Difference Between Flagella and Pili

The difference between flagella and pili is due to the difference in the properties like structure, origination and functional role. Both are the two common types of surface appendages, which aid in locomotion and attachment of the organisms.

  • Structure: The structure of flagellum appears whip-like, whereas pili appear thread or hair-like.
  • Origination: The emergence of flagella is from the cell-membrane, while pili emerge out from the cell-wall.
  • Functional role: The flagella aid locomotion, sensory organ, host invasion etc. and pili assist gene transfer, cell adhesion etc.

In this context, we will discuss the key differences between the two locomotory appendages (flagella and pili) along with the comparison chart. You will also get to know the definition, structure and some similarities between the two.

Content: Flagella Vs Pili

  1. Comparison Chart
  2. Definition
  3. Key Differences
  4. Similarities
  5. Conclusion

Comparison Chart

PropertiesFlagellaPili
ShapeWhip like and helical in structureHair like and non-helical in structure
SizeLong about 15 µmShort about 0.2-20 µm
ThicknessThicker (15-20 nm in Diameter)Thinner (3-10 nm in Diameter)
OriginFlagella originate from the cell membrane Pili originate from the cell wall
OccurrenceOccur both in gram positive and gram negative bacteriaOccur only in gram negative bacteria
ArrangementIt’s arrangement can be polar, lateral or peritrichousIt’s arrangement is throughout the cell surface
CompositionComposed of flagellin proteinComposed of pilin protein
RigidityMore rigidComparatively less rigid
Motor systemPresentAbsent
NumberFound less in numberNumerous
Organ of adhesionAbsentPresent
Involvement in conjugationAbsentPresent
TypesThey are particularly of three types, namely prokaryotic, eukaryotic and archaeal flagellaThey are particularly of two types, namely conjugative and type-IV pili
MovementWave-like or sinusoidal movementTwitching mobility
Receptors of virusAbsentThese acts as receptors of different viruses
ExamplesEscherichia coli, Vibrio cholera Salmonella sp. etc.Neisseria gonorrhoeae, Myxococcus xanthus, Salmonella and Shigella sp. etc.

Definition of Flagella

Flagella are the locomotary appendages, whose functional role is to provide cell-motility. They appear as a long, spiral and whip-like structures, which protrude outside the cell membrane. The length of the flagellum is nearly 15 µm, and the diameter is about 15 to 20 nm.

Flagella are of three types (based on its occurrence), namely prokaryotic, eukaryotic and archaeal. However, the thickness, number, position and the arrangement of flagella vary from organism to organism. The structure of bacterial flagella possesses three elements, namely basal body, hook and filament.
diagram of flagella
The basal body comprises of the four annular rings (S, M, P and L). A gram-negative bacteria’s flagellum possesses S, M, P and L rings, whereas a gram-positive bacteria’s flagellum possesses only S and M ring in the cell membrane. The rotation in the rings of the basal body cause propeller-like motion and results in movement of the organism.

Definition of Pili

Pili are the locomotary element, which plays a functional role in providing cell mobility, cell attachment and gene transfer. They appear short, straight and hair-like appendages, which protrude outside the cell wall. The length of pili is nearly 0.2 to 20 µm, and the diameter is about 3 to 10 nm. Pili are found on the surface of gram-negative prokaryotes and absent in eukaryotes.

They also differ in thickness, position, arrangement and number from individual to individual species. There are two types of pili, namely conjugative and type-IV pili. Conjugative pili aid gene transfer mechanism through conjugation, whereas type-IV pili help in cell attachment and movement.
pilus structure

Key Differences Between Flagella and Pili

  1. The shape of the flagella is whip-like, rigid and helical in structure, whereas the shape of pili is hair-like, less rigid, and non-helical.
  2. A flagellum originates from the cell membrane and exists in both gram-positive and gram-negative bacteria. Pili originate from the cell wall and only exists in gram-negative bacteria.
  3. The flagellum arrangement can be polar, lateral or peritrichous, and it is found less in number (usually one), and the arrangement of pilus is throughout the cell surface.
  4. Flagellin protein contributes to flagella structure, while pilin protein is an important cell constituent that makes up the structure called a pilus.
  5. The motor system (flagellar motor) in flagella assists cell movement, whereas absent in pili.
  6. Pili comprise an adhesion organ that facilitates cell attachment and gene transfer mechanism through the conjugation process. In contrast, flagella lack the organ of adhesion and do not participate in the conjugative gene transfer.
  7. Flagella show wave-like and sinusoidal movement and pili involve twitching mobility. Sinusoidal movement occurs from the base to the filament, leading to the organism’s wave-like motion. Conversely, type-IV pili aid in twitching motility by first attaching the cell to the solid substratum and then bringing forward motion.
  8. Escherichia coli, Vibrio cholera, Salmonella sp. etc. are the examples of the microorganisms possessing flagellar motility. Neisseria gonorrhoeae, Myxococcus xanthus, Salmonella sp. and Shigella sp. etc. are examples of the organisms having pili.

Similarities

  • Both flagella and pili are the proteinaceous filamentous structures.
  • Flagella and pili are the surface appendages emerged from cell-membrane and cell-wall, respectively.
  • Both are hollow and tubular in structure.

Conclusion

Flagella and pili are the filamentous structures composed of protein molecules, namely flagellin and pilin respectively. Thus, they differ in physical properties (like shape, size etc.) and chemical properties (like composition and functional role).

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