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.

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.

Periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide. The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a “biological clock.” When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour (circadian rhythm), monthly, or annual cycles. Seealso photoperiodism.

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Gradual changes in size, shape, and function during an organism's life that translate its genetic potentials (genotype) into functioning mature systems (phenotype). It includes growth but not repetitive chemical changes (metabolism) or changes over more than one lifetime (evolution). DNA directs the development of a fertilized egg so that cells become specialized structures that carry out specific functions. In humans, development progresses through the embryo and fetus stages before birth and continues during childhood. Other mammals follow a similar course. Amphibians and insects go through distinctive stages that are quite different. In plants, the basic pattern is determined by the arrangement of lateral buds around a central growing stem. Different rates of growth of the plant's component elements then determine its shape and that of various parts. In both animals and plants, growth is greatly influenced by hormones; factors within individual cells probably also play a role.

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or germ warfare

Military use of disease-producing or poisonous agents, and the means for defending against such agents. Biological warfare agents include many bacteria, such as those which cause anthrax, brucellosis, and typhus; viruses that cause diseases such as equine encephalitis; fungi such as rice blast, cereal rust, wheat smut, and potato blight; and toxins such as botulinum and ricin that are extracted from living organisms. Biological warfare dates from ancient times when warring groups would try to poison enemy soldiers with rotting or diseased corpses, infect cattle and horses, or spread contagion through civilian populations. Following the horrors of World War I, a 1925 Geneva Protocol prohibited the use of biological agents in warfare; however, this did not prevent Japan from using them in China during World War II. During the Cold War the Soviet Union as well as the U.S. and its allies built huge stockpiles of biological agents. Both sides signed the 1972 Biological Weapons Convention, which prohibits the production, stockpiling, or development of biological weapons and requires the destruction of existing stockpiles, but the Soviets conducted a clandestine program until the 1990s. Biological weapons programs can be concealed easily, and the 1972 convention contains no provisions for inspection and reporting. As a result, many states have been suspected of developing biological warfare agents, and some modern armed forces have prepared defensive measures. These include battlefield sensors, protective garments and masks, sterilizing agents, and vaccines.

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Study of the physiological basis of behaviour. Traditional specializations in the field cover perception, motivation, emotion, learning, memory, cognition, or mental disorders. Also considered are other physical factors that affect the nervous system, including heredity, metabolism, hormones, disease, drug ingestion, and diet. An experimental science, physiological psychology relies heavily on laboratory research and quantitative data.

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In biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles. Biological membranes have a dual function: separation of vital but incompatible metabolic processes conducted in the organelles; and passage of nutrients, wastes, and metabolic products between organelles and between the cell and the outside environment. Membranes consist largely of a double layer of lipids in which are embedded large proteins, many of which transport ions and water-soluble molecules across the membrane. Seealso cytoplasm, eukaryote.

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Quantity of plant and animal species found in a given environment. Sometimes habitat diversity (the variety of places where organisms live) and genetic diversity (the variety of traits expressed within a species) are also considered types of biodiversity. The estimated 3–30 million species on Earth are divided unequally among the world's habitats, with 50–90percnt of the world's species living in tropical regions. The more diverse a habitat, the better chance it has of surviving a change or threat to it, because it is more likely to be able to make a balancing adjustment. Habitats with little biodiversity (e.g., Arctic tundra) are more vulnerable to change. The 1992 Earth Summit resulted in a treaty for the preservation of biodiversity.

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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

• Biological warfare
• Organic (disambiguation)