Food Web

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Food Web, set of interconnected food chains by which energy and materials circulate within an ecosystem (see Ecology). The food web is divided into two broad categories: the grazing web, which typically begins with green plants, algae, or photosynthesizing plankton, and the detrital web, which begins with organic debris. These webs are made up of individual food chains. In a grazing web, materials typically pass from plants to plant eaters (herbivores) to flesh eaters (carnivores). In a detrital web, materials pass from plant and animal matter to bacteria and fungi (decomposers), then to detrital feeders (detritivores), and then to their predators (carnivores).

Food Web
The sun is the original source of energy in virtually all ecosystems. Producers (plants) convert the light energy into chemical energy, storing it in their cells. When primary consumers (herbivores) eat the producers, the energy changes into a form that can be stored in animal cells. Secondary consumers (carnivores) transform the energy once again. Decomposers may occupy several positions in the pyramid, both receiving energy from decaying plants and animals and supplying it to detrivores and fungus-eaters.

Generally, many interconnections exist within food webs. For example, the fungi that decompose matter in a detrital web may sprout mushrooms that are consumed by squirrels, mice, and deer in a grazing web. Robins are omnivores, that is, consumers of both plants and animals, and thus are in both detrital and grazing webs. Robins typically feed on earthworms, which are detritivores that feed upon decaying leaves.

TROPHIC LEVELS

The food web can be viewed not only as a network of chains but also as a series of trophic (nutritional) levels. Green plants, the primary producers of food in most terrestrial food webs, belong to the first trophic level. Herbivores, consumers of green plants, belong to the second trophic level. Carnivores, predators feeding upon the herbivores, belong to the third. Omnivores, consumers of both plants and animals, belong to the second and third. Secondary carnivores, which are predators that feed on predators, belong to the fourth trophic level. As the trophic levels rise, the predators become fewer, larger, fiercer, and more agile. At the second and higher levels, decomposers of the available materials function as herbivores or carnivores depending on whether their food is plant or animal material.

ENERGY FLOW

Through these series of steps of eating and being eaten, energy flows from one trophic level to another. Green plants or other photosynthesizing organisms use light energy from the sun to manufacture carbohydrates for their own needs. Most of this chemical energy is processed in metabolism and dissipated as heat in respiration. Plants convert the remaining energy to biomass, both above ground as woody and herbaceous tissue and below ground as roots. Ultimately, this material, which is stored energy, is transferred to the second trophic level, which comprises grazing herbivores, decomposers, and detrital feeders. Most of the energy assimilated at the second trophic level is again lost as heat in respiration; a fraction becomes new biomass. Organisms in each trophic level pass on as biomass much less energy than they receive. Thus, the more steps between producer and final consumer, the less energy remains available. Seldom are there more than four links, or five levels, in a food web. Eventually, all energy flowing through the trophic levels is dissipated as heat. The process whereby energy loses its capacity to do work is called entropy.

Metabolism

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Metabolism (chemistry), inclusive term for the chemical reactions by which the cells of an organism transform energy, maintain their identity, and reproduce. All life forms—from single-celled algae to mammals—are dependent on many hundreds of simultaneous and precisely regulated metabolic reactions to support them from conception through growth and maturity to the final stages of death. Each of these reactions is triggered, controlled, and terminated by specific cell enzymes or catalysts, and each reaction is coordinated with the numerous other reactions throughout the organism.

ANABOLISM AND CATABOLISM

Two metabolic processes are recognized: anabolism and catabolism. Anabolism, or constructive metabolism, is the process of synthesis required for the growth of new cells and the maintenance of all tissues. Catabolism, or destructive metabolism, is a continuous process concerned with the production of the energy required for all external and internal physical activity. Catabolism also involves the maintenance of body temperature and the degradation of complex chemical units into simpler substances that can be removed as waste products from the body through the kidneys, intestines, lungs, and skin.

Anabolic and catabolic reactions follow what are called pathways—that is, they are linked to produce specific, life-essential end products. Biochemists have been able to determine how some of these pathways weave together, but many of the finer intricacies are still only partly explored. Basically, anabolic pathways begin with relatively simple and diffuse chemical components, called intermediates. Taking their energy from enzyme-catalyzed reactions, the pathways then build toward specific end products, especially macromolecules in the forms of carbohydrates, proteins, and fats. Using different enzyme sequences and taking the opposite direction, catabolic pathways break down complex macromolecules into smaller chemical compounds for use as relatively simple building blocks.

When anabolism exceeds catabolism, growth or weight gain occurs. When catabolism exceeds anabolism, such as during periods of starvation or disease, weight loss occurs. When the two metabolic processes are balanced, the organism is said to be in a state of dynamic equilibrium.

HOW METABOLISM DERIVES ITS ENERGY

In keeping with the first two laws of thermodynamics, organisms can neither create nor destroy energy but can only transform it from one form to another. Thus, the chlorophyll of plants, at the foundation of almost all food and energy-transfer webs (see Food Web), captures energy from sunlight and uses it to power the synthesis of living plant cells from inorganic substances such as carbon dioxide, water, and ammonia. This energy, in the form of high-energy products (carbohydrates, fats, and proteins), is then ingested by herbivores and secondarily by carnivores, providing these animals with their only source of energy and cell-building chemicals.

Nylon

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Nylon, term applied to a synthetic resin widely used for textile fibers, characterized by great strength, toughness, and elasticity, and processed also in the form of bristles and molded articles. Nylon was developed in the 1930s by scientists of Eleuthère Irénée du Pont de Nemours, headed by the American chemist Wallace Hume Carothers. It is usually made by polymerizing adipic acid and hexamethylenediamine, an amine derivative. Adipic acid is derived from phenol; hexamethylenediamine is made by treating adipic acid catalytically with ammonia and hydrogenating the product Hydrogenation. Nylon is insoluble in water and in ordinary organic solvents; it dissolves in phenol, cresol, and formic acid, and melts at 263° C (505° F).

In making textile fibers, small chips of the nylon polymer, which is obtained as a tough, ivorylike material, are melted and forced through holes in a metal disk called a spinneret. The filaments are congealed by a blast of air and are then drawn to about four times their original lengths. The diameter of the filaments is controlled by changing the rate at which the molten nylon is pumped into the spinneret and the rate at which the filaments are drawn away. Filaments much finer than those of ordinary textile fibers can be made from nylon. Nylon fibers can have the appearance and luster of silk or can be made to resemble natural fibers such as cotton; their tensile strength is higher than that of wool, silk, rayon, or cotton. Dyes are applied either to the molten mass of nylon or to the yarn or finished fabric. Acetate rayon dyes are usually used for nylon.

Nylon is used in the manufacture of fabrics for such articles as hosiery, night garments, underwear, blouses, shirts, and raincoats. Nylon fabrics are water-resistant; they dry quickly when laundered and usually require little to no ironing. Nylon fibers are also used for parachutes, insect screening, medical sutures, strings for tennis rackets, brush bristles, rope, and fishing nets and line. Molded nylon is used for insulating material, combs, dishware, and machinery parts.