movement(redirected from molecular movement)
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(in biology), one of the manifestations of vital activity.
Animals and man. Movements enable an organism to interact actively with the environment—specifically, to move from place to place and to capture food. Movements are effected by specialized organs, the structure of which varies with the type of animal and depends on the type of locomotion and the nature of the habitat—terrestrial, aquatic, or aerial. The organs may be pseudopodia (the slow flow from one place to another of protoplasm; ameboid movement), cilia and flagella (ciliary and flagellar movement), or special body appendages by means of which the animals cling to a rough area of the substrate (setae, squamellae, scutella) or attach themselves to it (suckers). The most common construction of the locomotor organs, the limb, is a system of levers activated by muscular contractions. Some aquatic animals, such as sponges and corals, which maintain a sedentary mode of life, use cilia and flagella to set their immediate environment in motion and bring them food.
Animals can move about by (1) moving over a substrate, that is, upon solid or liquid support (walking, running, jumping, creeping, sliding), (2) moving freely in water (swimming), and (3) moving freely in air (flying). In all cases, the movements are the result of the interaction of forces external to the organism (gravity, environmental resistance) and internal forces (muscular tension, contraction of myofibrils, movements of protoplasm). Purposeful movements are possible only through the coordinated work of a large number of muscles, which is effected by the nervous system. Movements in water and air can also be passive. For example, certain spiders release their silk and are borne through great distances by the air currents. The soaring of birds, using air currents, is also a form of passive movement. Some aquatic animals have adaptations that maintain their bodies in a suspended state (for example, vacuoles in the external layer of protoplasm in radiolarians and air sacs in colonies of siphonophores). Active movements in water are accomplished by specialized remigial structures (from hairs and flagella to the modified limbs of aquatic turtles, birds, and seals), by flexure of the entire body (most fish, caudate amphibians), and by jet action, ejecting water from body cavities (medusoids and cephalopods). Active movement in air—flying—is characteristic of most insects, birds, and some mammals (bats). The movements in air of so-called flying fish, frogs, and mammals (for example, the flying squirrels) are not in fact flying but rather long, gliding jumps accomplished by means of such devices of support as elongated thoracic fins, interdigital membranes on the feet, and skin folds.
As animals evolved, the kinds of movement changed and became more complex. C. Darwin showed that in the course of evolution it was those kinds of movement and locomotor design that were vital and useful for the species that became fixed by natural selection. An important stage in the process was the development in vertebrates of the rigid skeleton and striated musculature. This entailed greater complexity in the structure of the nervous system and permitted a variety of movements, broadening the vital possibilities of the organisms.
In man, movements are the most important means of interacting with the environment and of influencing it actively, and they are highly varied: there are movements associated with the autonomic functions as well as with locomotion and the movements involved in work, everyday life, sports, speech, and writing. According to I. M. Sechenov, “all external manifestations of cerebral activity can in effect be reduced to muscular movement” (Izbr. proizv., 1953, p. 33).
There are two approaches to the study of animal and human movements. The first is the clarification of the biomechanical characteristics of the motor and support apparatus and a dynamic kinematic description of natural movements. The second (neurophysiological) approach studies the patterns of control of movements by the nervous system. The muscles that bring about movements have been found to be controlled reflexively by impulses from the central nervous system. The principal locomotor movements are inherited (unconditioned reflexes) and develop during the course of individual development (ontogeny) and as a result of constant exercise. The mastery of new movements is a complex process involving the formation of new conditioned-reflex connections and stabilizing them. After many repetitions voluntary movements are executed in a more coordinated and economical manner, and gradually they become automatic. Signals reaching the nervous system from proprioreceptors in the muscles, tendons, and joints are the most important factor in regulating movements. The proprioreceptors communicate information about the direction, magnitude, and speed of movements and activate reflex arcs in different parts of the nervous system whose interaction coordinates movements.
Plants. There are two basic types of movement in plants— passive and active. Passive, or hygroscopic, movements are caused by changes in the water content of the colloids that make up the cell membrane. In flowering plants, hygroscopic movements play an important role in spreading seeds and fruits. In the Jericho rose, a plant growing in the Arabian desert, the twigs are convolute when the air is dry but unfold when it is damp, break away from the substrate, and are borne off by the wind. Feathergrass and geranium fruits become buried in the ground because of hygroscopicity. In the Siberian pea tree the ripe pod dries out, two of its glumes become coiled into a spiral, and the seeds are forcibly scattered. Active movements are based mainly on the phenomena of irritability and the contractility of the proteins in plant cytoplasm and on the growth processes. Sensing the environment, plants react with intensified metabolic activity, acceleration of cytoplasmic movements, growth, and other movements. The stimulation perceived by the plant is transmitted along cytoplasmatic strands (plasmodesmata), after which the plant as a whole reacts to the stimulus. Weak stimulation intensifies, while strong irritation inhibits, the physiological processes. Active movements may be slow (as in growth) or quick (as in contractile movements). Growth movements include tropisms (the stimulation acts in a particular direction and there is unilateral growth, causing the organ to bend, such as in geotropism, phototropism, and chemotropism) and nastic movements (plant responses to stimuli in no single direction, such as in thermonasty and photonasty).
Contractile movements are often called turgor movements. These movements result from the interaction of adenosine triphosphate (ATP) and contractile proteins. Thus, the mechanism of contractile movements in plants is almost the same as that in the contraction of human muscle, the movements of slime mold, or the zoospores of algae. Active contractile movements include the spatial shifting of certain lower organisms—taxes—which, like tropisms, are caused by unilateral stimulation. Flagellate bacteria, some algae, and the antherozoids of mosses and ferns can react with taxis. Many algae (Chlamydomonadaceae) exhibit positive photo-taxis; the antherozoids of mosses collect in capillaries containing a weak sucrose solution, and those of ferns, in malic acid solution (chemotaxis). Seismonasty is a contractile movement probably caused by the contractions of cytoplasmic protein. Autonomous movements are similar to seis-monasties. For example, in the Indian telegraph plant (Des-modium gyrans) the compound leaf consists of a large blade and two smaller lateral blades that rise and fall like a semaphore. Under unfavorable conditions, such as darkness, these movements cease. In Biophytum sensitivum the leaflets fold up after strong stimulation, as in the mimosa, performing a number of rhythmic contractions. Apparently there is a breakdown and rapid restoration of ATP during this activity, which also causes leaves to move continuously under the influence of stimuli. The leaflets of the wood sorrel fold up in response to strong light, darkness, and elevation of temperature. They fold up toward evening and open again by night, apparently after the bond between the ATP and the contractile proteins is restored. ATP activity is high in plants capable of making nyctinastic (Acacia dealbata), seismonastic (Mimosa pudica), and autonomous (Desmodium gyrans) movements, but it is insignificant in plants that are incapable of movement (Desmodium canadensis). The ATP content is highest in tissues involved in the movements. The movements of mimosa leaves were once thought to be associated with a loss of turgor and an escape of water into the intercellular spaces in the leaf nodes. V. A. Engel’gardt (1957) has suggested that ATP participates in the osmotic phenomena associated with the movements of mimosa leaves and with the dehydration of its cells in the nodes.
P. A. GENKEL’
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