polarity(redirected from cell polarity)
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the specific orientation of processes and structures in space, which is inherent in organisms and leads to the appearance of morphological and physiological differences at opposite ends or on opposite sides of cells, tissues, organs, and the organism as a whole. Polarity is manifested with particular clarity in plants. Even the multicellular strands of green algae and the hyphae of fungi are marked by polarity: their constituent cells are oriented in one direction. Polarity arises in the spores of algae, fungi, mosses, horsetails, and ferns only after an appropriate external stimulus. The cells begin to break down, giving rise to a new organism oriented in a certain plane. In seed plants, polarity is displayed even in the zygote and developing seed, where two rudimentary organs form, namely, the leaf bud and the root. In a developing plant organism, polarity appears in the predominant direction of cell divisions, growth, and differentiation. The polarization and differentiation of each cell depend on the position the cell occupies with respect to other cells.
The leading role in the polarization of cells and tissues and in the orientation of organs in space belongs to plant hormones. Thus, the implantation of a lilac bud in an undifferentiated callus tissue causes the polar formation of xylem strands. The addition of auxins to the implantation zone sharply increases the polarity. The growth of aboveground parts is activated in stem grafts by the action of gibberellins, and the establishment and growth of roots are promoted by auxins. The polarity of organs already formed generally is preserved even when their normal position is sharply disrupted (experiments on the inversion of grafts). However, in some cases it is possible to disrupt the polarity by altering ambient conditions (light, heat, moisture, chemicals). This changes the gradient of hormonal and trophic processes, which in turn determine the polarization of morphological and physiological structures.
In animals, polarity is observed both in cells and in the entire organism. In epithelial cells a distinction is made between the basal and distal parts, with a characteristic disposition of individual structures, such as the nucleus, the Golgi apparatus, and secretory granules. In nerve cells, polarity is expressed by the location of the axon and dendrites. In Protozoa, polarity is manifested in the location of organoids along the anteroposterior or dorsoventral axis. Polarity sometimes is present in an egg cell before fertilization, but it more commonly arises as a result of the penetration by a spermatozoid. The presence of physiological polarity has been established in hydrozoan polyps and worms. This enabled the British scientist C. Child to formulate the theory of physiological gradients—the variation of physiological activity and sensitivity to damaging factors along the longitudinal axis. Polarity effects also are observed during vegetative multiplication and regeneration. It has been possible in experiment to observe polarity distortion. For example, in the axolotl, transplantation of a section of an extremity may result in the formation of digits not only at the distal end of the transplanted stump but also at the proximal end.
REFERENCESKrenke, N. P. “Poliarnost u rastenii.” Izv. An SSSR Seriia biologiches-kaia, 1940, no. 3.
Sinnot, E. Morfogenez rastenii. Moscow, 1963. (Translated from English.)
Molotkovskii, G. Kh. Poliarnost’ razvitiia i fiziologicheskaia genetika rastenii. Chernovtsy, 1968.
Leopold, A. Rost i razvitie rastenii. Moscow, 1968. (Translated from English.)
Child, C. “Physiological Dominance and Physiological Isolation in Development and Reconstitution.” Wilhelm Roux’ Archiv Entwick-lungsmechanik der Organismen, 1929, vol. 117.
L. IA. BLIAKHER and V. I. KEFELI
a characteristic of chemical bonds that shows the redistribution of electron density in the space near the nuclei compared to the initial density distribution in the neutral atoms forming a given bond.
The “effective charges” of the atoms are a quantitative measure of the polarity of chemical bonds; an effective charge is the difference between the electron charge concentrated in a certain region of space (of the order of atomic dimensions) near the nucleus and the nuclear charge. It is an approximate measure, since it is definitely impossible to single out regions in the molecule that relate to individual atoms and individual bonds (if there are several).
Bonds are highly nonpolar only in diatomic homonuclear molecules; in all other cases they are polar to a certain degree. Covalent bonds are usually slightly polar, whereas ionic bonds exhibit strong polarity. The polarity of chemical bonds is sometimes indicated by symbols of atomic charges (for example, H+δ—Cl-δ, where δ is a certain part of the elementary charge).
polarity(1) The direction of charged particles, which may determine the binary status of a bit.
(2) In micrographics, the change in the light to dark relationship of an image when copies are made. Positive polarity is dark characters on a light background; negative polarity is light characters on a dark background.