to myofibril.

A muscle contains bundles of muscle fibres (multinucleate cells). Each fibre contains many myofibrils made of repeating sarcomeres. Fibres have many mitochondria and a specialized ER (sarcoplasmic reticulum) that stores Ca2+.

WORKED EXAMPLE 3 Hierarchy (standard)

Arrange in order of size: sarcomere, muscle fibre, myofibril, muscle.

Solution

muscle > muscle fibre > myofibril > sarcomereA1

10 The sarcomere

Bands and lines.

A sarcomere runs from one Z line to the next. Thin actin filaments attach to Z lines; thick myosin filaments occupy the centre. The A band (dark) is the length of the myosin; the I band (light) contains only actin; the H zone contains only myosin.

Region	Contains	During contraction
A band	myosin (and overlapping actin)	stays the same
I band	actin only	shortens
H zone	myosin only	shortens
sarcomere	Z line to Z line	shortens

WORKED EXAMPLE 4 Bands (exam level)

Explain why the A band does not change length during contraction.

Solution

The A band is the length of the myosin filamentsR1

Filaments slide; they do not shortenR1

11 The sliding filament model

Cross-bridge cycling.

1. Ca2+ released from the sarcoplasmic reticulum binds to troponin, moving tropomyosin and exposing binding sites on actin. 2. Myosin heads (with ADP + Pi) bind to actin, forming cross-bridges. 3. Release of ADP and Pi causes the power stroke, pulling actin towards the centre. 4. ATP binds, myosin detaches. 5. ATP hydrolysis re-cocks the myosin head.

WORKED EXAMPLE 5 Role of ATP (exam level)

Explain the two roles of ATP in muscle contraction.

Solution

ATP binding causes myosin to detach from actinR1

ATP hydrolysis re-cocks (re-energizes) the myosin headR1

In the real world

Rigor mortis occurs after death because ATP runs out: myosin heads remain attached to actin, locking muscles rigid.

Quick check

1. What is the role of calcium ions?

Answers: 1. bind troponin, moving tropomyosin to expose binding sites

12 Measuring sarcomere length

From micrographs.

Sarcomere length can be measured from electron micrographs using magnification. Resting sarcomeres are about 2.5 μm; contracted about 1.8 μm.

WORKED EXAMPLE 6 Calculation (standard)

Five sarcomeres measure 45 mm in a micrograph at × 4000. Find the length of one sarcomere.

Solution

45 mm/5 = 9 mm = 9000 μmM1

9000/4000 = 2.25 μmA1

13 Titin and muscle elasticity

A molecular spring.

Titin is a giant protein running from the Z line to the M line. It acts as a spring, preventing overstretching and helping the sarcomere recoil to resting length after contraction.

Quick check

1. State one function of titin.

Answers: 1. prevents overstretching; provides elastic recoil

14 Antagonistic muscles

Muscles only pull.

Muscles can only contract (pull); they are relaxed and stretched by an opposing (antagonistic) muscle. At the elbow, the biceps flexes and the triceps extends the arm. In insects, flight muscles act antagonistically on the thorax.

WORKED EXAMPLE 7 Elbow (standard)

Describe the antagonistic action of the biceps and triceps.

Solution

Biceps contracts, triceps relaxes: arm bends (flexion)R1

Triceps contracts, biceps relaxes: arm straightens (extension)R1

15 Joints and the skeleton

Levers and synovial joints.

Bones act as levers, with joints as pivots. Synovial joints (elbow, knee, hip) have cartilage (reduces friction, absorbs shock), synovial fluid (lubricates) and a joint capsule with ligaments (holds bones together). Tendons attach muscles to bones.

Structure	Function
cartilage	smooth surface; shock absorption
synovial fluid	lubrication
ligaments	connect bone to bone; stability
tendons	connect muscle to bone; transmit force

16 Range of motion

Hinge and ball-and-socket joints.

Hinge joints (elbow, knee) allow movement in one plane; ball-and-socket joints (hip, shoulder) allow movement in many planes. Range of motion can be measured with a goniometer or from photographs.

WORKED EXAMPLE 8 Comparing (standard)

Why does the hip allow more movement than the knee?

Solution

Ball-and-socket allows rotation in several planesR1

Knee is a hinge: one planeR1

Try this

Use a protractor app on photographs to measure the range of motion of your elbow and shoulder. Compare the two joints.

17 Intercostal muscles and breathing

Antagonistic pairs in ventilation.

External intercostal muscles contract to raise the ribs (inhalation); internal intercostals contract to lower them in forced exhalation. Their fibres run in different directions, so each pulls the ribs in an opposite direction.

Quick check

1. Why must the two intercostal layers be antagonistic?

Answers: 1. each can only pull; opposite directions move ribs up or down

18 Adaptations for swimming

Locomotion in water.

Marine mammals show adaptations for swimming: streamlined bodies; flippers and flukes (modified limbs); blubber for insulation and buoyancy; nostrils (blowholes) on top of the head; the ability to hold breath (high myoglobin).

WORKED EXAMPLE 9 Adaptations (exam level)

Outline three adaptations of dolphins for swimming.

Solution

Streamlined body reduces dragR1

Fluke moves up and down for propulsionR1

Flippers for steering; blowhole for breathing at surfaceR1

19 Muscle fatigue and fibre types

Fast and slow twitch.

Slow-twitch fibres (many mitochondria, myoglobin, capillaries) contract slowly and resist fatigue: endurance. Fast-twitch fibres rely on anaerobic respiration, contract rapidly and fatigue quickly: sprinting. Training changes the proportions slightly.

Quick check

1. Which fibre type would dominate in a marathon runner?

Answers: 1. slow-twitch

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 10 Mixed problem (exam level)

A drug blocks the release of Ca2+ from the sarcoplasmic reticulum. Predict and explain the effect on muscle contraction.

Solution

Tropomyosin continues to block actin binding sitesR1

No cross-bridges form: muscle cannot contractR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 11 Paper 1 question (standard)

Which band shortens during contraction? A. A band B. I band C. myosin filament D. actin filament

Solution

BA1

WORKED EXAMPLE 12 Paper 2 question (exam level)

Outline the structure of a synovial joint.

Solution

Cartilage on bone ends; synovial fluidR1

Joint capsule; ligamentsR1

22 Motility in single cells

Cilia and flagella.

Single-celled organisms move using cilia (Paramecium) or flagella (sperm, Chlamydomonas). Eukaryotic cilia and flagella contain a 9 + 2 arrangement of microtubules; dynein motor proteins use ATP to bend them.

Quick check

1. Name the motor protein that bends eukaryotic flagella.

Answers: 1. dynein

23 Common misconceptions and links

Students often think…	Actually…
actin and myosin shorten	they slide; the sarcomere shortens
muscles push to extend joints	muscles only pull; antagonists reverse movement
ATP is used to make the myosin head bind	ATP detaches the head and re-cocks it
the A band shortens	the A band stays the same length

How B3.3 connects to the rest of the course

Topic	Connection
B2.3 Cell specialization	multinucleate muscle fibres
C2.2 Neural signalling	motor neurons and neuromuscular junctions
C1.2 Respiration	ATP supply
B1.2 Proteins	actin, myosin and titin

24 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Text A. An electron micrograph of a sarcomere at ×20 000 shows Z lines 46 mm apart. In a contracted sarcomere, the Z lines are 36 mm apart.

(a) Calculate the length of the relaxed and contracted sarcomeres. [2]

(b) State which bands shorten during contraction. [2]

(d) Explain the role of ATP in the cross-bridge cycle. [3]

Markscheme

Part	Answer	Marks
(a)	relaxed 2.3 µm	A1
	contracted 1.8 µm	A1
(b)	I band and H zone (and sarcomere)	A2
(c)	released from sarcoplasmic reticulum	R1
	bind to troponin	R1
	tropomyosin moves, exposing binding sites on actin	R1
(d)	ATP binding detaches myosin from actin	R1
	hydrolysis re-cocks the myosin head	R1
	allows repeated cycles / power strokes	R1

Examiner's comments. In (b), stating that the A band shortens lost the mark. In (d), many candidates said ATP 'causes binding'; it causes detachment.

25 Check your understanding

Section A: multiple choice

1. During contraction the A band

A. stays the same length B. shortens C. lengthens D. disappears

2. Calcium ions bind to

A. myosin B. actin C. troponin D. titin

3. Binding of ATP to myosin causes

A. the power stroke B. detachment from actin C. calcium release D. tropomyosin to cover sites

4. Tendons connect

A. bone to bone B. muscle to muscle C. cartilage to bone D. muscle to bone

5. The biceps

A. extends the elbow B. flexes the elbow C. rotates the knee D. connects radius to ulna

6. Synovial fluid

A. lubricates joints B. connects bones C. stores calcium D. contracts

Section B: structured

7. Draw and label a sarcomere. [4]

8. Explain the sliding filament model. [5]

9. Explain how antagonistic muscles move the forearm. [3]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Z lines, actin, myosin, A band, I band, H zone, M line.

8. Ca²⁺ release, troponin/tropomyosin, cross-bridges, power stroke, ATP detachment, filaments slide, sarcomere shortens.

9. Biceps contracts and triceps relaxes to flex; triceps contracts and biceps relaxes to extend; muscles only pull.

26 Summary and topic map

Idea	Remember
Motility	locomotion; food, escape, mates, migration
Muscle fibre	multinucleate; sarcoplasmic reticulum; mitochondria
Sarcomere	Z lines; actin; myosin; A, I, H
Sliding filaments	myosin pulls actin; I and H shorten
Control	Ca²⁺, troponin, tropomyosin; ATP for detachment
Joints	cartilage, synovial fluid, ligaments, tendons
Antagonistic pairs	biceps and triceps

Before the exam, make sure you…

•state which bands shorten and which do not
•sequence the cross-bridge cycle correctly
•distinguish tendons from ligaments
•explain antagonistic action

Muscle and Motility on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 4. Topic map for Muscle and Motility.

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