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Figure 5.22 Tumbleweeds move by rolling, but why don't we see macroscopic rotating structures in nature?

Source: Reproduced with permission of EriKolaborator.

The flagellum is remarkable from many perspectives, but one intriguing feature is that it is one of the few rotational structures in nature. The ATP synthase, which makes the energy storage molecule ATP, is another rotating structure which is introduced in the next chapter. We could ask at the larger scale: Why aren't there more rotating structures in nature? One question that biologists often ask is: Why don't animals have wheels? Let's briefly examine this question. Wheels are not very good on rough surfaces, such as most planetary surfaces, and they cannot easily climb vertical surfaces higher than their radius, making them less useful than legs. Of course, tumbleweeds (Figure 5.22) and some spiders roll across plains and down slopes, respectively, but they are not genuine rotating structures within an organism. One major problem with rotating structures in multicellular life is evolving a system to rotate an appendage without it forming knots in muscles, blood vessels, and other attached structures. One answer could be to make a rotor, like the flagellum. However, the flagellum relies on small protein units embedded in a membrane, and it relies on the diffusion of ATP to provide the energy. It is not immediately obvious how one could scale this up into a rotating macroscopic structure, since diffusion, because it is too slow, would be limiting to structures on centimeter or larger scales. This then brings us back to using blood vessels and other means to transport the necessary nutrients and energy, which itself returns us to the problem of how to make rotating blood vessels, etc. You might like to consider these ideas for yourself. Why are rotating structures in life on Earth limited to molecular machines? Why don't we see large-scale rotational structures in multicellular animals? Is this just a chance evolutionary outcome or are there good evolutionary reasons for their rarity in life on Earth? Is this a universally applicable observation?

Gould, S.J. (1983). Kingdoms without wheels. In: Hen's Teeth and Horse's Toes, 158–165. London: Penguin.

LaBarbera, M. (1983). Why the wheels won't go. The American Naturalist 121: 395–408.

In the simplest arrangement of the flagellum on the cell, there is one flagellum, or a group of flagella, at one end of the microbe, propelling the microbe forwards. To change direction, the flagella briefly rotate in the opposite direction, which causes the microbe to “tumble” and so change the direction in which it is facing (Figure 5.23). When the direction of original rotation is resumed, the microbe moves off in a new direction. In this way, a microbe can change its direction to move away from noxious substances or toward nutrients.

Figure 5.23 Microbial movements toward nutrients and away from toxins. A possible track of a microorganism that encounters a noxious substance or nutrients. Tumbling allows for the organism to randomize direction and, depending on whether it detects a greater concentration of noxious substance or nutrients after changing direction, it may initiate further tumbling events until it is moving either away or toward the concentration gradient.

Not all microbes have flagella just at one end of the cell; some have groups of them at both ends. Some members of the family Vibrionaceae, which are widely found in the environment and include the organism responsible for causing the disease cholera, either can have flagella at one end of the cell or can be all over the outside surface of the microbe, depending on whether they are attached to a surface or are free-swimming. Some of these different arrangements are shown in Figure 5.24.

Figure 5.24 Sketch showing different arrangements of flagella. (a) A single flagellum (monotrichous), (b) many flagella bundled at one end of the cell (lophotrichous), (c) one or more flagella at both ends (amphitrichous), (d) flagella distributed randomly over the surface (peritrichous).

Although flagella on the outside of cells are by far the most common means of achieving locomotion, it is by no means the only way microbes are able to move. Bacteria belonging to the group, the Spirochaetes, have numerous flagella located between the cell membrane and the cell wall. The microbes are helically shaped and can move using these internal flagella. However, to be able to move forwards, the flagella at both ends of the cell must move in opposite directions; if they moved in the same direction, the actions would cancel each other out, and the cells would be stationary.

Pseudomonas aeruginosa is a common and well-studied microbe, found in soil, water, and other moist locations. This species is an example of a type of microorganism that moves using much shorter structures than flagella, called pili (singular pilus). Pili can be extended from the cell surface, and they achieve movement by “twitching.” Short jerks can be observed as a slow movement of organisms when viewed under a microscope.