Kingdom without wheels
Do you recognize the image? It is a still from the film dramatization (Return to Oz) of L. Frank Baum’s 1907 text Ozma of Oz. These are Wheelers. If you look closely you will see that these fanciful folk sport wheels, rather than hands and feet, as terminal appendages. Isn’t it curious that Wheelers only exist in the make-believe land of Oz and that truly wheeled creatures are not part of the world’s realized menagerie? Although our hands and feet articulate freely about wrists and ankles, simple engineering precludes complete rotation about the joints. [Yes, the owl is able to rotate its head through 270º but only as the result of significant modifications to its circulatory and skeletal systems.] Because animal bodies are large, the movement of materials via simple diffusion isn’t very effective so they require all sorts of systems to do this more efficiently. Not only does the circulation pass the wrists and ankles, but so do bits of the nervous and lymphatic systems, not to mention bones and musculature. For obvious reasons then these articulations are not able to spin freely about the skeleton which supports them as an axle supports a wheel.
But what about the very smallest of living things? Because of their limited volumes and relatively large surfaces they are free of systems infrastructure (i.e., no plumbing) and are able to rely on diffusion to move things from place-to-place. Do we know of biological wheels among the bacteria, for example? The answer is yes and these include molecular rotary motors. These sound pretty complicated but they aren’t really. As analogy think of how the energy of falling water can be harnessed to drive a water wheel. Water flows from where it has higher potential energy (high up the mountain) to where it has lower potential energy and, as it passes the wheel, some of that loss in potential is passed to the wheel which may drive a hydroelectric power facility, for example. In much the same way molecular motors rely on gradients of protons which supply energy as a driving force. Remember that diffusion is the passive movement of molecules, in this case protons, from an area of high concentration to an area of lower concentration. As protons in areas of high concentration move down their gradient they provide the energy required to spin the motor at 1000 RPM. In the case of the bacterial flagellum, protons run through something called the stator which drives a free-spinning shaft which is connected to a hook (which acts as a universal joint) which is connected to the long flagellar filament. [Ok, so maybe it’s not quite that simple … energy from the cascading protons drives conformational changes in stator proteins which drive conformational changes in rotor proteins which cause rotation of the shaft, hook, and extended flagellar filament.] The image on the left shows a micrograph of the bacterial flagellar motor with a schematic on the right. The short video below shows an artist’s rendition of a moving bacterium and its incredible molecular motor. [The title of this post comes from an essay which may be found in the collected series by S.J. Gould entitled Hen’s Teeth and Horses Toes.]