I never actually liked that example of what an organism would be doing to move without a means of a motor like function. This here is just a bit of a side note before I move on to talking about the actual section of the text, but I thought that I would mention that this non-motor assumption relies on the organism stopping between each motion. I find that when I've gone swimming before, I've tried out consecutive forward and backwards motions, and if anything, I've never undergone zero net motion, unless I allowed myself to stop between the strokes. However, if I did periodic strokes, the first stroke moved me forward, with the second slowing me down, slightly moving me back, the third moving me further forward from where I was after the first, and this cycle continues on.
Ok, now on to the section of the chapter.
As it was read, the backwards and forwards zero net motion assumption had been made, and as it was, it actually led scientists to the discovery of how bacterial flagella work. The discovery came about as many theories had, starting as a heretical idea. However, Berg and Anderson's rotary motion theory was proven by Silverman and Simon, who used mutant E.Coli, missing the flagella, and anchoring the flagellum stumps to a cover slip, the bacteria started rotating about. I can certainly agree with the text when they say that the flagellar motor is a marvel of nanotechnology.
It is worth mentioning though that the text does go on to mention that the e.coli does in fact follow the stop and go movement assumption (that is, it stops moving before it makes its next stroke), which indicates to me that turning while moving isn't an easy task for the bacterium to do.
Finally, the last part to this blog, the uses of movement as determined by the bacterium are foraging, where the cell constantly "tastes" the environment and moves towards the highest concentration of food, attack where the cell accelerates to grab its food before it escapes, and escaping, where the cell high-tails itself out of danger.
Be careful when generalizing your experience in the pool to the experience of a bacterium in the same pool. Perhaps it is worth an exercise: what is the Reynolds number characteristic of your motion in a pool?
ReplyDeleteKeep in mind that the bacteria WILL, in general, move. It will undergo Brownian motion, though, so the expected displacement (i.e. average) is zero. This means that the bacterium will have a very hard time controlling where it goes... A bacterium cannot drive "toward" anything for this reason - even if it could turn, it would be useless to try doing so, because the fluctuations of the solvent would tend to randomize its motion faster than it could compensate. The bacterium just decides to move or not move, it has no control over its direction. It rolls the dice until there is enough food around.
Keep in mind that the force you feel when you are turning in motion is inertial in nature - it is the acceleration accompanying your velocity adjustment. When we say that the bacterium lives in a Low-Reynolds number world, we are saying that inertia is so inconsequential as to be meaningless!