Chemotaxis definition

The common denominator of all these mechanisms is that their end result is a directional change up a chemoattractant gradient or down a chemorepellent gradient. These mechanisms should be distinguished from chemokinesis (also termed orthokinesis [7]), which is a mechanism of response that does not involve directional changes and is, therefore, not a part of the broad definition of chemotaxis. In chemokinesis, the linear velocity of the cell or organism is altered by the stimulus [7]. Chemotaxis and chemokinesis may occur in parallel, as in the case of the response of sperm cells to substances secreted from the egg (Chapter 7). 

Gradient-stimulated modulation of the direction of movement (classical, narrow definition of chemotaxis) Amoebae (e.g., Dictyostdium discoideum), cells with amoeboid movement (e.g., tumor cells, white blood cells), spermatozoa (of, e.g., hydroids and tunicate) 5,6,7... 

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