Chemotaxis

CHEMOTAXIS, the phenomenon of the movement of lower organisms and mobile cells of higher animals toward or away from specific chemical stimuli. In the plant world, when there is no movement in space, but only a change in the direction of growth, this phenomenon is called chemotropism. If an organism moves or grows toward a stimulus, it is called positive chemotaxis; in the opposite case, it is called negative. Pfeffer developed a method for studying chemotaxis in microorganisms, which consists of the following: he introduced a liquid being tested for chemotactic effect into a capillary tube sealed at one end. Then, the capillary was placed with its open end into a drop of water containing microorganisms. In the case of positive chemotaxis, the microorganisms moved toward the capillary and even entered it; in the opposite case, they exhibited movement away from it. Using this method, it was possible to establish that for bacteria, peptone, meat extract, and asparagine are positively chemotactic; cane sugar, glycerin, and potassium salts are weaker; and sodium salts are even weaker. Opposite reactions are caused by free acids, alkalis, alcohols, and other toxic products. In the same way, positive chemotaxis was established in fern spermatozoa toward malic acid, in moss antherozoids toward a weak solution of cane sugar, and also for some other types of antherozoids. It is possible that in some organisms this phenomenon plays a role in the process of fertilization. Finally, by the same method, it was established that the movement of leukocytes to a focus of inflammation represents positive chemotaxis toward substances released by bacteria or toward products of tissue decay. Chemotaxis also plays a significant role in explaining the phenomena of phagocytosis. There is, however, a view that the chemotaxis of leukocytes during inflammation is a phenomenon of a purely physicochemical order (see Phagocytosis). An interesting example of chemotaxis is nerve regeneration, the direction of which is apparently determined by chemotactic irritations from the degenerating peripheral segment of the nerve. Among plants, chemotaxis has the greatest significance in the life of fungi and other saprophytic and parasitic plants, helping them direct their hyphae and suckers toward a source of nutrients. Some authors distinguish this form of chemotaxis under the name of trophotropism. Depending on the concentration of the irritating substances, the sign of chemotaxis can change, shifting from positive to negative at excessively high concentrations. For plant roots under natural conditions, as far as solutions are concerned, chemotropism is not of great importance, since the results of the influence of solutions on the entire root and on the root tip perceiving the irritation often cancel each other out. However, the action of water and air (aerotropism and hydrotropism) is significantly more important. If a vessel containing the root system of plants is sealed tightly, leaving only one opening, the roots will be directed toward this opening, moving toward the flow of oxygen.

V. Zamaraev.