7.1 Muscles & Movement — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

Section 1 of 8Muscles & Movement

1

Skeletal muscle → muscle fibres (multinucleate cells) → myofibrils → sarcomeres arranged end to end, giving a striated appearance.

2

Thin filaments are mainly actin, with tropomyosin wound round them and troponin attached at intervals; thick filaments are myosin, with protruding heads that have ATPase activity.

3

The A band is dark (myosin present); the I band is light (actin only); the H zone is the centre where only myosin occurs; the Z line anchors actin and marks sarcomere ends; the M line anchors myosin.

4

On contraction, I band and H zone shorten, the sarcomere shortens, the A band stays the same length – evidence that filaments slide rather than shorten.

5

At the elbow, biceps contracting flexes the joint while triceps relaxes; triceps contracting extends it. Tendons transmit force; ligaments hold bones together.

6

Synovial joint components

Synovial joint components cartilage (smooth, reduces friction, absorbs shock), synovial fluid (lubricates, nourishes cartilage), synovial membrane (secretes fluid), capsule and ligaments (stability).

7

Stimulation

Stimulation acetylcholine at the neuromuscular junction → action potential along sarcolemma and T-tubules → Ca²⁺ released from sarcoplasmic reticulum.

8

Cross-bridge cycle

Cross-bridge cycle bind → power stroke (ADP + P_i released) → ATP binds, head detaches → ATP hydrolysed, head re-cocked → rebinds further along actin.

9

Relaxation needs ATP for active transport of Ca²⁺ back into the sarcoplasmic reticulum; tropomyosin then blocks binding sites again. Muscle is stretched back by the antagonist.

10

Fast twitch fibres have more sarcoplasmic reticulum and myosin with faster ATPase activity; slow twitch fibres have more mitochondria, myoglobin and capillaries.

11

The proportion of fibre types is partly genetic and partly altered by training; elite sprinters have more fast twitch, endurance athletes more slow twitch fibres.

12

Muscle fibre terms

Muscle fibre parts have special names

sarcolemma (cell surface membrane), sarcoplasm (cytoplasm) and sarcoplasmic reticulum (ER).
T-tubules are infoldings of the sarcolemma that carry depolarisation deep into the fibre.
Fibres are multinucleate and packed with mitochondria to supply ATP.

13

Thick & thin filaments

Each myosin molecule has a fibrous tail anchored in the thick filament and a globular head; heads point away from the M line. A thin filament is two chains of globular actin twisted together, with fibrous tropomyosin wound round them and troponin attached at intervals.

14

Rigor mortis

After death, ATP is no longer produced, so ATP cannot bind to myosin heads to detach them from actin. Cross-bridges stay locked and muscles remain stiff, which is rigor mortis. This shows ATP is needed for cross-bridge release.

15

Fibre type distribution

Muscles needing sustained contraction, such as back (postural) muscles, have a high proportion of slow twitch fibres. Muscles making brief, rapid movements, such as eyelid muscles, are rich in fast twitch fibres, which tire quickly due to lactate build-up.

16

Role of the skeleton

Muscles are effectors stimulated by motor neurones. They can only produce effective movement by pulling on a rigid, incompressible skeleton; bones act as levers that do not bend or shorten, so contraction moves the bone around the joint.

Section 2 of 8Process & sequence

1

A nerve impulse arrives at the neuromuscular junction; acetylcholine diffuses across and depolarises the sarcolemma.

2

Depolarisation spreads along the sarcolemma and down T-tubules, causing the sarcoplasmic reticulum to release Ca²⁺.

3

Ca²⁺ binds to troponin, which changes shape and pulls tropomyosin away from the myosin-binding sites on actin.

4

Myosin heads (with ADP + P_i attached) bind to actin, forming cross-bridges.

5

ADP and P_i are released and the myosin head tilts (power stroke), pulling actin towards the centre of the sarcomere.

6

A new ATP binds to the myosin head, causing it to detach; ATPase on the head hydrolyses ATP to ADP + P_i, re-cocking the head.

7

The cycle repeats while Ca²⁺ is present;

when stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum and tropomyosin re-blocks the sites.

Cross-bridge cyclewrite these steps in this order; each step is a marking point.

1

Ca²⁺ binds troponin; tropomyosin moves; binding sites exposed.

2

Myosin head binds actin forming a cross-bridge.

3

Power stroke: head tilts, actin pulled; ADP + P_i released.

4

ATP binds; myosin head detaches from actin.

5

ATP hydrolysed; head returns to cocked position ready to rebind.

Section 3 of 8Compare

Slow twitch vs fast twitch fibres
Feature	Slow twitch (Type I)	Fast twitch (Type II)
Speed of contraction	Slow	Fast
Fatigue	Resistant	Fatigue quickly
Main respiration	Aerobic	Anaerobic (lactate fermentation)
Mitochondria	Many	Few
Myoglobin / colour	High / red	Low / pale
Capillaries	Many	Few
Stores	Little glycogen	Much glycogen and creatine phosphate
Sarcoplasmic reticulum	Less extensive	Extensive – rapid Ca²⁺ release
Use	Posture, endurance (marathon)	Sprinting, lifting

Bands of the sarcomere during contraction
Region	What it contains	Change on contraction
A band	Full length of myosin (with overlapping actin)	Stays the same
I band	Actin only	Shortens
H zone	Myosin only	Shortens / disappears
Sarcomere (Z to Z)	Whole unit	Shortens

Section 4 of 8Worked examples

Worked examples

2.3 µm

On a micrograph (magnification × 20 000), a sarcomere measures 46 mm. Calculate its actual length in µm.

46 mm = 46 000 µm. Actual = image ÷ magnification = 46 000 ÷ 20 000 = 2.3 µm.

A sarcomere shortens from 2.4 µm to 1.8 µm. Calculate the percentage decrease.

Change = 2.4 − 1.8 = 0.6 µm. % decrease = 0.6 ÷ 2.4 × 100 = 25%.

0.8 µm

An A band is 1.6 µm and a relaxed sarcomere is 2.4 µm. Calculate the total I-band length in the sarcomere, and state its length when the sarcomere contracts to 2.0 µm.

I band total = 2.4 − 1.6 = 0.8 µm (0.4 µm each side).
- A band remains 1.6 µm, so contracted I band = 2.0 − 1.6 = 0.4 µm.

Section 5 of 8Keep going

Muscles & Movement Edexcel A Level Biology SNAB: what to do next

These Muscles & Movement Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your Muscles & Movement Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any Muscles & Movement Edexcel A Level Biology SNAB point you were unsure about before moving on.

Muscles & Movement Edexcel A Level Biology SNAB: revision checklist

1

Revise the main Muscles & Movement Edexcel A Level Biology SNAB definitions until you can write them from memory.

2

Practise Muscles & Movement Edexcel A Level Biology SNAB exam questions under timed conditions.

3

Link each Muscles & Movement Edexcel A Level Biology SNAB idea to a diagram or a worked example.

4

List the common Muscles & Movement Edexcel A Level Biology SNAB mistakes and how to avoid them.

5

Explain one Muscles & Movement Edexcel A Level Biology SNAB idea out loud as if teaching a friend.

6

Summarise Muscles & Movement Edexcel A Level Biology SNAB in five key points on a revision card.

1

Practice: Muscles & Movement Edexcel A Level Biology SNAB practice questions

2

Previous lesson: 6.2 Forensics — 9BI0 Notes

3

Next lesson: 7.2 Respiration — 9BI0 Notes

4

Course: Edexcel A Level Biology A (snab) 9BI₀ — all units

5

Official syllabus: Pearson Edexcel A Levels

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Skeletal muscle · Tendon · Ligament · Synovial joint · Antagonistic muscles · Flexor / extensor · Sarcomere · Sliding filament theory · Cross-bridge · Tropomyosin and troponin · Sarcoplasmic reticulum · Slow twitch fibre · Fast twitch fibre · Myoglobin

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

4

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

6

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1

the actin and myosin filaments shortenthe filaments slide past each other so the sarcomere shortens; filament lengths stay the same.

2

ATP is used to make the myosin bindATP binding causes the myosin head to detach; its hydrolysis re-cocks the head.

3

calcium binds to tropomyosinCa²⁺ binds to troponin, which moves tropomyosin away from the binding sites.

4

the triceps pushes the arm straightthe triceps contracts, pulling on the ulna to extend the elbow while the biceps relaxes.

5

ligaments join muscle to bonetendons join muscle to bone; ligaments join bone to bone.

6

fast twitch fibres have more energyfast twitch fibres generate ATP rapidly by anaerobic respiration and creatine phosphate but fatigue quickly.