Flagella

Flagella are whip-like appendages that extend from the cell body of some prokaryotic and eukaryotic cells. Their primary function is locomotion, enabling cells to move through fluid environments.

1.0Types of Bacterial Flagellation

The arrangement of flagella on a bacterial cell is known as flagellation. This arrangement is a key characteristic used for bacterial classification.

• Monotrichous: A single flagellum at one end of the cell. (e.g., Vibrio cholerae)
• Amphitrichous: A single flagellum at both ends of the cell. (e.g., Spirillum minor)
• Lophotrichous: A tuft of flagella at one or both ends of the cell. (e.g., Pseudomonas fluorescens)
• Peritrichous: Flagella are distributed all over the entire surface of the cell. (e.g., Escherichia coli)
• Atrichous: Lacking any flagella.

2.0Flagella Structure

Prokaryotic Flagella Structure

Prokaryotic flagella, found in bacteria and archaea, are simpler than their eukaryotic counterparts. They are helical structures composed of a single protein, flagellin. The structure is divided into three main parts:

• Filament: The long, outermost part that extends into the surrounding medium. It's a hollow, rigid tube made of flagellin.
• Hook: A short, curved section that connects the filament to the cell surface. It acts like a universal joint, allowing the filament to rotate.
• Basal Body: The motor of the flagellum. It's a complex structure embedded in the cell wall and plasma membrane. In Gram-negative bacteria, the basal body has four rings (L, P, M, S), while in Gram-positive bacteria, it has two rings (M, S). The basal body is powered by a proton-motive force, which causes the hook and filament to spin like a propeller.

3.0Structure of Eukaryotic Flagella

Eukaryotic flagella are more complex and larger than prokaryotic flagella. They are enclosed within the cell's plasma membrane and have a characteristic 9+2 microtubule arrangement.

• Axoneme: The core of the flagellum. It contains two central single microtubules surrounded by nine peripheral pairs (doublets) of microtubules.
• Basal Body (Kinetosome): A structure at the base of the flagellum from which the axoneme arises. It has a 9+0 arrangement, consisting of nine triplet microtubules arranged in a ring without a central pair. This is structurally similar to a centriole.
• Plasma Membrane: The entire structure is enclosed by the cell's plasma membrane, making it a proper cellular organelle.

Movement in eukaryotic flagella is generated by the sliding of the microtubule doublets, powered by the motor protein dynein. This process uses ATP for energy. The beating motion is more wave-like, unlike the rotary motion of prokaryotic flagella.

4.0Flagella in Different Organisms

• Bacteria: Provide motility for survival and colonisation.
• Protozoa: Used for swimming (e.g., Euglena).
• Algae: Help in reproduction and movement.
• Animal cells: Found in sperm cells to assist in fertilisation.

5.0Differences Between Prokaryotic and Eukaryotic Flagella