DIGESTION

process by which food is dissolved and chemically converted so that it can be absorbed by the cells of an organism and used to maintain vital body functions. In vertebrates, the main organs of the digestive tract are the mouth, the stomach, and the small and large intestines. Also important are the liver and pancreas, which produce enzymes that contribute to the breakdown of food in the digestive tract. Complex carbohydrates (polysaccharides and starches), fats, and proteins must be converted into simpler compounds before they can be assimilated into cells. During digestion, these organic compounds are reduced by hydrolytic enzymes, which split the long molecular chains by adding water molecules. Vacuolar digestive systems, found mostly in unicellular organisms, are the simplest forms of digestion. Food is ingested and passed through a cell in bubblelike chambers called vacuoles. Enzyme-containing organelles, or lysosomes, fuse with the vacuoles, and the enzymes convert the food into simpler compounds that can be assimilated into cellular material. After a vacuole has crossed the cell, it is expelled along with any undigested matter. One of the simplest groups of multicellular animalsthe spongeshave slightly more elaborate digestive processes called channel-network systems. In these cases, whiplike structures called flagella direct water and food along channels lined with cells that capture food particles and engulf them in vacuoles in which digestion occurs. In all other multicellular animals, special cells digest food for the entire organism. The bodies of hydra, for example, are composed of two cell layers: an outer layer (epithelium), which performs protective and sensory functions, and an inner layer (gastrodermis), responsible for digestion. The prey of a hydra is partly digested in a central cavity and is completely broken down within the vacuoles of the gastrodermal cells. Such systems are called saccular (i.e., the digestive cavity is a sac with a single opening that serves as both a mouth and anus).