The flagellum

Axons of antennal ORCs project through the antennal nerve to enter the brain at the level of the ipsilateral antennal lobe (AL) of the deutocerebrum (52). ORC axons project from the flagellum to targets in the AL, but axons from antennal mechanosensory neurons bypass the AL and project instead to an "antennal mechanosensory and motor center" in the deutocerebrum posteroventral (with respect to the body axis of the animal) to the AL (52, 58, 64). In moths and certain other insect groups, sex-pheromonal information is processed in a prominent male-specific neuropil structure in each AL called the macroglomerular complex (MGC) (16, 52, 64, 65). 

In spermatozoa, the phosphocreatine shuttle is present to transfer energy from the mitochondria to the flagellum, which is essential for swimming of the sperm (Figure 9.21) (see also Chapter 19). 

Figure 9.21 The creatine/phosphocreatine shuttle in spermatozoa. This shuttle may not be present in all sperm it will depend upon the distance between the mitochondria and the flagellum. Mitochondria are present in the midpiece just below the head. ATP is required for movement of the flagellum which enables the sperm to swim. Dynein ATPase is the specific motor ATPase, similar to myosin ATPase, that transfers energy from ATP to the flagellum. A deficiency of creatine may explain low sperm motility in some infertile men. CK - creatine kinase. Deficiences of enzymes in the pathway for synthesis of creatine are known to occur (see Appendix 8.3).

Figure 9.30 Flow diagram of the energy chain from food to essential processes in human life. The ATP utilised by the NayK ATPase maintains the ion distribution in nerves that is essential for electrical activity and, in addition, maintains neurotransmitter synthesis, both of which provide communication in the brain and hence consciousness, learning and behaviour (Chapter 14). ATP utilisation by myosin ATPase is essential for movement and physical activity. ATP utilisation by the flagellum of sperm is essential for reproduction and ATP utilisation for synthesis of macromolecules is essential for growth.

The midpiece contains the mitochondria which are wrapped around the proximal part of the flagellum. The beating of the flagellum, and hence the swimming of the sperm involves the motor protein known as dynein, which requires ATP hydrolysis. In some species, the diffusion of energy in the spermatozoa is increased by the presence of the creatine/phosphocreatine shuttle (Chapter 9) that is, phosphocreatine and creatine diffuse throughout the cytosol... 

Figure 19.18 The role of cyclic GMP and vasodilation in provision and preparation of spermatozoa for fertilisation. Vasodilation is regulated by the concentration of cyclic GMP by relaxation of smooth muscle. The resultant increase in blood flow to the corpora cavernosa results in erection of the penis for the ejaculation of spermatozoa into the vagina. The increase in blood flow to the vaginal smooth muscle provides more oxygen for diffusion into the lumen. Here it provides for oxidative phosphorylation in the mitochondria of the-mid section of the spermatozoa, which provides the ATP for the beating of the flagellum and hence for swimming to the oviduct for fertilisation.

FIGURE 12-26 The two-component signaling mechanism in bacterial chemotaxis. When an attractant ligand (A) binds to the receptor domain of the membrane-bound receptor, a protein His kinase in the cytosolic domain (component 1) is activated and autophosphorylates on a His residue. This phosphoryl group is then transferred to an Asp residue on component 2 (in some cases a separate protein in others, another domain of the receptor protein). After phosphorylation on Asp, component 2 moves to the base of the flagellum, where it determines the direction of rotation of the flagellar motor. 

The shaft and rings at the base of the flagellum make up a rotary motor that has been called a "proton turbine." Protons ejected by electron transfer flow back into the cell through the turbine, causing rotation of the shaft of the flagellum. This motion differs fundamentally from the motion of muscle and of eukaryotic flagella and cilia, for which ATP hydrolysis is the energy source. 

Collar Cell (Choanocyte) A flagellate cell with a high membrane around the base of the flagellum. 

Such behavior raised many questions. What causes reversal of direction of the propellor Why do the bacteria tumble How does a bacterium "decide" when to tumble How is the flagellum changed from a left-handed to a right-handed superhelix How does this behavior help the bacterium to find food Most intriguing of all, what kind of motor powers the... 

Some bacteria boast a marvelous swimming device, the flagellum, which has no counterpart in more complex cells.8 In 1973 it was discovered that some bacteria swim by rotating their flagella. So the bacterial flagellum acts as a rotary propeller—in contrast to the cilium, which acts more like an oar. 

The bacterial flagellum uses a paddling mechanism. Therefore it must meet the same requirements as other such swimming systems. Because the bacterial flagellum is necessarily composed of at least three parts—a paddle, a rotor, and a motor—it is irreducibly complex. Gradual evolution of the flagellum, like the cilium, therefore faces mammoth hurdles. 

The discovery that the flagellum is more complex than first supposed, that it also contains an unpredicted, sophisticated protein-pumping mechanism, and that structures resembling the protein pump can occur independently, briefly set Darwinian hearts aflutter. The innocent optimism was based on Ken Miller s rhetorical redefinition of irreducible complexity, which decreed that parts of IC systems could not have any other function. Since a subset of the flagellum appeared to be part of the TTSS, then that violated Miller s dictum, and cheered some of the more unreflective Darwinists. 

But as I pointed out above, there s no reason that parts or subassemblies of irreducibly complex systems can t have one or more other functions, and wordplay can t masquerade as a real explanation. Neither the TTSS, the flagellum, nor any transitions between them have been soberly investigated in a Darwinian framework in the professional science literature. The best place to see this is in a recent paper entitled, Bioinformatics, genomics, and evolution of non-flagellar type-III secretion systems a Darwinian perspective.))27 In it we learn that A type-III secretion system is an exquisitely engineered [emphasis added] molecular pump,... 

Cilia and flagella are stable microtubule-based structures which project from the plasma membranes of particular eukaryotic cells. The energy-dependent oscillations of these structures can drive material over the surface of a cell or propel the cell along. For example, the whip-like motions of cilia on the cells at the head of the fallopian tube draw newly released ova from the ovaries into and along the oviduct. The snake-like movements of the flagellum on a sperm provide these cells with movement. 

FlhB A component of the flagellum-specific export apparatus in bacteria... 

Cilia—Similar to the flagellum except more numerous, delicate, and shorter. 

Figure 34.32. Proton Transport-Coupled Rotation of the Flagellum. (A) MotA-MotB may form a structure having two half-channels. (B) One model for the mechanism of coupling rotation to a proton gradient requires protons to be taken up into the outer half-channel and transferred to the MS ring. The MS ring rotates in a counterclockwise direction, and the protons are released into the inner half-channel. The flagellum is linked to the MS ring and so the flagellum rotates as well.

When Walker was a postdoc, people argued whether this was rotation, or whether it was beating in a sinusoidal fashion. The key experiment was that somebody made an antibody that recognized the end of the flagellum and they took a cover slip and coated it with the antibody and added the bacteria to it. This then trapped the bacteria attached by the tip of their tail, and one could look in the fight microscope and see the bacteria turning around in this fashion. This was the first demonstration of rotation in these motile bacteria. The ATP experiment mentioned above was a derivative from this bacteria motility experiment, a macroscopic demonstration of a microscopic chemical event. 

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