ABSTRACT
This chapter describes about the swimming and crawling movements of animal cells as aimless random walks. But the purpose of these movements—the reason they evolved in the first place—is to carry a cell in a specific direction. Bacteria and other microorganisms swim because this takes them closer to sources of food, or farther away from noxious chemicals. The repertoire of behaviors shown by single cells is astonishingly rich and complex. Protozoa also have a wide repertoire of tactic behavior. At first sight the responses to externally applied electric fields seem artificial and of little relevance to a living animal. However, surprisingly large electric fields do exist naturally in living tissues. Crawling cells are guided by the terrain over which they move. Although a surface is necessary to provide lamellipodia with a firm anchorage, not all surfaces are equivalent. Sunlight is a ubiquitous environmental stimulus and many free-living cells have evolved the capacity to move either toward or away from it.
