tricarboxylic acid cycle: see Krebs cycle.

proton-proton cycle: see nucleosynthesis.

nitrogen cycle, the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids, which form proteins of nucleic acids, and of many other organic materials.

Nitrogen Fixation

Although the earth's atmosphere is 78% nitrogen, free gaseous nitrogen cannot be utilized by animals or by higher plants. They depend instead on nitrogen that is present in the soil. To enter living systems, nitrogen must be "fixed" (combined with oxygen or hydrogen) into compounds that plants can utilize, such as nitrates or ammonia. A certain amount of atmospheric nitrogen is fixed by lightning and by some cyanobacteria (blue-green algae). But the great bulk of nitrogen fixation is performed by soil bacteria of two kinds: those that live free in the soil and those that live enclosed in nodules in the roots of certain leguminous plants (e.g., alfalfa, peas, beans, clover, soybeans, and peanuts). Among the free-living forms are species of Clostridium, discovered c.1893 by Sergei Winogradsky, and Azotobacter, discovered c.1901 by M. W. Beijerinck. Both Clostridium and Azotobacter are generally present in agricultural soils, and both are saprophytes, i.e., they use the energy from decaying organic matter in the soil to fuel soil processes, including nitrogen fixation.

Bacteria that live in the roots of legumes are of the genus Rhizobium, first isolated c.1888 by Beijerinck. These rod-shaped bacteria enter the roots chiefly through the root hairs and then work their way to the inner root tissues. There they stimulate the growth of tumorlike nodules. Within the nodules the bacteria develop into forms called bacteroids, which live in a symbiotic (mutually beneficial) relationship with the green plant. The bacteroids take carbohydrates from the plant for energy to fix nitrogen and synthesize amino acids; the plants take the amino acids elaborated in the nodule to build plant tissue. Animals in turn consume the plants and convert plant protein into animal protein. Rhizobia can be found free-living in the soil, but they cannot fix nitrogen in the free state, nor can the legume root fix nitrogen without Rhizobia.

The exact biochemistry of nitrogen fixation within the nodule is not yet understood. It is estimated that more than 300 lbs of nitrogen per acre (340 kg per hectare) can be fixed by fields of alfalfa and other legumes. After a harvest legume roots left in the soil decay, returning organic nitrogen compounds to the soil for uptake by the next generation of plants. For this reason crop rotation in which a leguminous crop is rotated with a nonleguminous one is a common practice for maintaining soil fertility.

Other Aspects of the Nitrogen Cycle

Decomposing animal remains and animal wastes also return organic nitrogen to the soil as ammonia. Many different kinds of decay microorganisms participate in ammonification. The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely without any nitrogen being returned to a gaseous state. But still another group of microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-cultivated soils.

Effects of Artificial Fixation

Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen). Humans annually fix vast amounts of nitrogen for industrial purposes and for use as fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be upsetting the natural nitrogen cycle in the biosphere. There is some question whether natural denitrification can keep pace with fixation. For one thing, run-off of nitrate fertilizer can cause eutrophication of lakes and streams (see water pollution) and can foul drinking supplies. Another environmental problem is that inorganic fertilizers tend to depress legume fixation. As a consequence, root tissue remaining after harvest is poorer, and thus more fertilizer must be applied the following year.

glyoxylate cycle: see Krebs cycle.

cycle, in astronomy, period of time required for the recurrence of some celestial event. The length of a cycle may be measured relative to the sun or to the fixed stars (see sidereal time). A frequently observed cycle is the day, during which the sun seems to circle around the earth due to the earth's rotation on its axis; although the length of the day varies, the average day is defined as exactly 24 hr of mean solar time. Another important cycle is the year, during which the earth completes an orbit of the sun. The solar year is measured from one vernal equinox to the next and is equal to 365 days, 5 hr, 48 min, 46 sec of mean solar time (see calendar). The sidereal year, measured relative to the stars, differs in length from the solar year due to the precession of the equinoxes. The moon goes through a cycle of phases as it orbits the earth, completing a cycle from one full moon to the next in about 291/2 days, or one lunar month (see synodic period). The moon completes an orbit of the earth relative to the stars in one sidereal month, which is about 2 days shorter than the lunar month. Every 18 years, 111/3 days the earth, moon, and sun are in very nearly the same relative positions; for this reason, solar and lunar eclipses recur in a cycle with this period. This cycle was known to the Chaldaeans (fl. 1000-540 B.C.) and was called the saros by them. Halley's comet reappears in a cycle whose period is about 75 years. Astronomers also make use of various other cycles, e.g., those of sunspots and variable stars.

citric acid cycle: see Krebs cycle.

carbon-nitrogen-oxygen cycle: see nucleosynthesis.

carbon cycle, in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances. Animals eat the plants and are then eaten by other animals. When these life forms die, they decay, breaking down into, among many other things, carbon dioxide, which returns to the atmosphere. Plants and animals also release carbon dioxide during respiration. Animals and some microorganisms require the carbon-containing substances from plants in order to produce energy and as a source of materials for many of their own biochemical reactions; this cycle is vital to them. The process of incorporating carbon dioxide into the molecules of living matter is called fixation. Nearly all carbon dioxide fixation is accomplished by means of photosynthesis, in which green plants form carbohydrates from carbon dioxide and water, using the energy of sunlight to drive the chemical reactions involved. Green plants use carbohydrates to build the other organic molecules that make up their cells, such as cellulose, fats, proteins, and nucleic acids. Some of these compounds require the incorporation of nitrogen (see nitrogen cycle). When carbohydrates are oxidized in cells they release the energy stored in their chemical bonds, and some of that energy is also used by the cell to drive other reactions. In the process of oxidation, or respiration, oxygen from the atmosphere (or from water) is combined with portions of the carbohydrate molecule, producing carbon dioxide and water, the compounds from which the carbohydrates were originally formed. However, not all of the carbon atoms incorporated by the plant can be returned to the atmosphere by its own respiration; some remain fixed in the organic materials that make up its cells. When the plant dies, its tissues are consumed by bacteria and other microorganisms, a process called decay. These microorganisms break down the organic molecules of the plant and use them for their own cell-building and energy needs; by their respiration more of the carbon is returned to the atmosphere. The carbon-containing molecules that an animal derives from consuming other organisms are reorganized to build its own cells or oxidized for energy by respiration, releasing carbon dioxide and water. When the animal dies it too is decayed by microorganisms, resulting in the return of more carbon to the atmosphere. Carbon-containing molecules in wood (or other dry, slow-decaying organic materials) may be oxidized by burning, or combustion, also producing carbon dioxide and water. Under conditions prevailing on earth at certain times, green plants have decayed only partially and have been transformed into fossil fuels—coal, peat, and oil. These materials are made of organic compounds formed by the plants; when burned, they too restore carbon dioxide to the atmosphere.

Ulster cycle: see Gaelic literature.

Passion cycle, in art, the depiction of the last events in the life of Jesus. The Passion was a favorite subject of medieval and Renaissance artists and was considered the most ambitious of projects. The scenes depicted generally include the entry of Jesus into Jerusalem, the washing of his feet, the Last Supper, the Agony in the garden, the betrayal, the denial of Peter, Jesus before Pilate, the flagellation, the mocking of Jesus, the road to Calvary, the 14 Stations of the Cross (developed in the 14th cent. as a separate Crucifixion cycle), the Deposition, the Pietà (or Lamentation), and the Entombment. The scenes may be represented singly, as in Michelangelo's Pietà, or as a suite, as in Giotto's frescoes in the Arena Chapel at Padua. The artists' interpretations of what was to be represented in each scene were strictly circumscribed by convention and were usually limited to biblical descriptions of the events.

Metonic cycle: see synodic period.

Krebs cycle, series of chemical reactions carried out in the living cell; in most higher animals, including humans, it is essential for the oxidative metabolism of glucose and other simple sugars. The breakdown of glucose to carbon dioxide and water is a complex set of chemical interconversions called carbohydrate catabolism, and the Krebs cycle is the second of three major stages in the process, occurring between glycolysis and oxidative phosphorylation. This cycle, also known as the citric acid cycle, was named in recognition of the German chemist Hans Krebs, whose research into the cellular utilization of glucose contributed greatly to the modern understanding of this aspect of metabolism. The name citric acid cycle is derived from the first product generated by the sequence of conversions, i.e., citric acid. The reactions are seen to comprise a cycle inasmuch as citric acid is both the first product and the final reactant, being regenerated at the conclusion of one complete set of chemical rearrangements. Citric acid is a so-called tricarboxylic acid, containing three carboxyl groups (COOH). Hence the Krebs cycle is sometimes referred to as the tricarboxylic acid (TCA) cycle. The Krebs cycle begins with the condensation of one molecule of a compound called oxaloacetic acid and one molecule of acetyl CoA (a derivative of coenzyme A; see coenzyme). The acetyl portion of acetyl CoA is derived from pyruvic acid, which is produced by the degradation of glucose in glycolysis. After condensation, the oxaloacetic acid and acetyl CoA react to produce citric acid, which serves as a substrate for seven distinct enzyme-catalyzed reactions that occur in sequence and proceed with the formation of seven intermediate compounds, including succinic acid, fumaric acid, and malic acid. Malic acid is converted to oxaloacetic acid, which, in turn, reacts with yet another molecule of acetyl CoA, thus producing citric acid, and the cycle begins again. Each turn of the citric acid cycle produces, simultaneously, two molecules of carbon dioxide and eight atoms of hydrogen as byproducts. The carbon dioxide generated is an ultimate end product of glucose breakdown and is removed from the cell by the blood. The hydrogen atoms are donated as hydride ions to the system of electron transport molecules, which allow for oxidative phosphorylation. In most higher plants, in certain microorganisms, such as the bacterium Escherichia coli, and in the algae, the citric acid cycle is modified to a form called the glyoxylate cycle, so named because of the prominent intermediate, glyoxylic acid.

Fenian Cycle: see Gaelic literature.

Cycle that involves the continuous circulation of water in the Earth-atmosphere system. Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

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or Krebs cycle or citric-acid cycle

Last stage of the chemical processes by which living cells obtain energy from foodstuffs. Described by Hans Adolf Krebs in 1937, the reactions of the cycle have been shown in animals, plants, microorganisms, and fungi, and it is thus a feature of cell chemistry shared by all types of life. It is a complex series of reactions beginning and ending with the compound oxaloacetate. In addition to re-forming oxaloacetate, the cycle produces carbon dioxide and the energy-rich compound ATP. The enzymes that catalyze each step are located in mitochondria in animals, in chloroplasts in plants, and in the cell membrane in microorganisms. The hydrogen atoms and electrons that are removed from intermediate compounds formed during the cycle are channeled ultimately to oxygen in animal cells or to carbon dioxide in plant cells.

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Period in which several important kinds of solar activity repeat, discovered in 1843 by Samuel Heinrich Schwabe (1789–1875). Lasting about 22 years on average, it includes two 11-year cycles of sunspots, whose magnetic polarities alternate between the Sun's northern and southern hemispheres, and two peaks and two declines in the phenomena (e.g., solar prominences, auroras) that vary in the same period. Attempts have been made to connect the solar cycle to various other phenomena, including possible slight variations in the diameter of the Sun, sequences of annual growth rings in trees, and even the stock market's rise and fall.

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Circulation of nitrogen in various forms throughout nature. Nitrogen is essential to life, but in the atmosphere it is in a form (the diatomic molecule N₂) unavailable to most organisms. Nitrogen fixation by microbes turns this nitrogen into nitrates and other compounds, which plants or algae assimilate into their tissues. Animals that eat plants in turn incorporate the compounds into their own tissues. Microbes decompose the remains and waste of all living things into ammonia (ammonification); the ammonia may leave the soil through vaporization into the air or leaching into water. Ammonia remaining in soil may be transformed by bacteria into nitrates (nitrification), which then can be reassimilated into living organisms, or into free nitrogen (denitrification), which reenters the atmosphere. Hence, once fixed from air, some nitrogen goes through the cycle repeatedly without returning to the gaseous state.

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Periodic fluctuation in the rate of economic activity, as measured by levels of employment, prices, and production. Economists have long debated why periods of prosperity are eventually followed by economic crises (stock-market crashes, bankruptcies, unemployment, etc.). Some have identified recurring 8-to-10-year cycles in market economies; longer cycles have also been proposed, notably by Nikolay Kondratev. Apart from random shocks to the economy, such as wars and technological changes, the main influences on the level of economic activity are investment and consumption. An increase in investment, as when a factory is built, leads to consumption because the workers employed to build the factory have wages to spend. Conversely, increases in consumer demand cause new factories to be built to satisfy the demand. Eventually the economy reaches its full capacity, and, with little free capital and no new demand, the process reverses itself and contraction ensues. Natural fluctuations in agricultural markets, psychological factors such as a bandwagon mentality, and changes in the money supply have all been proposed as explanations for initial changes in investment and consumption. After World War II many governments used monetary policy to moderate the business cycle, aiming to prevent the extremes of inflation and depression by stimulating the national economy in slack times and restraining it during expansions. Seealso productivity.

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Circulation through nature of carbon in the form of the simple element and its compounds. The source of carbon in living things is carbon dioxide (CO₂) from air or dissolved in water. Algae and green plants (producers) use CO₂ in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO₂ to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO₂ in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO₂ via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).

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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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or Ulaid cycle

In early Irish literature, a group of legends and tales dealing with the heroic age of the Ulaid, a people of northeast Ireland from whom the modern name Ulster derives. The stories, set in the 1st century BC, were recorded from oral tradition between the 8th and 11th century and are preserved in the 12th-century manuscripts The Book of the Dun Cow and The Book of Leinster and later compilations. Reflecting the customs of a free pre-Christian aristocracy, they combine mythological and legendary elements. Among the stories are “Bricriu's Feast,” containing a beheading game that appeared in medieval narratives, and “The Tragic Death of the Sons of Usnech,” dramatized in the 20th century by William Butler Yeats and John Millington Synge.

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