The Real Thing
OK - let's talk about this thing known as a flagellum (if there are more
than one, together they are called, flagella - the plural form of
flagellum). The word "flagellum" is Latin for whip. This
structure is present as an appendage on certain bacteria and allows the
bacteria to move around. If a bacterium is capable of movement, the
bacterium is said to be motile. The ability to move is a very
important thing to a bacterium - offers great advantages - just as the
ability to move is important to us. In a moment, I'll
talk more about this ability to be motile in relation to the process called
chemotaxis, nutrient acquistion, etc., but first I want to talk some
about just what a flagellum actually is.
How Many Flagella Does a Bacterium Have - How Are They Arranged?
While an individual bacterial cell may be no longer than 10 µm (10
micrometers - or, 10 millionths of one meter), a flagellum may be 10-times
longer. Some bacteria have one flagellum, some have a few, and some
have many - depends upon the bacterial species. The location of the
flagellum or flagella can also be distinctive. There are basically four
different types of flagellar arrangements:
What's a Flagellum Made of - What Does a Flagellum Do?
The flagellum structure is a remarkable thing. The long part of the flagellum
is made of a protein called flagellin - who would have guessed? This
protein is arranged as individual helical threads which wrap around one
another to form a braid and the braids wrap around one another to form a
filament (like threads and braids which form a rope).
However, there is an overall, relatively inflexible helical shape to the
filament - kind of like a stretched-out corkscrew (relatively
wide-angled bends). This shape, when rotated clockwise or
counter-clockwise (depending on the direction of the turn of the helix), acts
as a propeller and will propel the bacterium forward in a straight line
through the medium (similar to how a drill bit will enter an object as
it rotates). If the structure rotates in the opposite direction, the cell will
do what is called, tumbling, - kind of irregularly moves around - kind of like
trying to guide a ship going backwards....
What Are the Parts of a Flagellum
A flagellum consists of: a filament, a hook, and a basal body. The
filament is the long part, the hook is a wider region at
the base of the filament - where the surface of the cell is located, and
the basal body anchors the flagellum to the cell.
How Does a Flagellum Work?
Energy generated by the cell is used to cause the flagellum or flagella
to rotate. If there are flagella all around the cell, each flagellum
intertwines with the others to form a single filament which extends from
one end of the cell - quite amazing. The basal body consists of the
filament which at this point has become a more narrow shaft which
goes through the cell
wall and the plasma membrane underneath, and a series of rings around the
shaft. The rings which surround the shaft appear similar to and appear
to have the same function as bearings which surround the shaft of any
mechanical motor. The rings are static - as is the basal body. However, as
energy is applied the shaft rotates inside the rings which causes the
helically-shaped filament to rotate - as a result, the bacterium will
move forward in a straight line. No one yet understands how the energy
is applied and transferred as a physical force to cause the shaft to
rotate. It is not yet undertstood how protein molecules can
interact to cause something to actually turn. However, it is known that
quite a lot of energy is required - several hundred ATP molecules-worth
of energy are used per one complete rotation.
From Where Does This Energy Come?
The energy is based upon the molecule called ATP - adenosine
triphosphate. When a phosphate group is enzymatically released from this
molecule, approximately 7,300 calories of energy are made available for
useful work (see "What the Heck is an
Enzyme?"). The actual
energy is not directly in the form of ATP usage, but rather from
what is known as the Proton Motive Force. In this case, energy from the
Proton Motive Force is used for flagellum rotation instead of for the
synthesis of ATP - which simply means a net loss of potential ATP
available for other processes - therefore - the use of ATP. This
force is generated when hydrogen ions (protons) are generated and extruded to
the outside of the cell during electron transport (electrons have come
from the "burning" of sugar nutrient - protons come along with
the electrons). The build-up of this positive charge on the outside of
the cell creates a voltage gradient analogous to water pressure. As the
protons re-enter the cell through special molecules, the dissipation of
the voltage potential across the membrane releases energy - which can be
used to synthesize ATP or in this case, to rotate the flagellum. An
analogy would be that of opening the water tap, and using the water
pressure to turn a wheel. If you wish to learn
more about Electron Transport, the Proton Motive Force, and ATP, please see:
Selected Microbiology
Lectures.
So, What's the Big Deal About The Ability to Move?
If bacteria cannot move, they must be in a place where food can get to
them, otherwise they'd die. If bacteria can move however, the cells can
go to where the food is located. Bacteria cannot "see" in the
way that we can; but, bacteria can chemically "see". Movement
in response to chemical signals is called chemotaxis. Bacteria
have protein molecules within the plasma membrane which respond to the
presence of nutrients. So, when a bacterium is in a liquid environment,
there may be nutrients within the medium. These nutrients will be at
different concentrations - higher at the source of the nutrient, and
lower far away from the source.. If a bacterium encounters an
appropriate concentration of one of these nutrients, the nutrient can
bind to the cell's protein. This binding results in an intracellular
signal to somehow be generated. This intracellular signal ultimately
results in activation of flagellum rotation
and the bacterial cell will begin to move in a straight line.
Eventually, the bacterium will swim out of the nutrient - just as if you
detected perfume and began to walk in a straight line.. if you were very
lucky to walk directly toward the source of the perfume, you would
eventually arrive at the source. However, chances are that you might
walk at an angle - then - you would eventually move out of the perfume
molecules. You would then of course simply turn and move in different
directions until you again detected the perfume.. and you would start
walking in a straight line again.
A bacterium cannot make these consious decisions; however, as the bacterium moves out of the nutrient molecules the nutrient-binding proteins in the membrane will soon not be occupied any longer, and the intracellular signal will cease. This cessation of signal will cause the flagellum to reverse rotation and to rotate slowly. When rotation reverses, the bacterial cell will no longer travel in a straight line, but will randomly move about. Eventually, the cell will "accidentally" enter the nutrient molecules again.. and - the cell will reverse flagellar rotation and begin to again move in a straight line. Eventually, through this process called a "random walk" the bacterial cell will arrive at the rich source of the nutrient, be surrounded, and swim around in there until the nutrient is gone. The random walk looks sort of like the following:
/---\
--------------------------------
/ \ /
____/ \ / -------> eventually.. *** Source
--------------------------------
\___ \
|/
Some Bacteria Use Magnets
I'm not kidding. Some ocean-dwelling bacteria with flagella are known
to have magnets (called magnetosomes) which contain an iron-type
substance. The bacteria orient and can move along earth's magnetic
field lines. One end of the bacterium acts as the
north, and the other end as the south magnetic pole - when the bacteria
are not attached to something, the bacteria will move rapidly by
flagellar rotation along a magnetic field line until they run into a
place for attachment. Interestingly, north-seeking bacteria seem to be
the most prevalent. If the bacteria are placed
onto a microscope slide, and a magnet applied, they will move in unison
like crazy - either away from or towards the pole of the magnet. One
such bacterium is: Aquaspirillum magnetotacticum. This process
of movement is called magnetotaxis. Honey bees have such
magnet-like things, as do some birds and some dolphins.
Bacteria Aren't the Only Cells Which Can Move
We, too, have cells which respond to chemotactic signals. Some very
inmportant cells such as neutrophils, macrophages, etc., which help us
to fight infection respond to chemical substances released as a result
of trauma or inflammation, and will move toward the source of the
inflammation. In these cases, no flagella-like things are present on
these cells; instead, the cells move by actually extending their
membrane and kind of "crawl" along. These cells must
physically move from inside a vein, cross the vein wall and enter the
tissue. This type of movement across a vein is called
diapedesis. And, you of course know that sperm move in response
to chemical signals... in this case, the whip-like appendage of the
sperm rapidly undulates (doesn't rotate) which propels the sperm
forward. This appendage is not called a flagellum, but is truly called
a "tail". So, we are back to where we started, a tail of
Thucidydes.
Book: Don't Touch That Doorknob!