Write the mechanism of bending of flagellum and explain effective and recovery strokes.

Explanation

Bending movement of a flagellum is brought about by the sliding of microtubules past each other due to the functioning of .dynein arms. utilising ATP. A flagellum pushes the fluid medium at right angles to the surface of its attachment, by its bending movement.
Bending movement of flagella and cilia : Dynein arms show, a complex cycle of movements using energy provided by ATP (dynein arms are the sites of ATPase activity in the cilia and flagella). The dynein arms of each doublet attach to an adjacent doublet and pull the neighbouring doublet. So the doublets slide past each other in opposite directions. The arms release and reattach a little further on the adjacent doublet and again .pull.. As the doublets of a flagellum orcilium are physically held in place by the radial spokes, the doublets cannot slide past much. Instead they curve and cause bending of flagellum or cilium. Such bending movements of flagella and cilja play an important role in the flagellar and ciliary locomotion:
The flagellar movement of many organisms is a sidewise-lash which consists of two strokes namely the effective or propulsive stroke and the recovery, stroke.

i) Effective stroke : Flagellum becomes rigid and starts bending to one side beating against the water. This beating against water is at right angles to the body axis and the organism moves forwards.
ii) Recovery stroke : Flagellum becomes comparatively soft so as to offer least resistance to water and moves backwards to its original position. It is called .recovery stroke..