The basal structure consists of a series of rings connected by a rod, the rings are the L ring (embedded in
the lipid bilayer of the outer membrane), the P ring (embedded in the periplasm), the S and the M rings
(the rotating motor or rotor) and the C ring. The L and P rings act as a bushing (a bushing is a ring-like
structure that constrains moving mechanical parts, in this case the rotating rod, and may also be lubricated
to reduce friction). The motor proteins (Mot) conduct electric current carried by positively charged
protons ('positive electricity' as opposed to negative electricity in which the current is carried by negatively
charged electrons as in a metal wire) from the periplasm into the cell cytoplasm. The electric charge is
thought to flow into the M (motor) ring where it is converted into rotary mechanical motion, causing the
M-ring to rotate. The M ring is attached to the rod, causing the rod to rotate. The M ring acts against the
fixed Mot (stator) ring, which rotates slowly in the opposite direction to the M ring - slowly because it is
fixed to the bacterial cell wall and so causes the whole bacterial cell to rotate in a direction opposite to the
M ring and rod. The S ring is now known to be part of the rotor, along with the M ring, and forms a socket
for the rod, but is not part of the stator. Confusion may arise when the stator ring is referred to as the
S-ring.
The rod is attached to the filament via a flexible hook (which acts as a universal joint, transferring rotary
motion to the filament via the hook associated proteins (HAPs)). The filament is actually much too long
to show more than a tiny segment of it in these diagrams, it is about 20 nanometres in diameter, but 10 -
15 micrometres (10 - 15 thousand nanometres) long, which is longer than a typical bacterial cell which is
about 2 micrometres long. The filament is made up of about 30 000 subunits of a protein called flagellin
and is a corkscrew or helix shape. (The flagellin is arranged into typically 11 strands that are twined
together). This shape is important, mutants with straight filaments are immotile - the filament is the
propeller driven by this remarkable microscopic electric motor! (The cell body may contribute to thrust in
some forms in which the body is also helical). The helical filament is hollow, and flagellin is transported
from inside the cell, through the C ring (which has a hole in its centre) and along the filament, in its hollow
core, to its tip to which they are added - the filament constantly grows, as it must do to compensate for
breakages. A cap protein forms the tip and stabilises the filament.