DIGESTIVE PROCESS

sequence by which food is broken down and chemically converted so that it can be absorbed by the cells of an organism and used to maintain vital bodily functions. In order to sustain themselves, all organisms must obtain nutrients from the environment. Some nutrients serve as raw materials for the synthesis of cellular material; others (e.g., many vitamins) act as regulators of chemical reactions in living cells; and still others, upon oxidation in living cells, yield energy. Not all nutrients, however, are in a form suitable for immediate use by an organism; some must undergo physical and chemical changes before they can serve as energy or cell substance. Through the act of eating, or ingestion, nutrients are taken from the environment. Many nutrient molecules are so large and complex that they must be split into smaller molecules before they can be used by the organism. This process of breaking down food into molecular particles of usable size and content is called digestion. Unusable components are expelled from the organism by a process called egestion, or excretion. Some plants, many microorganisms, and all animals perform these three functionsingestion, digestion, and egestion (often grouped under the term alimentation)but, as expected, the details differ considerably from group to group. The problems associated with nutrient intake and processing differ greatly depending on whether the organism is autotrophic or heterotrophic. Autotrophic organisms are those that can manufacture the large energy-rich organic compounds necessary for life from simple inorganic raw materials; consequently, they require only simple nutrients from the environment. By contrast, heterotrophic organisms cannot manufacture complex organic compounds from simple inorganic ones, and so they must obtain preformed organic molecules directly from the environment. Green plants constitute by far the majority of the Earth's autotrophic organisms. During the process of photosynthesis, they use light energy to synthesize organic materials from carbon dioxide and water. Both compounds can be absorbed easily across the membranes of cellsin a typical land plant, carbon dioxide is absorbed from the air by leaf cells, and water is absorbed from the soil by root cellsand used directly in photosynthesis; i.e., neither of them requires digestion. The only other nutrients needed by most green plants are minerals such as nitrogen, phosphorus, and potassium, which also can be absorbed directly and require no digestion. There are, however, a few green plants (e.g., sundew, Venus's-flytrap, pitcher plant) that supplement their inorganic diet with organic compounds (particularly protein) obtained by trapping and digesting insects and other small animals. Heterotrophy characterizes all animals, most micro-organisms, and plants and plantlike organisms (e.g., fungi) that lack the pigment chlorophyll, which is necessary for photosynthesis. These organisms must ingest organic nutrientscarbohydrates, proteins, and lipids (fats)and, by digestion, rearrange them into a form suitable for their own particular needs. Additional reading William T. Keeton, James L. Gould, and Carol Grant Gould, Biological Science, 4th ed. (1986), is a comprehensive study that includes an examination of digestion. Charles J. Flickinger et al., Medical Cell Biology (1979); and Walter Hoppe et al. (eds.), Biophysics (1983; originally published in German, 1977), examine the structure of cells and cell membranes, the molecular mechanics of peptides, and the function of enzymes. Specialized studies include H.J. Vonk and J.H.R. Western, Comparative Biochemistry and Physiology of Enzymatic Digestion (1984); Thomas H. Wilson, Intestinal Absorption (1962); D.H. Smyth (ed.), Intestinal Absorption, 2 vol. (1974); B.P. Babkin, Secretory Mechanisms of the Digestive Glands, 2nd rev. ed. (1950); A.S.V. Burgen and N.G. Emmelin, Physiology of the Salivary Glands (1961); Arnold V. Wolf, Thirst (1958); Geoffrey H. Bourne and George W. Kidder (eds.), Biochemistry and Physiology of Nutrition, 2 vol. (1953); E.F. Annison and D. Lewis, Metabolism in the Rumen (1959); and J.B. Jennings, Feeding, Digestion, and Assimilation in Animals, 2nd ed. (1972). Chapters on digestion can be found in such comprehensive texts as Henry Gray, Anatomy of the Human Body, 30th American ed., edited by Carmine D. Clemente (1985); B.I. Balinsky, An Introduction to Embryology, 5th ed. (1981); and Leslie Brainerd Arey, Developmental Anatomy, 7th rev. ed. (1974). Frank H. Netter, The Digestive System: A Compilation of Paintings on the Normal and Pathologic Anatomy , 3 vol. (195762), is a part of the Ciba Collection of Medical Illustration series. Specialized works include Leonard R. Johnson et al. (eds.), Physiology of the Gastrointestinal Tract, 2nd ed. (1981); Horace W. Davenport, Physiology of the Digestive Tract, 5th ed. (1982); Charles F. Code (ed.), Alimentary Canal, 5 vol. (196768); John Morton, Guts: The Form and Function of the Digestive System, 2nd ed. (1979); R.J. Last, Anatomy, Regional and Applied, 7th ed. (1984); Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986); C. Ladd Prosser (ed.), Comparative Animal Physiology, 3rd ed. (1973); J.A. Colin Nicol, The Biology of Marine Animals, 2nd ed. (1967); and Robert D. Barnes, Invertebrate Zoology, 4th ed. (1980).