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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

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

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

Part 1 of 4Muscles 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.

There are over 600 skeletal muscles in the human body.
Tendons
Muscles are attached to bones by tendons, and bones are joined at joints by ligaments.
Antagonistic pairs
Muscles can only pull (by contracting), not push, so they work in antagonistic pairs that pull in opposite directions.
When one contracts, the other relaxes.
Flexor
A muscle that bends a joint is a flexor (e.g. biceps bends the elbow); a muscle that straightens a joint during contraction is an extensor (the triceps).

Part 2 of 4Muscle fibres and myofibrils

Skeletal muscle is attached to the skeleton and is made of muscle fibres.

Sarcolemma
A muscle fibre is a highly specialised, long, multinucleate cell-like unit surrounded by a membrane called the sarcolemma; its cytoplasm is the sarcoplasm,
  • which contains many mitochondria and a specialised endoplasmic reticulum that stores calcium ions.
    • (the sarcoplasmic reticulum)
Myofibrils
Within the sarcoplasm lie bundles of myofibrils.
Myosin
Each myofibril is made of two types of protein filament: thick filaments of myosin and thin filaments of actin.
They are arranged in a repeating pattern, giving light and dark bands under the microscope.
Sarcomere
The unit of contraction is the sarcomere, the section between two Z-lines.
PartDescription
A bandDark band; the length of the myosin filaments (and overlapping actin)
I bandLight band; actin only, no myosin
H zoneCentre of the A band; myosin only
Z lineBoundary of a sarcomere where actin is anchored
M lineMiddle of the sarcomere, linking myosin

Part 3 of 4Fast 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.

FeatureSlow twitchFast twitch
ContractionSlow, sustainedFast, powerful, in short bursts
RespirationAerobicAnaerobic (glycolysis)
MitochondriaManyFewer
CapillariesManyFewer
MyoglobinHigh (red)Low (pale)
FatigueSlow to tireTire quickly (lactic acid)
Found inPostural muscles, long-distance runningLimbs 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.

Part 4 of 4Worked examples

  1. Explain why muscles work in antagonistic pairs. [2]2 marks

  2. Describe the ultrastructure of a myofibril. [3]3 marks

  3. Explain why marathon runners have many slow twitch fibres. [2]2 marks

Key terms

TermMeaning
Antagonistic pairTwo muscles that pull in opposite directions.
MyofibrilBundle of actin and myosin filaments in a muscle fibre.
SarcomereContractile unit of a myofibril.
SarcoplasmCytoplasm of a muscle fibre.
Slow twitch fibreFatigue-resistant fibre using aerobic respiration.
Fast twitch fibreFibre that contracts quickly using anaerobic respiration.

Avoid these mistakes

Avoid these mistakes
Saying muscles push on bones.
Saying actin is the thick filament.
Saying fast twitch fibres respire aerobically with many mitochondria.

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