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Cilia and Flagella

Cilia and flagella are motile cellular appendages occurring in most microorganisms and animals, except higher plants. They are composed of microtubules and remain covered by the plasma membrane.

Cilia are slender, microscopic, hair-like structures or organelles extending in large numbers from the surface of eukaryotic cells.

Flagella are the complex filamentous cytoplasmic structure protruding through the cell wall. These are unbranched, long, thread-like structures, mainly composed of the protein flagellin, intricately embedded in the cell envelope.

Cilia and Flagella

Compare and Contrast

Cilia and flagella are protrusions of cell membrane having similar internal structures. However, they differ in their mode of beating, number, size, and function.

Cilia vs. Flagella

The critical differences between cilia and flagella are as follows:

BasisCiliaFlagella
1. Found InEukaryotic cellsEukaryotic and prokaryotic cells
2. Composed OfLinear polymers of globular proteins, tubulin, motor protein dynein, and another protein nexinTubulin and dynein. Along with these, bacterial flagella also contain a protein called flagellin
3. StructureOrganization Possess a central bundle of microtubules, called the axoneme, in which 9 outer doublet microtubules surround a central pair of singlet microtubules. When viewed under an electron microscope, the cross-section of the axoneme shows the characteristic “9 + 2” arrangement of microtubules. Length Short, hair-like organelle (5-10µ) Thickness Cilia is around 0.3 to 0.5 μm in thickness.Organization Eukaryotic flagella are remarkably similar in their organization to cilia. Prokaryotic flagella are simpler structures made up of flagellin     Length Long, whip-like, or thread-like organelle (150µ) Thickness Flagella is around 0.02 to 0.025 μm in thickness.
4. FunctionsPlay a primary role in locomotion, feeding circulation, aeration, and respiration Help mainly in locomotion
5. Distribution in CellIn protozoans of the class ciliate Ciliated epithelium of the metazoan In larva of certain platyhelminthes, echinodermata, mollusk, and annelidIn some bacterial cells Protozoans of the class Flagellata Choanocyte of the sponges Spermatzoa of the metazoan Some algae Sperms of Ginkgos and cycads
6. DensityNumerous, approximately hundreds per cellFewer than cilia, less than 10 per cell
7. NumberAbout 300-14000About 1-8
8. Energy RequirementUse ‘kinesin,’ having an ATPase activity that produces energy to perform the movementPowered by the proton-motive force by the plasma membrane in prokaryotes ATP-driven in eukaryotes
9. Position In CellPresent throughout the surface of the cellEither present at both the ends of the cell or all over the surface
10. TypesTwo types: primary or non-motile cilia and motile ciliaThree types: bacterial, archaeal, and eukaryotic
11. ExamplesAn example of tissue cells with cilia is the epithelial lining of the lungs that sweep away fluids or particles. Some examples of ciliated protozoans are Colpoda, Coleps, Paramecium, and VorticellaSome examples of flagellated protozoans are Trypanosoma, Leishmania, Giardia, and Salmonella
12. Types of MotionRotational, like a motor, very fast-movingSlow, wave-like, sinusoidal, and undulating movement
13. Beating PatternBeat in a coordinated rhythm either simultaneously (synchronous) or one after the other (metachronous)Beat independent of each other  
14. How Do they BendThe cross-link of the nexin arm in the doublets prevents the sliding of microtubules over one another. The force thus generated by the movement of dynein causes the cilium to bend.The bending of eukaryotic flagella is caused by dynein-driven microtubule sliding in the axoneme.

How are Cilia and Flagella Alike

Here are the similarities between cilia and flagella:

  • They both arise from the basal body, a centriole-like structure.
  • They both outgrow from the cell’s plasma membrane.
  • They both have an axoneme, a central filament. Axoneme has 11 microtubules of which, 9 are in pairs (doublet), and the remaining 2 are located in the center (singlet).
  • Their primary function is locomotion.

Conclusion

Cilia and flagella are two significant microscopic appendages of cells. Cilia are present in both animals and microorganisms but not in most plants. In contrast, flagella are used for mobility in bacteria as well as gametes of eukaryotes. Both cilia and flagella serve locomotory functions but in different manners. Both rely on dynein, a motor protein, and microtubules to work.

Article was last reviewed on Friday, October 1, 2021

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