Eukaryotic Cilia and Flagella
Eukaryotes are those organisms whose cells possess certain characteristics such as a
membrane-bound nucleus. This includes animals, plants, fungi and
protoctistans. Prokaryotes (bacteria
and archaebacteria) have very different motility mechanisms, which may utilise flagella that operate on
very different principles; to see prokaryotic motility devices
click here.
flagellum structure
cross section of flagellum or cilia
Metachronal rhythm
protofilament structure
Euglena
Euglena
Above: top - the base of a eukaryotic flagellum; bottom - a detailed cross-section of the flagellum. The flagellum consists of
a long whip-like appendage, one or more of which emerge from one end of the cell. Undulations of the flagellum propel the
cell through liquid media. The flagellum consists of a core of tubular proteins, called
microtubules (MTs) that form
protofilaments that are bundled into fibres. The most common arrangement of fibres is a 9+2 arrangement, in which 9 pairs
of fibres form an outer cylinder and a central pair (enclosed within a sheath) form an inner core. These fibres are covered
by membrane, continuous with the cell membrane and the interior of the flagellum contains cytoplasm. The example below
shows the single-celled flagellate Euglena - a single-celled organism belonging to the Kingdom Protoctista:
Above: Euglena. The species on the left has chloroplasts, similar to those in certain plant cells, and is a single-celled
protophyte (proto-plant) whilst the species on the right has no chloroplasts but contains many vesicles in its cytoplasm and is
a protozoan (proto-animal). Some species can alternate between plant cell-like photosynthetic forms and animal cell-like
non-photosynthetic forms, depending on the availability of light and nutrients.
F, flagellum; E, eyespot (part of a light sensor);
C, chloroplast; P, pyrenoids; and CP; caudal peduncle. Smaller diagrams illustrate the observed ranges of movement. Both
forms rotate as they swim using their flagellum. The form on the left was quite stiff and rigid, and somewhat elastic (rather like
a nematode worm) and is probably stiff due to the pressurised contents of the cell cytoplasm. A tough 'skin' or pellicle is
necessary to contain this pressure and is a typical feature of such cells. The form on the right was much more flexible,
capable of amoeboid-like movements, characteristic of animal cells, or so-called euglenoid movements. Presumably its
membrane and pellicle are much more flexible and less rigid.
Flagella beat or undulate as waves pass along them from base to tip. These waves may be planar (flat) or circular, but the
flagellum itself does not rotate, in total contrast to the propeller mechanism of bacterial flagella.

Cilia have the same structure as flagella, but tend to be shorter and more numerous (the distinction between a cilium and a
flagellum is not clear cut). Cilia, like flagella, are used by some single-celled protoctistans for locomotion. Typically rows of
cilia beat in a metachronal rhythm, driving a stream of water past the cell. In multicellular animals, cilia are frequently
employed for transport of fluids - fields or tracts of cilia propel water over the gills of many creatures, such as the gills of the
mussel (
Mytilus) or the radial canals that transport fluids around the bodies of jellyfish, or the cilia lining your bronchi that
drive mucus out from the lungs. The diagram below illustrates the metachronal rhythm:
Above: stages in the beat of a single cilia (the stages are numbered in temporal order). During the power stroke the cilium
actively propels fluid, but during the recovery stroke it flexes so as to restore its position whilst displacing the minimum fluid,
ready for the next power stroke. This ensures that the bulk movement of fluid is in one direction (rather than simply
oscillating from side to side).

The key to understanding how flagella and cilia work lies in the structure of the protein fibres that form the central core of
each appendage. Each fibre is a hollow cylinder of protofilaments (microtubules) made of a protein called tubulin. A protein
called dynein is attached at regular intervals along the length of the peripheral fibre doublets. These form dynein arms.
The dynein arms are movable and work much like tiny legs and walk one doublet along its neighbouring doublet (see
the cross-section at the top of the page), causing the fibres to slide relative to one another. Nexin bridges and radial
links (spokes) convert this sliding movement into bending movement, causing the whole appendage to bend.
vorticellid
Left: a ciliate protozoan. This single-celled
organism was obtained in large numbers by placing
seaweed in salt water. The arrow indicates the
direction of locomotion - the cell rotates as it swims
forward. Several tracts of cilia cover the cell. Inside
the cell are numerous vesicles and food vacuoles
(which contain ingested food particles undergoing
digestion). The small diagram inset shows a
number of these cells moving past one another,
exhibiting considerable flexibility as they squeeze
past one another. One of the larger vacuoles
functions as a contractile vacuole - a vacuole which
periodically swells with water and then contracts as
it ejects the water from the cell, a process which
removes excess water from the cell.

The salt water was kept in a jar with a loose fitting
lid, which meant that some water slowly evaporated.
Eventually (after several days) conditions became
unfavourable for this organism, possibly because of
rising salt concentration as the water evaporated,
and these cells became dormant - rounding up and
absorbing their cilia and becoming motionless, with
the various vesicles inside them ceasing to move
around inside the cell. Another ciliate protozoan,
presumably better suited to the new conditions,
took its place. This ciliate is shown below and is a
species of
Euplotes.

Euplotes has appendages called cirri. These are
bundles of 40-120 cilia, tightly packed in an
hexagonal array and locked together. Similar
bundles, called membranelles consist of three rows
of 10-40 cilia locked together. This particular
species had 10 frontoventral cirri (4 frontal + 6
ventral), 5 anal cirri at the rear, and 4 caudal cirri,
or 10+5+4 = 19 cirri. The number of cirri varies with
species but is typically: (6-10)+5+(3 or 4). The
small caudal cirri constantly vibrate, even when the
cell is stationary but the ventral cirri are used like
legs, enabling
Euplotes to crawl over vegetation
(like seaweed).
Above: the ciliate Euplotes deploys bundles of cilia called cirri as legs. Other cirri may be sensory or
involved in flushing water across the cell surface, possibly to bring in food particles and oxygen or to expel
waste. The large vacuole corresponds to the contractile vacuole seen by other authours, but was not
seen to contract here, possibly it was not needed to expel water as the medium by this point was already
very salty and so not much water would be entering the cell. These diagrams were based on repeated
observations of a number of living cells as seen with a light microscope - which is tricky since the cells
beat their cirri and move almost constantly! The ventral bristle cilia have an unknown function.
It is remarkable to think that Euplotes is a single-cell when it has leg-like appendages, distinct sensory systems, a
distinct digestive system and a definite front end, rear end, dorsal surface (back), ventral surface (under-belly),
left side and right side; indeed, it is an entire animal-like creature packaged into a single cell! Under the electron
microscope a much higher degree of complexity is revealed - including a complicated system of microtubules that
anchor the various cilia and cirri and form an internal cell skeleton (or cytoskeleton) with a definite and precise,
but very complicated architecture. There is also a complicated pellicle with three layers (which may correspond to
the three layers seen with the light microscope and indicated in portions of the above figures?) and various
sub-pellicular layers and structures. Indeed, some ciliates rank amongst the most complicated single cells known.

The diagram below shows another ciliate that uses its cilia in quite a different manner. This ciliate is a vorticellid
(or vorticellid-type) protozoan. It can be found in pond water and attaches to a solid surface via a long contractile
stalk. It's front end is equipped with whorls of cilia whose beating drives a current of water towards the creature's
'mouth' so that it can suck in food particles which become ingested and incorporated into the large vacuoles
visible throughout its body. Common food items include bacteria. The length of the body, excluding tail, is
indicated in the figure.
Click here to see more single-celled organisms that belong to the kingdom Protoctista.

Click here to learn about an animal that builds its body from flagellate and amoeboid cells.

Click here to learn about motility in bacteria.
More views of Euplotes, click to
enlarge.
Pellicle 1
Pellicle 3
Pellicle 4
Pellicle 2
Left and below: a 3D computer model of the pellicle (or
'skin') of a generic ciliate of the paramecium type. The
cilia, the bases of which are shown as cylinders
projecting upwards, are not bundled into cirri but form
tracts along the surface of the cell (often in a helical
arrangement). The cell surface consists of a series of
membranes arranged into membranous sacs or
alveoli,
with one or two cilia passing through the pellicle in the
middle of each alveolus. Underneath the pellicel run
helical bands of contractile cytoskeletal filaments called
kinetodesma (shown in orange) and connected to the
bases of the cilia (basal bodies). These filament bundles
may also be cross-connected by protein filaments to
one another and in some ciliates there is a hexagonal
mesh of cytoskeletal filaments beneath the pellicle. As
well as supporting the pellicle with its powerfully beating
cilia, the kinetodesma are also involved in changing cell
shape and some ciliates have very flexible pellicles and
are very pleomorphic (able to change into many
different shapes).
Positioned underneath the pellicle are flask-like trichocysts which can open between adjacent alveoli, firing out a long thread
capped by a barbed harpoon-like tip. Trichocysts are discharged for anchorage purposes when paramecium stops swimming
and are also used in defence.

The arrangement of cytoskeletal filaments beneath the pellicle of ciliates can be very complicated, but is always regular and
precise. It is sometimes possible to see these patterns under the microscope. These structures have been characterised in detail
for a number of ciliate, such as
Paramecium, Euplotes and Stentor. An example of such a structure is the cirrus base in Euplotes
shown below:
The cirrus consists of a field or 2D array of cilia that behave as a single unit.
The cilia terminate in complex basal structures. A, axosome; Alv, alveolus (a
membranous sac below the cell membrane); Bf, basal fibre system; Cf,
complex fibres; Df, distal fibre system; K, kinetosome (the base of a cilium
embedded in the cytoplasm); L, link between distal fibre system and
peripheral fibres; P, parasomal sac; Pf, peripheral fibres; O; outer cell
membrane or plasmalemma; Rf, rootlet fibres; Sm, subpellicular
microtubules; V, vacuole or vesicle. Note that the cell surface is covered by
the semi-rigid pellicle with its triple membrane system (outer membrane plus
alveoli) and subpellicular microtubules. [Diagram based on Grim, 1982.]
It is truly remarkable that ciliates can possess such complex cell structures with such well-ordered and arranged parts on such a
minute scale! The forms seen in ciliates perhaps represent the ultimate sophistication achieved by single-cells on Earth. These
organisms really do deserve the title of animalicules (lit. 'little animals').
References:

J.N. Grim, 1982. Subpellicular microtubules of Euplotes eurystomus: their geometry relative to cell form, surface contours and
ciliary organelles.
J. Cell Sci. 56, 471-484.
One example of a cell that uses flagella for locomotion is the animal spermatozoan. Spermatozoa differ greatly in architecture,
indeed the sperm of the fruit-fly
Drosophila is longer the body of the adult fly! The spermatozoa of the vast majority of animals are
driven by flagella, usually a single flagellum, but biflagellate animal spermatozoa are not unknown. The spermatozoa of
nematodes
is an exception, their spermatozoa are non-flagellated amoeboid cells that crawl by a novel mechanism.