What is chemotaxis?
Before explaining chemotaxis, I feel I should explain what taxis is to begin with.
Taxis is the movement of a motile cell or organism (such as a bacterium) toward or away from a specific stimulus. Knowing that, you might infer that chemotaxis is the movement of a motile cell toward or away from a chemical stimulus—and you'd be right!
In the case of bacteria, receptors are present in various locations, such as in or under the cell wall. The receptors receive chemical stimuli (e.g. Oxygen, ribose) and that stimulus is relayed to the flagella, which are whiplike filamentous appendages that propel the bacteria.
If the received chemotactic stimulus is positive, it is referred to as an attractant, and the bacterium moves toward the it quickly and with few “tumbles”. If the chemotactic stimulus is negative, it is called a repellent, and the bacterium moves away from it quickly, resulting in an increase in frequency of “tumbles”.
How does chemotaxis occur?
Its function is either chemical attraction (positive chemotaxis) of one cell or individual organism to another cell of organism or to any source of odor, or movement away from any such source of odor (negative chemotaxis). The general function of positive chemotaxis is to locate something of benefit to the responding individual, and the general function of negative chemotaxis is to escape a noxious or potentially harmful situation.
Positive chemotaxis is so widespread in nature that it’s difficult to generalize beyond that. Examples include attraction of bacteria to nutrients, attraction of sperm to eggs, attraction of white blood cells to sites of infection, attraction of a bee to a flower, attraction of salmon to the stream they were born in when they return to mate, attraction of a male moth to a female that’s advertising for a mate, attraction of mosquitoes to a person, wolves homing in on the scent of prey on the wind, birds migrating along “olfactory maps” of the terrain below, and you or I following our nose to an aromatic bakery up the street.
Examples of negative chemotaxis come to mind less easily. Animals’ defensive odors that repel enemies (like skunk odor) induce a negative chemotaxis in the fleeing individual. A fish turning away from an inflow of polluted water, antelope fleeing when they catch the scent of a lion, or someone stepping away from a person who hasn’t bathed in a long time, would be others.
Caution must be exercised in any such examples, because there are also behaviors called positive and negative chemokinesis that can achieve the same or similar purposes. In chemotaxis, the scent releases a movement directly toward or away from the source. In chemokinesis, the scent releases an increase or decrease in the animal’s motion, but the direction of motion is random. The response may involve walking more rapidly or slowly, or turning more frequently or less so. I know of cases of parasite larvae finding their hosts this way (blood fluke larvae attracted to snails), even though the larva doesn’t travel on a path directly toward its target.
This gets into such terminologies as chemo-orthokinesis and chemoklinokinesis, which is more than I care to spend additional time explaining in writing and maybe more than the reader cares to know anyway.
What is the difference between chemotactic and chemotropic movement?
Chemotactic: movement toward the chemicals without growth eg. Movements of leucocytes in our body
Chemotropic: movement with growth eg. Movement of pollen tube toward the micropylar end due to chemical secretion of synergid cell!