7.1Topic 7 · Run for your Life
Muscles and Movement — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
10 min read 11 parts Lesson 2 of 25 1 quiz
Skeletal Muscle: Movement and Muscle Fibres — EDEXCEL A LEVEL BIOLOGY A (SNAB) 9BI0 — REVISION NOTES notes: Explain antagonistic muscle action, describe muscle fibre ultrastructure and compare fast and slow twitch fibres.Effective movement of the human body requires both muscle a…
- antagonistic pairs
- sarcolemma
- sarcoplasm
- myofibrils
- Worked example 1
- Worked example 2
Section 1 of 11Muscles and the skeleton
Effective movement of the human body requires both muscle and an incompressible skeleton: muscles produce effective movement only if they pull on a structure that does not shorten or bend, i.e. bone.
Section 2 of 11Muscle fibres and myofibrils
Skeletal muscle is attached to the skeleton and is made of muscle fibres.
- which contains many mitochondria and a specialised endoplasmic reticulum that stores calcium ions.
- (the sarcoplasmic reticulum)
| Part | Description |
|---|---|
| A band | Dark band; the length of the myosin filaments (and overlapping actin) |
| I band | Light band; actin only, no myosin |
| H zone | Centre of the A band; myosin only |
| Z line | Boundary of a sarcomere where actin is anchored |
| M line | Middle of the sarcomere, linking myosin |
Section 3 of 11Fast and slow twitch fibres
There are two types of muscle fibre, and human muscles contain both. Some muscles have higher proportions of one type because of their different properties.
| Feature | Slow twitch | Fast twitch |
|---|---|---|
| Contraction | Slow, sustained | Fast, powerful, in short bursts |
| Respiration | Aerobic | Anaerobic (glycolysis) |
| Mitochondria | Many | Fewer |
| Capillaries | Many | Fewer |
| Myoglobin | High (red) | Low (pale) |
| Fatigue | Slow to tire | Tire quickly (lactic acid) |
| Found in | Postural muscles, long-distance running | Limbs for sprinting, e.g. cheetah legs, robin wings, human eyelids |
Fast twitch fibres are often found in high proportions in the limbs of animals that flee a predator or hunt at high speed, such as the wings of a robin and legs of a cheetah, and in human eyelids, which contract in short bursts and need not sustain rapid movement.
Section 4 of 11Worked examples
Explain why muscles work in antagonistic pairs. [2]2 marks
Muscles can only pull; one muscle of the pair contracts to bend the joint, while the other contracts to straighten it, so movement in both directions is possible.
Describe the ultrastructure of a myofibril. [3]3 marks
Repeating sarcomeres containing thick myosin and thin actin filaments overlapping in a pattern of A, I and H bands, with actin anchored at Z lines.
Explain why marathon runners have many slow twitch fibres. [2]2 marks
Slow twitch fibres use aerobic respiration, with many mitochondria and a good blood supply, so can contract for a long time without fatigue.
Section 5 of 11Structure of the filaments in a myofibril
Skeletal muscle fibres contain bundles of myofibrils. Each myofibril is divided along its length into repeating units called sarcomeres, bounded by Z discs. Two types of protein filament lie inside a sarcomere and give the muscle its striped appearance.
- Each myosin molecule is a fibrous protein with a globular head.
- The heads project outwards from the filament and each head has a binding site for actin and a binding site for ATP.
- The head also acts as an ATPase enzyme.
- Wrapped around the actin is a long fibrous protein, tropomyosin, with smaller troponin molecules attached at intervals.
- Actin has myosin-binding sites along it.
When the muscle is relaxed, tropomyosin lies across the myosin-binding sites on actin and blocks them, so myosin heads cannot attach.
- and the H zone is the central part of the A band containing only thick filaments.
Section 6 of 11Triggering contraction: calcium ions and the sarcoplasmic reticulum
A nerve impulse arriving at the neuromuscular junction depolarises the muscle fibre membrane (sarcolemma). The depolarisation spreads through the fibre and stimulates the sarcoplasmic reticulum (SR), a specialised endoplasmic reticulum that stores calcium ions, to release Ca2+ into the sarcoplasm.
Muscle contraction therefore needs both calcium ions and ATP. Calcium ions switch the binding sites on, and ATP supplies the energy for movement and detachment.
Section 7 of 11The cross-bridge cycle
Once the binding sites are exposed the following repeating cycle takes place. It occurs many times per second in every sarcomere.
- forming an actomyosin cross-bridge between the thick and thin filaments.
- This pulls the actin filament towards the centre of the sarcomere, so the muscle contracts a very small distance.
- The energy released moves the head back to its original, cocked position.
The filaments themselves do not shorten; they slide past each other. This is why the model is called the sliding filament theory.
Section 8 of 11Changes in the sarcomere and relaxation
When a sarcomere contracts, the actin filaments are pulled inwards towards the middle. As a result:
| Region | Change on contraction |
|---|---|
| Sarcomere length (Z disc to Z disc) | Decreases |
| I band (thin filaments only) | Gets shorter |
| H zone (thick filaments only) | Gets shorter or disappears |
| A band (length of thick filaments) | Stays the same |
Relaxation happens when stimulation stops. Calcium ions leave their binding sites on troponin and are actively transported back into the sarcoplasmic reticulum, which needs ATP. Troponin and tropomyosin return to their resting positions and cover the binding sites again, so cross-bridges cannot form and the muscle relaxes.
The ATP for contraction comes from respiration. Mitochondria are packed between myofibrils, and a short store of phosphocreatine can rapidly regenerate ATP from ADP during intense activity.
Section 9 of 11Worked examples
Describe the role of calcium ions in muscle contraction. [4]4 marks
Calcium ions are released from the sarcoplasmic reticulum when the muscle fibre is stimulated. They bind to troponin, causing it to change shape. This moves tropomyosin away from the myosin-binding sites on actin. Myosin heads can then bind to actin and form cross-bridges, so contraction can occur.
Explain the role of ATP in the cross-bridge cycle. [3]3 marks
ATP binds to the myosin head and causes it to detach from actin. ATPase then hydrolyses the ATP to ADP and Pi, releasing energy that returns the myosin head to its original position (recovery stroke) so it can bind to a new site. ATP is also used by active transport to pump calcium ions back into the SR.
A sarcomere shortens during contraction. State what happens to the lengths of the A band and the I band, and explain your answer. [3]3 marks
The A band stays the same length because it is the length of the thick myosin filaments, which do not shorten. The I band gets shorter because the actin filaments slide inwards between the myosin filaments, reducing the region containing only actin.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Antagonistic pair | Two muscles that pull in opposite directions. |
| Myofibril | Bundle of actin and myosin filaments in a muscle fibre. |
| Sarcomere | Contractile unit of a myofibril. |
| Sarcoplasm | Cytoplasm of a muscle fibre. |
| Slow twitch fibre | Fatigue-resistant fibre using aerobic respiration. |
| Fast twitch fibre | Fibre that contracts quickly using anaerobic respiration. |
| Term | Meaning |
|---|---|
| Sarcomere | The repeating contractile unit of a myofibril, lying between two Z discs. |
| Myosin | The thick filament protein whose globular heads bind actin and act as ATPase. |
| Actin | The thin filament protein that has myosin-binding sites. |
| Tropomyosin | Fibrous protein that covers the myosin-binding sites on actin in resting muscle. |
| Troponin | Protein bound to tropomyosin that binds calcium ions and moves tropomyosin. |
| Sarcoplasmic reticulum | Specialised endoplasmic reticulum in muscle that stores and releases calcium ions. |
| Cross-bridge | Temporary link formed when a myosin head binds to actin. |
| Recovery stroke | Return of the myosin head to its original position after ATP hydrolysis. |
Section 11 of 11Exam technique
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