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.