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Explain the structure of cilia and flagella.

GeneralClass 12AllAnswered 27 Mar 2026
Answer

Structure:

(i) Origin and Attachment:

  • Arise from basal bodies (modified centrioles)
  • Basal bodies anchor structures and control movement
  • Membrane-bound extensions of plasma membrane

(ii) Axoneme (Core Structure):

  • Central cylinder running length of cilium/flagellum
  • "9+2" arrangement:
    • 9 peripheral doublets: Microtubule pairs arranged radially
    • 2 central singlets: Single microtubules in center

(iii) Microtubule Organization:

Peripheral Doublets:

  • A-tubule: Complete (13 protofilaments)
  • B-tubule: Incomplete (10–11 protofilaments fused to A-tubule)
  • Dynein arms: Motor proteins projecting from A-tubule

Central Pair:

  • Connected by central bridge
  • Enclosed in central sheath (protein structure)

(iv) Connecting Elements:

  • Radial spokes: 9 spokes connecting peripheral doublets to central sheath
  • Nexin links: Connect adjacent peripheral doublets (maintain structure)

(v) Movement Mechanism:

  • ATP-powered dynein arms cause microtubule sliding
  • Coordinated sliding → bending motion
  • Wave-like (cilia) or whip-like (flagella) beating patterns

Comparison:

Feature

Cilia

Flagella

Number/cell

Numerous (300–14,000)

Few (1–4)

Length

Short (2–10 µm)

Long (up to 150 µm)

Distribution

Cover entire surface

Usually one end

Beat pattern

Coordinated, oar-like

Undulating, propeller-like

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