It was Michael Behe who really put the bacterial flagellum on the map (Darwin’s Black Box 1996). Who among us has not seen images of this elaborate and beautiful nanomachine? Dr. Behe’s arguments that this tiny molecular machine is irreducibly complex are certainly familiar to many. Boasting about 30 different proteins including a rotary motor driven by proton power (H+) and sodium ions (Na+ ), the device does not work when only some of the components are present. How then could it have assembled spontaneously when all the components must be present to function? The proteins are so large and the order of the amino acids so specialized that blind processes could never happen upon such molecules when they were needed to assemble the machine.
Here are details of the flagellum architecture (excluding the parts like the motor inside the cell). The flagellar filament is made up of one protein only (about 500 amino acids long) that self-assembles at the growing tip of the flagellum, guided by a revolving five-sided cap. Eleven such filaments coil around each other in a tight left-handed spiral. After it is synthesized in the cell, this protein is pushed through a tiny tube towards the growing flagellum tip. The energy source for this and the turning five-sided cap, all come from the rotary motor inside the cell. Once out at the tip, the flagellum protein self-organizes into the correct structure which, of course, is a necessary (and very special) design feature.
The fame of the bacterial flagellum has quite eclipsed our familiarity with the eukaryotic flagellum. This latter molecular machine is actually far more complex than the bacterial flagellum and far more interesting. Instead of being composed of about 30 different specialized proteins (as in the bacterial flagellum) the eukaryotic flagellum is composed of approximately 300 highly specialized proteins! The structure is more elaborate and so are the building and maintenance systems. Since there are no even remotely similar systems in bacteria and archaea, biologists are hard pressed to explain how such a molecular machine could have originated by natural processes.
The eukaryotic flagellum exhibits a very characteristic pattern in cross section. In the centre are two stand-alone tubules, around which are nine pairs of tubules encased by a plasma membrane. The eukaryotic flagellum is a remarkable feat of engineering because the structure is very long. One commentator thus remarked: “Unlike most organelles, which are surrounded by cytoplasm, the flagellum protrudes from the cell surface extending tens or even hundreds of microns into the external medium. The elongated organelle must import all the macromolecules required for its assembly, maintenance and function … as well as a prodigious amount of ATP to supply the thousands of dynein motors that drive flagellar motility.” [Joel Rosenbaum et al. 1999. Intraflagellar Transport: The Eyes Have It. J. of Cell Biol. 144 #3 pp. 385-388. See p. 385.]
There are spokes between the peripheral tubules, some spokes move up along an adjacent tubule, others keep the system from moving too far and so prevent the system from falling apart. This mechanism contributes to the beating of the flagellum. Some organisms like the green alga Chlamydomonas, use the flagella to move forward as do sperm cells. Other, shorter flagella (called cilia) line branchial passages in land animals and humans, and there are sensory cilia like the kinocilium in hair cells in the organ of Corti in the inner ear. The importance of this very distinctive cell structure to many kinds of eukaryotic life forms is readily apparent.
Perhaps the most amazing aspect of the eukaryotic flagellum is the supply and maintenance system. The solution to the problem of carrying building supplies and energy out to the farthest tips is proteinaceous rafts which move along the tubules. After making a trip out to the tip, the rafts move back in toward the cell proper. A raft carries resources, while two motor proteins, Kinesin II and dynein 1 b, respectively, propel the raft on its trip out and back, respectively. Even when the flagellum is no longer growing, the transport system is needed to maintain the system, especially providing energy to keep things moving.
The flagellar precursors are all manufactured in the cytoplasm of the cell.
The obvious significance of the prokaryotic and the eukaryotic flagella is that they are both very complicated and irreducibly complex. However, the eukaryotic flagellum is not only very unlike the bacterial flagellum, but many times more complex. The finesse of the transport system in the eukaryotic cell is a delight to contemplate. There are no ancestral structures or systems which could have acted as precursors for them. Even parts of a cell like this have a story to tell.