Difference Between Flagella and Pili

introduction image

Flagella and pili are the two surface appendages found commonly in prokaryotes. There are some significant differences between the flagella and pili which distinguishes both of them by the following properties like structure, emergence and functional role.

Structure: The structure of flagellum is whip-like whereas the structure of pili is thread or hair-like.

Emergence: The emergence of flagella is from the cell-membrane whereas pili emerge out from the cell-wall.

Functional role: Both flagella and pili play a fundamental role in the locomotion and attachment of the microbial cell respectively.

Content: Flagella Vs Pili

  1. Comparison Chart
  2. Definition of Flagella
  3. Definition of Pili
  4. Similarities between Flagella and Pili
  5. Key Differences Between Flagella and Pili
  6. 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 originates from the cell membrane Pili originates from the cell wall
OccurrenceOccurs both in gram positive and gram negative bacteriaOccurs only in gram negative bacteria
ArrangementIt’s arrangement can be polar, lateral or peritrichousIt’s arrangement can be throughout the cell surface
CompositionComposed of “Flagellin” proteinComposed of “Pilin” protein
RigidityMore rigidityComparatively less rigid
Motor systemPresentAbsent
NumberFound less in numberNumerous
Organ of adhesionAbsentPresent
Involvement in conjugationAbsentPresent
TypesIt is particularly of three types namely prokaryotic, eukaryotic and archaeal flagellaIt is particularly of two types namely conjugative and type-IV pili
MovementIt shows wave-like and sinusoidal movementIt involves “Twitching 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 can define as the locomotary element which functional role is to provide cell-motility. These are a long, spiral and whip-like structure which protrudes outside the cell membrane. The length of the flagellum is long about 15 µm, and the diameter is about 15 to 20 nm.

diagram of flagella

Flagella are of three types based on its occurrence namely prokaryotic, eukaryotic and archaeal but the thickness, number, position and the arrangement can vary from organism to organism. The structure of flagellum comprises of three elements namely basal body, hook and filament.

The basal body comprises of annular rings S, M, P and L which is present in gram-negative bacteria whereas only two rings, i.e. S and M are present in gram-positive bacteria. When the rings of the basal body rotate, it causes “Propeller-like motion” in the organism.

Definition of Pili

Pili can define as the locomotary element which plays a functional role in providing cell mobility and cell attachment. These are short, straight and hair like appendages which protrudes outside the cell wall. The length of pili is short about 0.2 to 20 µm, and the diameter is about 3 to10 nm.

pilus structure

Pili are found on the surface of gram-negative prokaryotes and absent in eukaryotes. It also differs in thickness, position, arrangement and a number from individual to individual species.

There are two types of pili namely conjugative and type-IV pili. Conjugative pili are involved in gene transfer mechanism whereas type-IV pili help in attachment and movement of the microorganism.

Similarities Between Flagella and Pili

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

Key Differences Between Flagella and Pili

  1. The shape of the flagella is whip-like and helical in structure whereas the shape of pili is hair like and non-helical in structure.
  2. The length and thickness of flagella are about 15 µm long and 15-20nm in diameter whereas the length and width of pili are about 0.2-20 µm short and 3-10nm in diameter.
  3. A flagellum originates from the cell membrane which is found both in gram-positive and gram-negative bacteria. Pili originates from the cell wall which is found only in gram-negative bacteria.
  4. The arrangement of flagellum can be polar, lateral or peritrichous and it is found less in number usually one and the arrangement of pili can be throughout the cell surface but usually 3-5 in number.
  5. Flagella are composed of a protein refers as “Flagellin” and are more rigid whereas the pili are composed of a protein refers as “Pilin” and are less rigid.
  6. The motor system is present in flagella also refers to as “Flagellar motor” that helps in the movement of the cell whereas absent in pili.
  7. Pili comprises of adhesion organ which helps in the cell attachment and also involved in the gene transfer mechanism like in conjugation process whereas flagella lack organ of adhesion and do not participate in conjugation.
  8. Flagella show wave-like and sinusoidal movement and pili involve “Twitching mobility”. Twitching motility is mediated by type-IV pili which first attaches the cell to the solid substratum and helps in the forward movement. The flagella mediate sinusoidal movement from the base to the filament which leads to the wave-like motion of the organism.
  9. Escherichia coli, Vibrio cholera Salmonella sp. etc. are the examples of the microorganisms which are motile by having flagella. Neisseria gonorrhoeae, Myxococcus xanthus, Salmonella and Shigella sp. etc. are the examples of organisms having pili in their cell surface.

Conclusion

Therefore, flagella and pili are the filamentous structures which are composed of protein molecules namely flagellin and pilin respectively. There are certain similarities and differences both in physical properties like shape, size and chemical properties like composition and functional role.

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