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Learn7402Paper 2 · Paper 2: Topics 5–8Organisms respond to changes in their internal and external environmentsMuscle contraction and plant responses
Organisms respond to changes in their internal and external environments · Subtopic 4 of 4

Muscle Contraction and Plant Responses

Skeletal muscle fibres contain myofibrils made of repeating sarcomeres (Z line to Z line): thin actin and thick myosin filaments overlap. At the neuromuscular junction, ACh depolarises the sarcolemma; the impulse travels down T-tubules; the sarcoplasmic reticulum releases Ca²⁺. Ca²⁺ binds troponin, moving tropomyosin off the binding sites on actin. Myosin heads bind (cross-bridges), perform the power stroke (releasing ADP + Pᵢ), then ATP binds to detach them and is hydrolysed to recock the head. Actin slides over myosin: the I band and H zone shorten; the A band stays the same. Phosphocreatine regenerates ATP rapidly. Plants: auxin (IAA) causes cell elongation (acid growth) and phototropism; gibberellin causes stem elongation and stimulates amylase in seed germination; abscisic acid closes stomata in drought; the Venus fly trap closes by rapid changes in turgor after two hair touches.

Cambridge 9700 Paper 4Edexcel YBI11 WBI15AQA 7402 Paper 2≈ 55 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe sarcomere structure.
  • Explain the sliding filament model.
  • Describe the neuromuscular junction.
  • Describe plant responses controlled by hormones.
Pause and think

Why does rigor mortis happen after death?

Section 2 of 14

Core idea

Start hereSliding, not shrinking

When you lift a bag, millions of protein filaments slide past each other, powered by ATP.

The big idea

Calcium exposes binding sites on actin; myosin heads pull, release and recock using ATP. Plants respond with hormones instead of muscles.

The cross-bridge cycle

Repeats while Ca²⁺ and ATP are present.

Worked exampleCounting sarcomeres

Myofibril 1.0 cm; sarcomere 2.5 µm.

Goal
Number.
Strategy
Convert units.
  1. 1.0 cm=10 000 μm1.0\text{ cm}=10\,000\ \mu\text{m}
    1 cm = 10⁴ µm.
  2. 10 0002.5=4000\frac{10\,000}{2.5}=4000
    Divide.
Answer4000 sarcomeres.
✓ CheckEach shortening a little adds up ✓.

Summary

  • A band constant; I and H shorten.
  • Ca²⁺, troponin, ATP.
  • Auxin, gibberellin, ABA.
Key words
Sarcomere
Contractile unit between two Z lines.
Auxin
Plant hormone causing cell elongation.
HistoryTwo Huxleys. In 1954 Andrew Huxley and Hugh Huxley (unrelated) independently published the sliding filament model in the same issue of Nature.
Fun factCounting to two. A Venus fly trap shuts only if a trigger hair is touched twice within about 20 seconds, so raindrops don't waste a closure.
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