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The ability to move either toward or away from an increasing chemical concentration is a coordinated activity that many single-cell organisms can do. Single-cell animals, and bacteria, typically have some mechanical means of movement. Some bacteria use long external whip-like filaments called flagella. Flagella are rotated by a molecular motor to cause propulsion thru water. The larger single-cell animals may use flagella similar to bacteria, or they may have rows of short filaments called cilia, which work like oars, or they may move about as amebas do. Amebas move by extruding themselves in the direction they want to go.
The Escherichia coli bacterium has a standard pattern of movement when searching for food: it moves in a straight line for a while, then it stops and turns a bit, and then continues moving in a straight line again. This pattern of movement is followed until the presence of food is detected. The bacterium can detect molecules in the water that indicate the presence of food. When the bacterium moves in a straight line, it continues longer in that direction if the concentration of these molecules is increasing. Conversely, if the concentration is decreasing, it stops its movement sooner, and changes direction. Eventually this strategy gets the bacterium to a nearby food source.
Amebas that live in soil, feed on bacteria. One might not think that bacteria leave signs of their presence in the surrounding water, but they do. This happens because bacteria make small molecules, such as cyclic AMP and folic acid. There is always some leakage of these molecules into the surrounding water thru the cell membrane. Amebas can move in the direction of increasing concentration of these molecules, and thereby find nearby bacteria. Amebas can also react to the concentration of molecules that identify the presence of other amebas. The amebas themselves leave telltale molecules in the water, and amebas move in a direction of decreasing concentration of these molecules, away from each other.
The ability of a cell to follow a chemical concentration gradient is hard to explain using chemistry alone. The easy part is the actual detection of a molecule. A cell can have receptors on its outer membrane that react when contacted by specific molecules. The other easy part is the means of cell movement. Either flagella, or cilia, or self-extrusion is used. However, the hard part is to explain the control mechanism that lies between the receptors and the means of movement.
In the ameba, one might suggest that wherever a receptor on the cell surface is stimulated by the molecule to be detected, then there is an extrusion of the ameba at that point. This kind of mechanism is a simple reflexive one. However, this reflex mechanism is not reliable. Surrounding the cell at any one time could be many molecules to be detected. This would cause the cell to move in many different directions at once. And this reflex mechanism is further complicated by the need to move in the opposite direction from other amebas. This would mean that a stimulated receptor at one end of the cell would have to trigger an extrusion of the cell at the opposite end.
A much more reliable mechanism to follow a chemical concentration gradient is one that takes measurements of the concentration over time. For example, during each time interval—of some predetermined fixed length, such as during each second—the moving cell could count how many molecules were detected by its receptors. If the count is decreasing over time, then the cell is probably moving away from the source. Conversely, if the count is increasing over time, then the cell is probably moving toward the source. Using this information, the cell can change its direction of movement as needed.
Unlike the reflex mechanism, there is no doubt that this count-over-time mechanism would work. However, this count-over-time mechanism requires a clock and a memory, and a means of comparing the counts stored in memory. This sounds like a computer, but such a computer is extremely difficult to design as a chemical mechanism, and no one has done it. On the other hand, the bion, an intelligent particle, can provide these services. The memory of a bion is part of that particle’s state information.