plasma, biological: see blood.
rhythm, biological, cyclic pattern of physiological changes or changes in activity in living organisms, most often synchronized with daily, monthly, or annual cyclical changes in the environment. The exact nature of the internal mechanism, or "biological clock," that controls such rhythms is not understood. Rhythms that vary according to the time of day, called circadian rhythms, include such phenomena as the opening and closing of flowers and, in humans, changes in body temperature, blood pressure, and urine production. In diurnal animals, activity increases in daylight; in nocturnal animals nighttime activity predominates. Activity of many marine organisms varies according to the tide. Monthly rhythms include weight changes in men and the menstrual period in women. Annual cycles, or circannual rhythms, include bird migrations, reproductive activity, and mammalian hibernation. Daily cycles, or circadian rhythms, are in part a response to daylight or dark, and annual cycles in part responses to changes in the relative length of periods of daylight. However, environmentally determined cyclical changes, such as changes in daylight, temperature, and availability of food, serve primarily to refine and adjust physiologically determined circadian or circannual rhythms: in the absence of external cues, the internal rhythms gradually drift out of phase with the environment. Physiological rhythms are also present in the activity of individual organs, e.g., the beating of heart muscle and the activity of electrical waves of the brain.

See G. G. Luce, Biological Rhythms in Human and Animal Physiology (1971); J. Brady, ed., Biological Timekeeping (1982); L. Glass and M. C. Mackey, From Clocks to Chaos: The Rhythms of Life (1988).

nomenclature, biological: see classification.
symmetry, biological, similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed. Symmetry in body shapes is related to the lifestyles of organisms. Asymmetry, or the absence of symmetry, most often occurs in sessile organisms or in slow-moving forms such as amebas. Most other organisms can generally be classified in three groups with respect to symmetry type.

In spherical, or point, symmetry, any straight cut through the central point of a sphere divides it into mirror-image halves. Point symmetry, often called universal symmetry by biologists, is seen in some floating animals with radiating parts, such as the single-celled protozoans of the order Radiolaria.

Radial, or line, symmetry, as exemplified by a cone or a disk that is symmetrical about a central axis, is especially suitable for sessile or floating animals. Most radially symmetrical animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. Radial symmetry is seen in sessile organisms such as the sea anemone, floating organisms such as jellyfish, and slow-moving organisms such as sea stars, or starfish. Many jellyfish have four radial canals and are said to have tetramerous radial symmetry; sea stars, with five arms, have pentamerous radial symmetry. Many flowers, such as dandelions and daffodils, are radially symmetrical. Nonradial parts, such as the slit-shaped gullets of sea anemones, are often present in otherwise radial animals.

In plane, or bilateral, symmetry, one particular plane, termed the sagittal plane, divides the body into two equal halves, usually right and left halves that are mirror images of each other. Flowers such as orchids and sweet peas are bilaterally symmetrical. Bilateral symmetry is most suitable for actively moving organisms, as it permits streamlining, and is the most common symmetry among animals. In animals this symmetry type also favors the formation of main nerve centers and special sense organs and contributes to cephalization, or the evolutionary development of a head.

The word biological may refer to:

  • Adjectival form of "biology", the study of life
  • Biological (noun), a biological preparation (e.g., a vaccine or antitoxin) that is synthesized from living organisms or their products and used medically as a diagnostic, preventive, or therapeutic agent. (Often used in distinction to a pharmaceutical (drug)).
  • Biological agent, an infectious disease or toxin that can be used in bioterrorism or biological warfare.
  • Biological tissue
  • Biological process
  • Organism, a biological entity
  • Life, the characteristic state of organisms
  • Consanguinity, being descended from the same ancestor

Materials and substances:

Main disambiguation page: Biological material

See also

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