What part of a prokaryotic flagellum allows the filament to rotate freely?

The hook of a prokaryotic flagellum is a crucial structure that connects the filament, which extends outward into the surrounding environment, to the basal body that anchors the flagellum to the cell wall and plasma membrane. It is designed to provide flexibility and allow for the rotation of the filament. When the basal body rotates, the hook acts as a universal joint that transmits this rotation to the filament, enabling it to turn freely.

The structure of the hook allows it to bend and adapt to the movements of the filament, ensuring that the rotational motion generated by the motor proteins in the basal body can be effectively transferred. This unique feature is essential for motility in prokaryotes, as it allows the organism to swim through liquid environments by creating a propelling motion with the flagellum.