Flagellum

The human sperm flagellum (≈50 µm) shares the 9 + 2 axoneme and dynein-driven mechanism of motile cilia but is much longer and beats with a symmetrical, undulating, wave-like motion that propels the cell (≈1–4 mm per minute) rather than moving fluid over a surface. It grows from the distal centriole of the spermatid.

Accessory structures strengthen and power it: in the middle piece a helical sheath of mitochondria surrounds the axoneme and supplies ATP, together with nine outer dense fibres; the principal piece is surrounded by a fibrous sheath of two longitudinal columns joined by circumferential ribs, which also carries glycolytic enzymes; the short end piece contains only the axoneme. Capacitation in the female tract changes the beat to a vigorous "hyperactivated" pattern needed for fertilisation.

Bacterial flagella are entirely different: rotating helical filaments of flagellin driven by a proton-powered motor, not homologous to the eukaryotic axoneme.