Cilia and Flagella: The Basics

Ethan Blackwood, Anna Menkis, Jeremy Cohen
Elenbaas Block 3
Thin tubular structures protruding from membranes
of some eukaryotic cells
 Aid in movement
 Structurally and functionally identical
 short in size
 usually found in great quantities, covering entire cell
Flagella (eukaryotic)
 much longer
 usually one or two in a cell
Membrane: extensions of
cell membrane
Composed of microtubles,
organized as following:
 Central axoneme with 2
 Surrounded by 9 microtuble
 Axoneme and doublets
connected by radial spokes
Dynein molecules bridge
gaps between doublets
Figure 1: Interior structure of cilia and
flagella (Credit: Florida State
Basal body (Kinetosome)
located at base
 9 sets of 3 microtubules in radial
Extensions of internal cytoskeleton
Composed of the globular protein tubulin
Doublet: one full microtubule sharing a wall
with a smaller, partial microtubule
Basal body: structurally identical to centriole
(part of cytoskeleton of an animal cell)
1. Protein dynein is activated
by ATP
2. Dynein arms on one
microtubule doublet “grab”
an adjacent doublet and
“walk” along its length.
3. Doublets slide past each
other and bend
4. Cilium/flagellum bends
Figure 2: Movement of cilia and flagella
(Credit: Benjamin/Cummings)
In protista: movement of organism through
 Same function applies to motile (flagellated)
sperm cells
In multicellular eukaryota: transport
extracellular substances (e.g. water, food)
 Protista (e.g. paramecium)
 Human respiratory tract, keeping dust and debris
out of lungs
 Digestive systems (e.g. of snails)
Flagella (eukaryotic)
 Protista (e.g. euglena)
 Motile (flagellated) sperm cells of animals and some
(Bacterial and archaeal flagella exist in some
Primary ciliary dyskinesia
 Recessive genetic disorder
 Defective cilia lining respiratory tract and fallopian tube
 Susceptibility to chronic, recurring respiratory infections
Polycystic kidney disease
 Defective cilia in the renal tube cells (kidney)
 Cysts, enlargement of kidneys, possible damage to other organs
Bardet-Biedl syndrome
 Problem in basal body caused by genetically mutated cilia
 Numerous consequences, ranging from obesity to mental retardation
Ectopic pregnancy
 Fallopian cilia fail to move fertilized ovum to uterus
Endosymbiotic model
 Proposed by Lynn Margulis
 Cilia and flagella are results of symbiosis between ancient eukaryotic and
prokaryotic cells
 Supporting evidence (weaker)
▪ Mitochondria and chloroplasts as endosymbiotic examples
▪ Some eukaryotes symbiotically use bacteria as motility organelles
Endogenous model
 Cilia and flagella developed from existing eukaryotic cytoskeleton
 Supporting evidence (stronger)
▪ Both cilia/flagella and cytoskeleton contain dynein and tubulin
▪ All eukaryotic kingdoms contain motile, 9+2 flagella, suggesting that the
common eukaryotic ancestor had this trait
Campbell, Neil A. et al. Biology: Concepts & Connections. San Francisco:
Benjamin/Cummings, 2000.
“Cilium-related disease.” Cilium. New World Encyclopedia.
Davidson, Michael W. “Cilia and Flagella.” 2004. Molecular Expressions:
Exploring the World of Optics and Microscopy.
“How important is endosymbiosis?” Understanding Evolution. University of
California Museum of Paleontology and National Center for Science
Kaiser, Gary E. “Composition and Functions of Eukaryotic Cellular Structures:
Cilia and Flagella.” The Eukaryotic Cell. Community College of Baltimore
(For further reading) Mitchell, David R. The Evolution of Eukaryotic Cilia and
Flagella as Motile and Sensory Organelles. 2006. SUNY Upstate Medical

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