6.3Organisms Respond to Changes in their Environments (A Level only)
Skeletal Muscles (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions
5 min read 12 parts Lesson 26 of 44 Unit quiz
Antagonistic Muscles, Sarcomere Structure and the Sliding Filament Theory — AQA A LEVEL BIOLOGY 7402 notes: Explain how skeletal muscle is organised, how sarcomeres change during contraction and how actin and myosin slide.Skeletal muscles enable movement when they pull on the rig…
- antagonistic pairs
- Worked example 1
- Worked example 2
- Worked example 3
- Worked example 4
- Calcium ions bind to troponin, which pulls tropomyosin away and exposes myosin-binding sites on actin
Section 1 of 12Antagonistic muscles
Skeletal muscles enable movement when they pull on the rigid, incompressible skeleton. They are attached to bones by tendons, which do not stretch when the muscle contracts. Muscles can only pull and cannot push, so they often work in antagonistic pairs: when one contracts the other relaxes (for example biceps and triceps at the elbow).
Section 2 of 12Structure of skeletal muscle
Skeletal muscle is also called striated (striped) muscle; it moves the skeleton.
Avoid common pitfalls
- myofibril (muscle) is not a microfibril (cellulose);
- muscle fibres are not protein filaments;
- myofibrils are not sarcomeres.
| Feature of sarcomere | Description | During contraction |
|---|---|---|
| Z-line | Boundary between sarcomeres | Z-lines get closer together |
| A band | Whole length of the myosin filaments (dark) | Stays the same length |
| I band | Only actin (light) | Becomes shorter |
| H zone | Only myosin (centre of A band) | Becomes shorter |
Section 3 of 12Examining muscle under a microscope
Pre-prepared slides of skeletal muscle tissue can be examined under an optical microscope, but the resolution is too low to see the detail of muscle structure; the visible features are the striped (banded) appearance of the fibres and the nuclei at the edge. The detailed structure of fibres, including sarcomeres, is visible using a TEM because electron microscopes have a higher resolution.
Section 4 of 12The sliding filament theory
During contraction actin and myosin filaments slide over each other, causing sarcomeres to shorten. The filaments themselves do not shorten. Myosin molecules have globular heads that can bind to actin; actin has binding sites covered at rest by tropomyosin, which is held by troponin.
This continues for as long as the fibre is stimulated by nerve impulses. When the muscle is no longer stimulated, calcium ions are removed, tropomyosin covers the binding sites again and the muscle relaxes.
Section 5 of 12Worked examples
Explain what happens to the A band, I band and H zone when a muscle contracts. [3]3 marks
A band stays the same length; I band and H zone become shorter, as the actin filaments slide in between the myosin filaments.
Explain why muscles work in antagonistic pairs. [2]2 marks
Muscles can only pull, not push; one muscle contracts to move a bone and the other contracts to pull it back.
Explain the role of calcium ions in muscle contraction. [3]3 marks
Calcium ions bind to troponin, moving tropomyosin and exposing binding sites on actin so myosin heads can attach and form cross-bridges.
Explain why a TEM is needed to see sarcomeres. [1]1 mark
TEMs have a higher resolution than optical microscopes.
Section 6 of 12Relaxation and ATP supply
After contraction, calcium ions return to the sarcoplasmic reticulum via active transport, which needs ATP. ATP for contraction can come from:
Section 7 of 12Slow and fast skeletal muscle fibres
There are two types of fibre in skeletal muscle. The relative proportion present in a muscle determines its properties, and muscle composition reflects the activities the muscle usually does.
| Feature | Fast muscle fibres | Slow muscle fibres |
|---|---|---|
| Contraction | Contract rapidly | Contract slowly |
| Fatigue | Fatigue quickly (lactate) | Resist fatigue; used for long periods |
| ATP source | Rely mainly on anaerobic respiration | Aerobic respiration |
| Mitochondria | Fewer, because aerobic respiration provides energy too slowly for their fast contraction | Many; also much myoglobin, many capillaries |
| Phosphocreatine | Large stores | Small stores, as aerobic respiration supplies enough ATP for slow contraction |
| Found in | Muscles for rapid movement, e.g. eyelids, biceps | Postural muscles, e.g. back and legs |
Link each feature to function.
Section 8 of 12Investigating muscle fatigue
Repeated contraction of muscles may result in muscle fatigue, due to the production of lactate during anaerobic respiration (lowering pH) and depletion of ATP and phosphocreatine.
Section 9 of 12Applying fibre types to examples
A sprinter's leg muscles contain a high proportion of fast fibres, giving a rapid burst of power fuelled by phosphocreatine and anaerobic respiration, followed by fatigue.
Section 10 of 12Worked examples
Explain why fast muscle fibres have fewer mitochondria than slow fibres. [2]2 marks
They rely mainly on anaerobic respiration and phosphocreatine; aerobic respiration provides energy too slowly for their rapid contraction.
Explain why slow fibres have small stores of phosphocreatine. [2]2 marks
They produce enough ATP for slow contraction by aerobic respiration.
Describe the results expected in an experiment on muscle fatigue and explain them. [3]3 marks
The number of successful squeezes decreases over time because lactate builds up and ATP supply falls.
Suggest one limitation of using only hand muscles in the fatigue investigation. [1]1 mark
Other muscle groups may fatigue at different rates, so conclusions cannot be generalised.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Antagonistic pair | Two muscles that work in opposition, one contracting as the other relaxes. |
| Myofibril | Rod-like structure in muscle fibres made of actin and myosin filaments. |
| Sarcomere | The repeating contractile unit of a myofibril, between two Z-lines. |
| Actin | Thin protein filament in muscle. |
| Myosin | Thick protein filament with heads that bind to actin. |
| Tendon | Inelastic tissue connecting muscle to bone. |
| Term | Meaning |
|---|---|
| Phosphocreatine | Store in muscle that rapidly phosphorylates ADP to ATP. |
| Fast muscle fibre | Fibre that contracts rapidly using anaerobic ATP supply. |
| Slow muscle fibre | Fibre that contracts slowly and depends on aerobic respiration. |
| Muscle fatigue | Reduced ability to contract after repeated contractions. |
| Lactate | Product of anaerobic respiration that contributes to fatigue. |
Section 12 of 12Exam technique
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